[[p. (i)]]
THE DYNAMICS OF ANIMAL DISTRIBUTION:
AN EVOLUTIONARY/ECOLOGICAL MODEL
BY
CHARLES HYDE SMITH
B.A., Wesleyan University, 1973
M.A., Indiana University, 1980
THESIS
Submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy in Geography
in the Graduate College of the
University of Illinois at Urbana-Champaign, 1984
Urbana, Illinois
[[p. (ii)]]
THE DYNAMICS OF ANIMAL DISTRIBUTION: AN EVOLUTIONARY/ECOLOGICAL MODEL
Charles H. Smith, Ph.D.
Department of Geography
University of Illinois at Urbana-Champaign, 1984
Investigators have sometimes assumed that the factors underlying animal distribution are too complex to lend themselves to normative modelling. In this work, a model of environment and community interaction is constructed which lends itself to the latter kind of interpretation. It is posed that spatially-varying rates and magnitudes of availability of moisture at given locations act as stresses on the nature of community infrastructure, and secondarily on the rates at which new populations may become integrated into these. Populations are thereby viewed as tending to change range in common directions, though at overall rates remaining peculiar to each. This interpretation is shown to lend itself to a synthesis of ecological and evolutionary approaches to distributional controls, and to suggest some novel viewpoints on the nature of evolutionary change and ecological interaction in general. The model is tested through a series of empirical studies of distribution patterns of all mammal and herptile species inhabiting the middle one-half of the United States. The results of the tests are in general consistent with expectations; for example, highly stressed community structures are shown to be correlated with smaller range sizes, greater variation in range sizes, and direction of dispersal. Spatial variation in stress magnitude is also shown to influence rates of faunal interaction between locations. The study concludes with a discussion of means whereby ecological modes of analysis can be extended to the study of evolutionary change over longer periods of time through the present model.
[[p. iii]]
ACKNOWLEDGMENTS
The author is indebted to a number of individuals who contributed time and attention to the development of this study. I should like to individually thank Gila Shoshony, Richard Chorley, Dave Swofford, Leigh Van Valen, Wayne Wendland, Donn Rosen, Torsten Hägerstrand, James Burt, Bruce Hannon, Judith Hill, Gareth Nelson, John Endler, and Mindy Wade for their various kinds of input, and to congratulate the members of my Committee for having the patience and confidence to see this work through to its completion.
[[p. iv]]
TABLE OF CONTENTS
Section Page
I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
II. CRITICISM OF THE LACK OF EMPHASIS ON SPATIAL INTERACTION
MODELLING IN ZOOGEOGRAPHY. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 9
An Historical Perspective on Trends of Study in Zoogeography . . . . . 9
Two Recent Innovations in the Study of Zoogeographic Patterns . . . . . 20
A Summary of Past Perspectives . . . . . 28
The Present Study . . . . . 31
III. MODEL DERIVATION: SYSTEM FRAMEWORK. . . . . . . . . . . . . . . . . . . . . . . . . .34
Energy and Mass Flow Through the Earth's Surface System . . . . . 34
System Controls and Exchanges . . . . . 37
Spatial Interaction and Evolution . . . . . 46
Spatial Interaction in the Community Context . . . . . 55
IV. SPATIAL INTERACTION AND RANGE CHANGE: AN INNOVATION
DIFFUSION MODEL. . . . . . . . . . . . . . . . . . . . . .. . . . . .
. . . . . . . . . . . . . . . . . . . . . . . .67
Innovation Diffusion Models . . . . . 67
Range Change Modelled as Innovation Diffusion . . . . . 73
V. FITTING THE MODEL FOR EMPIRICAL PURPOSES . . . . . . . . . . . . . . . . . . . . . . 80
A Measurable Surrogate for Stress . . . . . 80
Data Used in the Empirical Studies . . . . . 90
VI. SOME EMPIRICAL TESTS OF THE MODEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Introduction . . . . . 98
Analysis One . . . . . 104
Analysis Two . . . . . 107
Analysis Three . . . . . 110
Analysis Four . . . . . 112
Analysis Five . . . . . 114
Analysis Six . . . . . 115
Analysis Seven . . . . . 118
Analysis Eight . . . . . 122
Analysis Nine . . . . . 124
Analysis Ten . . . . . 126
Analysis Eleven . . . . . 129
Summary . . . . . 130
[[p. v]]
VII. EVOLUTION AND THE RECURSIVE DEVELOPMENT OF DISTRIBUTION
OF DISTRIBUTION PATTERNS . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . .136
Vicariance Events, Speciation, and Evolutionary Equilibrium . . . . . 137
Adaptability and the "Adaptive Landscape" . . . . . 158
The Analysis of Cumulative Pattern Development . . . . . 163
Altitudinal Zonation and Relict Populations . . . . . 173
A Regional Case Study . . . . . 179
Simulation Studies . . . . . 186
APPENDICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
VITA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 241
[[p. 1]] I. INTRODUCTION
[[p. 1]] Biogeography is often considered one of the most highly interdisciplinary sciences. Its complexities have sometimes prompted its students (see Myers, 1937: 340; Darlington, 1957: ix; Wilson, 1970: 1193; Elton, 1958: 34-35; Davies, 1961: 416) to complain that no one person can master all the cognate studies necessary to a full understanding of its subject matter, the distribution of organisms. Many have nonetheless entered into speculation regarding the causes of existing distribution patterns. Related discussions have usually arisen as a logical outgrowth of interest in the evolution and systematics of particular organismal lineages (the "geographical zoology" of Wallace: 1860, 1876) or through a desire to view community conditions at a given location in somewhat more general historical (Wallace's "zoological geography": 1860, 1876) and/or environmental (MacArthur's "geographical ecology": 1972) terms.
Regardless of the particular motivation for carrying them out, biogeographic studies have usually had as their goal increasing our knowledge of the biology of organisms (or groups of organisms). It is therefore not surprising that there is little theory that can be identified as elementally "bio-geographic;" that is, whose level of departure is organism-environment interrelation as opposed to organisms alone. This "geography as handmaiden to science" use of distributional data is not without its drawbacks. When the links among distribution, environment, and organismal function and change are stated in non-recursive terms, the resulting geographic perspective is predictable: distribution characteristics are demoted to the status of results--that is, to a kind of knowledge that cannot be used to examine future system change. Moreover, an [[p. 2]] idiographic position is forced. If one begins with the proposition that each population of organisms occupies unique and discrete spatial and temporal coordinates in the history of life, it is difficult to view associated causal conditions as being other than population-specific.
While it is often useful to think in population-specific terms, it seems counterproductive to argue that the study of organismal distribution must proceed from this starting point alone. At best this attitude is narrow-minded; at worst it is inconsistent, because it leads to the picture of a biosphere ordered at the level of its parts, but not in sum. We do not consider the concept of natural selection internally contradicting merely because the exact forces of selection on each population are different; neither should we dismiss the idea that the spatial distribution of populations might be interpreted as a single kind of response to some set of environmental influences. The natural selection model has provided a means whereby biological diversity can be ordered in logical fashion; similarly, we might be able to develop a general spatial interaction model relevant to the depiction of organismal distribution and its characteristics of change through time. Two main reasons can be identified for believing that the distribution of organisms (and the evolution of distribution) might be dealt with on such a wholly normative basis.
First, if we acknowledge that there has been a general ordered change in the level of complexity of organisms, we should also suspect that this has borne an ordered relationship to the general spatial setting within which it has taken place. The reason for this is fundamentally that the earth's surface is limited in extent, a constraint leading to a continual replacement of older forms by newer ones as lifestyle opportunities come and go. This process goes on and on, independent of the particular [[p. 3]] organisms involved. It is unthinkable that the spatial conditions which sponsor such turnover of populations can be considered as less than the necessary and sufficient causes for the characteristic patterns of distribution that are produced. It seems difficult to argue that a world with spatially-unlimited resources would promote competition for these; needs that could not be met at one location could always be satisfied simply by going elsewhere. A spatially-limited environment, on the other hand, stimulates competition by rewarding with survival those organisms that can most efficiently utilize available resources (Darwin, 1859a, 1859b; Wallace, 1859; Tilman, 1982).
The fact of a generalizable competition process among organisms provides a normative basis for the theory of natural selection. But while the results (biotic diversity) of competition/natural selection--or whatever biological evolution-inducing process we wish to imagine--can easily be understood to exist "in space," it is quite another matter to try to use such aspatial understandings to predict specifically what the spatial relationships constituting "in space" should be. There is nothing explicit in biological evolution models that leads, for example, to an understanding of the way instances of competition at a given time and place feed back to affect the nature of competition at later times in other places. To say simply that there is a "geographic component" to the process of evolution is to say nothing; where the matter of interest is the diffusion of an influence through space and time, this component and its recursive underpinnings must be specified to make possible hypothesis-testing and predictive science. Preferably, we should think in causal terms in this endeavor. Organisms are non-randomly distributed, and we should therefore expect the influences contributing to this order to be themselves [[p. 4]] non-randomly distributed in space (Getis and Boots, 1978; Baker, 1982; Sheppard, 1979; Rich, 1980); that is, to be linked to "preferred" routes through time and space for organismal change. Evolution is not just a process that takes place "in space", it is a process that takes place because of (or even as) space. Any regularities in the evolution of organisms should be referable to the spatial relationships promoting them.
Second, evolutionary outcomes are as much a function of input from the inorganic world as they are input from the organic world. Lest limited material resources be rapidly depleted and evolution abruptly stop at an early stage (see Hanson (1977), Cloud (1974, 1976), and Windley (1975) for related discussions), the first requirement of an evolutionary process occurring within a finite space is that it involve mechanisms by which resources can be used over and over, each time in a slightly different way, and often (or usually) by many different types of organisms. Nothing that is alive remains so for very long; indeed, even the elements comprising individual organisms when they are alive are continually being replaced. Given the fact that living things share materials that are both necessarily derived from the physical environment and passed back to it for recycling ("turned over"), regularities in the spatial structure of the physical environment must at some level be reflected in the way organisms evolve. Spatial variation in the way elemental resources turn over, for example, is certainly linked to spatial variation in competition regime types (E.P. Odum, 1969, 1971; Tilman, 1982; Hutchinson, 1964, 1978; Ford, 1982). Inasmuch as evolution: 1) is an irreversible process (Conrad, 1983; Prigogine, 1961) and 2) is characterized by changing patterns of interaction occasioned by the mobility of organisms/populations, such constraints on biological evolution in one setting can be expected to have delayed impact [[p. 5]] on other settings at later times. The study of the properties of organic diversity should therefore also be concerned with the regular way in which the unfolding of that diversity is constrained and/or enhanced by the distribution properties of exogenous physical influences. To exclude a consideration of the latter from biogeographic modelling is to make the mistake of thinking that the biosphere is an isolated system.
The present work is organized in three parts. The object of the first (Chapters II through V) is to develop a model of biotic-abiotic interaction that can serve as a generalized starting point for the study of the distribution of animal (or plant) populations. The first step toward this goal is a brief treatment (in Chapter Two) of biogeography's major philosophical positions of the last two hundred years; the reason for this analysis is not so much to criticize these with respect to their present domains of application, but instead to point out their inflexibility regarding the study of distribution as spatial interaction (see below for a definition of this term). From this beginning a model is developed which, it is hoped, will prove more adaptable in this regard.
A major portion of the first part of the work is devoted to characterizing changing distribution patterns as the reflection of a general progression in the ecosphere toward steady-state conditions. Two key elements in the discussion are the development of the concept of the "stress field," and an emphasis on community evolution-level constraints on range change in individual species populations. The stress field concept is introduced in an effort to specify how spatial variation in a critical physical environmental factor might influence range change in populations of organisms. It is argued that spatial variation in this factor must affect rates of "flow" (range change) of species populations--and therefore their [[p. 6]] collective spatial pattern--accordingly; a simplistic but somewhat useful analogy may be drawn with the relationship between shallow ocean bottoms of varying depth and the surface wave front patterns relatable thereto. The community level emphasis is productive in that community change is more easily linked to combinations of historical and environmental variables than is the evolution of an individual population to these; whereas the historical interaction of environment and populations is only implicit in the biology of speciation and but remotely connected to phylogenetic studies, the historical interaction of environment and communities might be explicitly interpreted as rates of integration and loss of populations into/from the latter. The "flows" of populations implied can then be dealt with on normative terms; likewise, the distribution patterns resulting. The spatial interaction process argued to underlie this flow is represented here through a modified innovation diffusion model in which the rate of "acceptance" of new populations into a given community structure is viewed as being in effect constrained by the local characteristics of the stress field.
An inherent feature of the discussion is that most of its conclusions arise from deductive arguments pertaining to systems concepts only indirectly related to the mainstream of biological research. Where possible I have attempted to provide a biological context, but at no point should the reader forget that the object of the work is to deduce from general system principles--and not the characteristics of specific organisms, populations, or communities--conclusions leading to predictions regarding the nature of distribution patterns. I am, in fact, logically obliged in all but a few instances to refrain from introducing any information gained from the study of the biology of organisms into my arguments; the specific reason for this [[p. 7]] will only become apparent by the third chapter. The general rationale is to provide a discussion in which process terms and structure terms are kept separate from one another as much as possible. I believe, in concert with other opinion (see, for example, Grene, 1971; Eldredge, 1981; Gould and Lewontin, 1979; Washburn et al., 1963; Ball, 1983; Ghiselin, 1966, 1984; Teichert, 1958; Hull, 1974; Gould and Vrba, 1982), that such separation is central to reducing the regressive thinking that can result when terms describing causal structures are synonymized with those referring to morphological structures. In Chapter III this strategy--a deliberate avoidance of individualistic "functional statements" (Nagel, 1961; Ruse, 1973)--will be shown to have implications that are both philosophically attractive and scientifically useful.
Much use of the term "spatial interaction" is made in this work, especially in the model development of the first part. We can define spatial interaction as both a general process and the events contributing to that process. In general terms, it can be viewed as a system of flows of various magnitudes connecting locations in space. The individual events that maintain such flows can be understood to have two fundamental properties. First, they must be recurring; i. e., their instances of occurrence in time and/or space must be non-unique and non-random. Second, they must occur in response to causal associations that develop between or among comparably-defined entities. Recurrence of events is a property needed to specify persistence; without the latter it is impossible to recognize the notion of "flow" or a system maintained thereby. The "comparably-defined entities" clause represents an effort to restrict the depiction of events to unambiguous cause and effect relationships (Nagel, 1961; Wolvekamp, 1982). I would argue, for example, that wars are fought [[p. 8]] between nations or the people making up nations, but not between one nation and the people making up another. Similarly, it is logically difficult to specify terms of causality between cells and organs or between organisms and whole communities because the entities involved are defined in such a way that they occupy the same space at the same time. A good example of spatial interaction in both its macro- and micro-level forms is afforded by international trade of goods. Here, the flows are stated in terms of "commerce" (dollars exchanged) among countries, and the events making these up, in terms of individual instances of purchase of goods.
The second part of the work (Chapter VI) relates the results of some empirical tests of the ideas developed earlier. The fundamental tenets of the model are combined in such a way as to yield predictions regarding the characteristics of pattern of distributional ranges (and boundaries thereof) in a particular study area. These predictions are then tested through the aid of univariate and multivariate statistical methods.
In the last part (Chapter VII), some topical issues within biogeography are addressed from perspectives grounded in the current model. Much of the early discussion in this section is of speculative nature, but in addition tests of some of the ideas presented are suggested and further empirical studies bearing on relevant matters introduced. The general object of this third part is to show that the model developed here can be used to consider longer-term implications of the evolutionary/ecological process underlying distribution patterns as well as its more immediate dynamics.
[[p. 9]] II. CRITICISM OF THE LACK OF EMPHASIS
ON SPATIAL INTERACTION MODELLING IN ZOOGEOGRAPHY
An Historical Perspective on Trends of Study in Zoogeography
Two of the most fundamental questions asked by zoogeographers are how populations of animals have come to be located where they are, and why those populations do not exist in other places (where one might for various reasons expect to find them). The complementarity of these two questions seems obvious enough now, but before the 1700's, not enough was known about the distribution of animals to even suggest they should be asked. In that century, two important developments took place.
First, it was discovered (thanks to the collective efforts of field naturalists) that ecologically-similar but geographically-distant areas tended to be populated by entirely different suites of species. This fact, first described in detail by Buffon (Buffon, 1749-1803; see discussion by Nelson, 1978; Nelson and Platnick, 1981) and now known as "Buffon's Law", was instrumental in forcing naturalists to think more carefully about the causal factors underlying the present distribution of organisms. Around the same time, knowledge of the fossil record began to congeal into a systematic understanding of the history of life. Paleontology suggested three further bits of information to be taken into account before the present distribution of animals could be understood: 1) that at present many organisms cannot be found in places where they obviously once flourished, 2) that most forms now living do not show up as fossils, and 3) that most forms known through fossils seem not to be represented by living organisms.
With the latter clues in hand, naturalists began to consider whether [[p. 10]] the biosphere might change through time in a fashion explaining Buffon's Law. There appeared to be two ways that such change could take place. The more conservative explanation--because it didn't conflict with Biblical teachings--was that distributional ranges alone vary with time. As the world was more fully explored, however, it became apparent that many forms known only as fossils were now truly extinct. Moreover, presently-existing forms seemed to appear on the average rather late in the fossil record. This latter fact suggested a second possible vehicle of change for the biosphere: that organisms themselves might have come into being at various points in time. Attention was drawn to how this might take place.
Two general kinds of causal models seemed appropriate. In the first, environment was viewed as forcing change; that is, as somehow directly specifying those adaptations that were needed to survive. The same kind of determinism could be invoked to explain distributional range changes in the shorter term sense; were the climate of an area to rapidly change, many existing occupants would be rendered unfit and forced out. On the other hand, it was also possible to imagine range shifts as being adaptive rather than forced; that is, as constituting a means of exploiting new opportunities. The trouble with this approach was that no one could propose a mechanism to explain constructive responses that was not inherently teleological (as had been, for example, the earlier "Great Chain of Being" understanding: Lovejoy, 1936). As a result, the first organismal change models proposed followed the idea that environment--and climate in particular--must "shape" organisms in a manner allowing them to exist under given conditions. This view was sponsored by most of the important thinkers of the period, notably Buffon (1749-1803), Maupertuis (1750), Forster (1777), Malthus (1798), Fabricius (1804), Montesquieu (1802), the older [[p. 11]] Candolle (1817, 1820), and, most of all, Lamarck (1809), who proposed a specific mechanism for organic change: the adoption of acquired characters.
Not everyone during the pre-Darwinian period was satisfied that organic change provided a satisfactory base for explaining current distribution characteristics, however. Many ignored both Lamarck and fossil evidence (Brooks, 1984; Kinch, 1980) and continued to believe that present patterns of diversity were simply a function of Divine Will, a position seemingly supported by the existence of disjunct distributions (Kinch, 1980). Some (e.g., P.L. Sclater, 1858), seeking a descriptive understanding of the Creation, adopted a quasi-scientific approach to the matter by systematically searching for "Centers of Creation" through faunal region delineation methods. Others (e.g., Charles Lyell, 1830-1833, 1972) were willing to accept the Creationist stance but still believed that the matter could be addressed using a scientific approach extending beyond mere description. To complicate matters further, some (e.g., Edward Forbes, 1846) defended the notion of Centers of Creation while at the same time arguing that post-Creation dispersals had rearranged original patterns of distribution.
The introduction of the Darwin/Wallace theory of natural selection (Darwin, 1859a, 1859b; Wallace, 1859) accelerated discussion further. Natural selection avoided an explicitly teleological stance on organic change but was still flexible enough to treat adaptation as a dynamic response to environmental/community conditions. Wallace (1860, 1863, 1866, 1869, 1876, 1880) led in applying the natural selection concept to the realm of zoogeography per se; by methodically drawing together the existing data of several different fields (notably, climatology, paleontology, paleogeography, and oceanography) and considering this information in light [[p. 12]] of past and present distributional records, he was able to evaluate the relationship of spatial differentiation to evolution (George, 1964; Smith, 1980, 1984; Fichman, 1977, 1981; Brooks, 1984). Specifically, once it was granted that organisms could evolve and had (varying) powers of dispersal, their occurrence in different regions over spans of time could be viewed as initial evolutionary events followed by dispersals away from place of origin (oftentimes leading to subsequent radiations in the new areas reached). This approach could be used to understand the evolution of biogeographic regions (and, ultimately, Buffon's Law), since periods of geographic isolation (whether locationally- or environmentally-induced) would prevent an area from receiving flows of new outside forms, thereby promoting the development of unique faunas.
The Darwin-Wallace synthesis was not immediately embraced by all workers interested in the study of distribution. A number continued to favor the earlier position that climatic influences largely dictate how (and where) a particular organism could evolve. This was seen as being especially so in areas of harsh environmental conditions (such as the arid American Southwest). The underlying logic was reasonable: it was difficult to believe that an organism unadapted to a certain climate would be able to disperse through it or into it. This argument had also been used by Agassiz (1850) and others to defend Creationist views on animal distribution (and is still used from time to time in modern contexts: note Lovtrup's (1981) criticism of Eldredge and Gould, 1972). This understanding was not as evolutionarily short-sighted as it may initially appear, since most of its advocates also believed in the inheritance of acquired characters (the "neo-Lamarckism" especially championed by Lester Ward, Edward Cope and Alpheus Hyatt--see Campbell and Livingstone, 1983; Livingstone, 1984). [[p. 13]] Climate could thus directly induce evolution (at the same time, however, the role of dispersal was rendered somewhat obscure).
These leanings extended to the way some thought zoogeographic regions should be portrayed. Allen's (1871, 1877) scheme, for example, was more of an ecological classification than a zoogeographic one, and was heavily criticized by Wallace (1876) and others for ignoring the evolutionary interrelationships of faunas. Merriam's "life zones" model (1890, 1894, 1898) was a similar effort to treat regionalization from a limiting factors perspective. Around the same time, the Russians (starting with Dokuchayev, 1951) began developing the analogous system of "zonal" classification, use of which has continued to the present day (Stegmann, 1938; Grigor'yev, 1936; Grigor'yev, 1961; Matveyev, 1972; Grishankov, 1973; Berg, 1947-1952).
The ecological approach to zoogeographic classification competed well with the historical approach-based Sclater/Wallace system (Sclater, 1858; Wallace, 1876) among biogeographers of the late nineteenth century. But even before the end of the 1800's it had become apparent that neither alone provided a wholly adequate basis for the study of distribution (Gill, 1885; Blanford, 1890). Lydekker (1896), apparently rediscovering the work of Candolle (1817, 1820), made an important attempt at reconciliation with his differentiation between the notions of "distributional area" and "station." This separation of concepts provided a useful tool through which the properties of distribution could be viewed from either a historical or ecological, respectively, perspective. Shortly thereafter, the acquired characters approach began to fall into general disfavor, and the environmental determinists were faced with attributing greater importance to the role of natural selection in evolution and dispersal or coming up with a better causal model than natural selection. Matthew (1915) eased many out [[p. 14]] of this dilemma by forging an argument for the role of dispersal in evolution that linked the nature of present and historical organismal distribution data to the distribution of climatic conditions. Most, unfortunately, found Matthew's correlations-based discussion so seductive that in the long run the development of biogeographic thought may actually have been retarded by its early uncritical acceptance (see comments by Croizat, 1981; Nelson, 1978).
Around the same time, the pace of investigation of distributional anomalies began to increase, partially in response to the comments of Wallace (Wallace, 1863, 1876, 1894; see Fichman, 1977, 1981). The elucidation of past and present constraints on avenues of dispersal became a major focus of interest. Such efforts often reduced to explaining present patterns via simplistic paleogeographic reconstructions. Many of these attempts were permeated by poor logic. For one thing, supporting geologic evidence was often meagre or entirely lacking--as in the land bridge theories of Joleaud (1924) and Ortmann (1910). Perhaps more embarrassing were the logical inconsistencies created by these ad hoc explanations. For instance, land bridge dispersal routes sometimes postulated to explain a supposed extension of range of certain organisms from place A to place B were often unable to reconcile the curious fact that no dispersals by other species had apparently taken place in the opposite direction. Simpson (1940, 1943) provides wonderful critiques of such excesses. Through the critical efforts of Simpson and others (for example, Schuchert, 1932; Myers, 1937; and Darlington, 1938) the transparency of many such ad hoc explanations was justly exposed.
The nineteenth and twentieth centuries have also witnessed a long line of progress in the study of the microclimate interface between organism and [[p. 15]] environment. Following the initial work of Liebig (1840) and extensions by Shelford (1911, 1913), the theory of limiting factors was expanded in many directions to account for all kinds of ranges of organismal tolerance of the environment. At times, biogeographers have attempted to apply such knowledge directly to a causal understanding of regional distribution patterns, but nothing of much worth has ever resulted (typical was the failure of Merriam's "life zones" approach). Unfortunately, while telling us a great deal about what evolution has accomplished at the individual level, the limiting factors/physiological ecology perspective falls short of identifying the role in evolution of causal processes of spatial and temporal magnitude greater than those referable to the lives of individual organisms. Gates (1970: 132) has stated:
"I have contended for a long time that if I knew the properties of a particular animal I could predict the climate within which the animal must live in order to remain in thermodynamic equilibrium....This will give us insight to the geographical distribution of animals throughout the world and their adaptation to various climates."
But Gates, a foremost modeller of the organism-environment interface (for example, Gates, 1962, 1980) does not consider the matter of the relation of organic change to geographical distribution. In his work (and that of many others: for example, Geiger, 1955; Shelford, 1911; Taylor, 1970; McNab, 1971, 1979, 1982; Kleiber, 1932; Vernberg, 1975; Scholander et al., 1950; Lowry, 1970) the focus is on the individual organism; again, this systems-analytical approach ignores larger-scale spatial and temporal components of the biological continuum. As a result, environmental, community, and population turnover properties (not to mention the evolutionary interaction of these) are not dealt with.
The extension of the limiting factors approach to the modern theory of [[p. 16]] the niche (MacArthur, 1955, 1968; Hutchinson, 1957, 1959; Savage, 1958; Preston, 1960, 1962; Whittaker, 1953, 1962) has been based largely on the idea that an organism's (or population's) sphere of activities can be expressed as a hypervolume defined by ranges of values of physical and biological variables. While this development opened the way for more sophisticated treatments of population- and community-level processes (examples include: 1) the gradient analysis studies of Whittaker and his followers: Whittaker, 1967, 1973, McIntosh, 1967; 2) ecological succession modelling: Horn, 1975, 1976, Odum, 1969, Pickett, 1976, Gutierrez and Fey, 1980; and 3) extensions of the general Lotka-Volterra model: May, 1976, Gilpin, 1975, Pielou, 1969, Schoener, 1976), it has been less successful in suggesting biogeographic models. This is probably because it is no less difficult to view an organism's niche as being other than the individualistic result of the evolutionary process that put it where it is than it is its suite of adaptations. Gould and Lewontin (1979) argue in this context that it is unproductive to dwell on the study of "unitary traits" because it becomes too easy to construct credible but untestable "just-so stories" accounting for these. As a result, evolution is trivialized, and possible cumulative structural controls on the general process of selection are ignored. This argument would seem to hold for the biogeographic context as well. When distributional range is viewed as being limited by combinations of factors specific to given populations, we are restricted to making geographical associations that can be stated only as ecological truisms or simple historical narratives (after Goudge, 1961). In so doing we also trivialize the meaning of geographical distribution: uniqueness is emphasized and we ignore the possibility that the individual distributional histories we can document may have evolved in response to the [[p. 17]] operation of non-population- and location-specific environmental influences.
I therefore believe that a dynamic and generally applicable model of the way distribution patterns evolve cannot be based on information derived from the study of the particular biological properties of organisms (i.e., their niche and phylogenetic relationships, morphology, and behavior); such attempts invariably force us into thinking in terms of correlations with the environment rather than recursive processes (a similar argument has been posed by Maze and Bradfield, 1982). Yet zoogeographers continue to begin their comparative historical and ecological studies of distribution with the study of the biological attributes of organisms. This approach is reasonably effective when interest centers on historical reconstruction of distributional change or ecological understandings of the relation between adaptation and environment, but is not so useful to promoting a synthesis of these two approaches. As a result, the spatial interaction processes directly linking ecological constraints to historical outcome are invariably weakly specified in biogeographic studies (a complaint also raised by Deignan, 1963; Croizat, 1958). As a substitute, pseudo-terms such as "dispersal" have been invented that put labels on distributional change rather than explicate same; that is, that reduce to narrative description the structural dynamics of the spatial interaction underlying such change. (Nelson (1983: 484) makes a similar point regarding the use of descriptive terminology in evolutionary studies.)
As long as narrative remains the preferred means by which the nature of distributional patterns and change in same is specified, I believe it cannot be stated fairly that the level of zoogeographic thought has significantly advanced beyond Wallace's synthetic philosophy of the mid-nineteenth century (see Gould and Lewontin (1979) and Nelson (1983) for further criticism [[p. 18]] linking Wallace to the present discussion).
There seem to be two immediate and related factors contributing to this impasse. First, it is now virtually taken for granted by most historical zoogeographers that the evolution of distribution patterns is to be viewed in terms of the individual evolutionary histories of those organisms involved. Nelson (1983: 490), for example, referring to "future prospects" in biogeography, comments: "The methodological approach will employ cladistics....The empirical approach (will concern) the use of biogeographical data as viewed in the cladistic aspect." I have already suggested that approaches beginning with the study of organismal traits are more likely to conceal the interaction processes serving evolution than explicate these. But this position can also be criticized on at least two other grounds. For one, it is in no sense obvious that models of change at the organismal and population levels will appropriately serve a synthetic understanding of distribution patterns (for example, the way regional faunas and floras evolve). Uncritical application of theory in this direction invites the individualistic fallacy, and criticisms of the type offered by Gould and Lewontin (1979). Workers involved in suborganismal-level research seem to have perceived an analogous danger and have responded accordingly: for example, by largely abandoning the idea of natural selection in the development of their problem-solving approaches. Instead, they treat it as a unifying concept to which they can refer as a means of relating the processes they study to other contexts (Kimura, 1983a). We should expect no more from evolutionary theory--or any other aspatially-based understanding--as a contributor to zoogeographic understandings.
A second criticism regards the overall strategy/object of historical biogeography. It has been suggested to me by members of the historical [[p. 19]] school that recent innovations (see below) in the methodology of zoogeography will make possible more accurate reconstructions of the history of past speciation events and associated distribution pattern evolution than had ever been thought possible (see also the comments of Platnick and Nelson, 1978). But what then? While this effort cannot in itself be condemned, it reflects something of a case of short-sightedness on the part of the historical school. Both geographers and historians are well acquainted with the long-term results of pursuing lines of study focusing on results rather than underlying generative processes: a general dwindling of interest. Geographers, at least, have been able to re-orient themselves toward process-oriented approaches (James, 1981; Johnston, 1981; 1983; Amedeo and Golledge, 1975; Davies, 1972; Griffith and Lea, 1983); historians, however, have had greater difficulty (Graubard, 1972; Gilbert, 1972; Cochran and Hofstadter, 1960). It is worthwhile to note, moreover, that geographers have found descriptive historical approaches unprofitable bases for normative modelling (note the present rejection, for example, of the geographical cycle notion of Davis (1899), the sequent occupance model (Mikesell, 1973), and the finalistic plant succession interpretations of Clements (1916)). An explanation for this is suggested in the following passage from Maruyama (1963: 174):
"....(when) the rules (of system evolution) are unknown, the amounts of information needed to discover the rules is much greater than the amount of information needed to describe the rules. This means that there is much waste, in terms of the amount of information, in tracing the process backwards than in tracing it forward."
In short, normative models provide more efficient description than does historical narrative. Thompson (1983: 168-169) has even gone so far as to argue that "generalizations which relate some present property to a [[p. 20]] developmental sequence of properties or events" (i.e., 'historical laws') "are not possible within current biological theory."
Efficient description, however, does not necessarily provide the kind of detail that is needed for many purposes of investigation (especially where regional evolution is involved). Ideally, we might wish to develop zoogeographic theory that is efficient in its generalization of process yet still capable of specifying unique conditions of interaction. This brings up the second problem that has retarded expansion of zoogeographic theory: the lack of a synthesis of historical and ecological approaches that can be used at an elemental level in the study of animal distribution patterns (see relevant remarks by Endler, 1982b; Eldredge, 1981). On the basis of preceding comments, it seems that such a synthesis should be forged from: 1) efficient treatment of the general processes organizing distribution patterns; 2) an awareness of the need to specify not only that such processes "occur in space," but precisely where in space under any given conditions as well; and 3) a distributional (spatial) emphasis rather than an organismal (biological) emphasis. Regarding the third element, it must be added that this emphasis should be capable of producing lines of thought that can be related to properties of biological organization (in a commensal fashion similar to that which has been so successful in, for example, the allied fields of genetics and population genetics).
Two Recent Innovations in the Study of Zoogeographic Patterns
The preceding comments, while critical in some instances, are not meant to suggest that zoogeography has been a static field of late. On the contrary, over the last fifteen years or so interest has risen to a relative level nearly comparable to that of the time of Darwin and Wallace. [[p. 21]] Nonetheless, I would argue that this interest can be attributed more to methodological advances and refinements in the available databases (for example, the effect that the development of plate tectonics theory has had on our reconstructions of paleogeography) than it can theoretical advances. It is important to understand the difference here; to improve the means through which data are reconciled is not necessarily--or even usually--equivalent to providing a new interpretative context for these. This is true whether or not such methods have genuine explanatory power, since explanation can be provided just as easily in immediate terms as in more general ones. (Kuhn (1962: Chapters Four and Five) develops a similar argument.) These remarks especially apply to the two most important recent contributions to the method of zoogeographic inquiry: the "island biogeography" of MacArthur and Wilson (1963, 1967) and "vicariance biogeography." A few comments on each should be made before we proceed.
MacArthur and Wilson's equilibrium theory of island biogeography comes closer to making normative statements about observed patterns of distribution than has any other work within the discipline of biogeography. It describes--through logic combining gravity model (distance-decay) principles and the notion of the evolution over time of an equilibrium between colonization and extinction rates--the diversity characteristics of biota on islands of varying sizes located at varying distances away from an assumed source of biotic propagules. As such, it provides a means of explanation that, importantly, is specific to neither setting nor organism type (Endler, 1982a). It has been applied and extended in many important ways; here, however, we must be concerned more with its main relevant limitations.
First, its sphere of application extends only to island or island-like [[p. 22]] situations (for examples of studies concerning the latter see Brown, 1971; Vuilleumier, 1970; Carlquist, 1974; Simberloff, 1974; Vuilleumier and Simberloff, 1980): generalization of the approach to continental conditions is not implicit (but see Smith, 1983b). Second, it implicitly treats islands as communities ipso facto; that is, the only condition of entry of colonizing groups into community infrastructure is the mere ability to physically reach and remain there. Distance and area factors become the main subjects of analysis, and while these can be used to provide portrayals of diversity relationships (May, 1975; Simberloff and Wilson, 1970; Simberloff, 1974; MacArthur and Wilson, 1967; Diamond, 1972; Preston, 1962; Connor and Simberloff, 1978), they are less useful to studying organism-environment interactions leading to evolution at the individual population level. Regarding this last point, Williamson (1981: 82-84) has outlined four basic weaknesses inherent in the theory in its original form: 1) it deals "explicitly only with the numbers of species, not with the numbers of individuals in species;" 2) the species of a given system of islands are dealt with as a lump sum, rather than as a functioning community; 3) historical factors influencing given sets of conditions are not taken into account; and 4) it does not take evolution in situ into account. Moreover, Williamson presents evidence that the main predictions made by the theory have not always been borne out in empirical tests. In sum, while the island biogeography approach is conducive to controllable analytical application, its use as more than an accounting framework that can only be used under special conditions is in some doubt.
The ideas leading to the development of vicariance biogeography may be traced to work by Willi Hennig, a German entomologist, and Leon Croizat, a Venezuelan botanist. Hennig's "phylogenetic systematics" (1965, 1966), or [[p. 23]] "cladism," started a true philosophical revolution within the field of biological systematics by suggesting that classification should be based on "natural methods"--the study of the order of origin of derived character traits--rather than the simpler correlative comparative morphology approach. Croizat started a second revolution (1958, 1962) by pointing out that in many instances regional faunal and floral units appear to center on oceans and other barriers instead of being separated by them. From this he inferred that species populations tend to be passively split over time by intervening environmental events, an idea conflicting with traditionalist views that speciation occurs as a result of active dispersal episodes. (It should be noted, however, that related views have actually been with us for some time: Wood (1860), for example, suggested that geologically-based post-Cretaceous isolation events were responsible for the maintenance of disjunct distributions of primitive birds and mammals; Wallace (1860) expressed similar ideas early in his career (Fichman, 1977, 1981).) It was a familiarity with both his work and Hennig's (which has implicit biogeographical ramifications) that led several workers in the early 1970's, most notably Gareth Nelson, Donn E. Rosen, J. S. Farris, and Norman I. Platnick, to forge a synthesis they tagged "vicariance biogeography." Summarized briefly, the approach deals with the study of the spatial arrangement of "sister groups", geographically-separated descendents of a former single species population (see Cracraft (1983) and Nelson and Platnick (1981) for overviews of the subject).
Vicariance biogeography has attracted a large group of vocal supporters, almost all of whom are comparative anatomists/systematists. I have no argument with the general approach itself espoused by workers in the field (e.g., Croizat et al., 1974; Platnick and Nelson, 1978; Nelson and [[p. 24]] Platnick, 1980, 1981; Cracraft, 1982; Rosen, 1978) beyond an unenthusiastic appreciation of the sometimes reactionary criticisms that are levelled at cladism in the more general sense (note the comments of Van Valen, 1978; Simpson, 1975; Mayr, 1974; also see Hull, 1979, 1983; Craw, 1983, 1984; Mayr, 1981). However, I greatly object to the inference seemingly taken by some vicariance biogeographers that this is the way by which historical biogeographic studies may most profitably advance (as is suggested, for example, by the title of Platnick and Nelson's 1978 work). Apart from the surficial fact that the method may have little relevance to many or most biogeographic concerns, historical or otherwise (for example, cultural biogeography, the history of domestication, extinctions, archaeo-zoogeography, the colonization and evolution of island systems, faunal dynamics under glacial regimes, physiological/geographical ecology, environmental conservation/disturbed habitats studies, dispersal/invasion studies, the more recent interaction characteristics of mammalian faunas (Flessa, 1976, 1981; Smith, 1983a, 1983b), etc.), it forces a manner of thinking that is not conducive to study of the way organism-environment feedback loops evolve and later influence events of organic change in other places. The problem may be described as follows.
Phylogenies can best be imagined as tree-like in structure, with bifurcations in a given tree representing instances of divergence of groups. In the elucidation of phylogenies, evolution must be viewed as irreversible and causally unambiguous, with particular descendents necessarily being derived from particular ancestral groups and complete reversion to an ancestral state not being possible. Nonetheless, all descriptions of evolutionary process through time are inferred, being a result of the way we interpret particular combinations of facts collected within given spatial [[p. 25]] settings. The problem is that interaction in space, unlike time, is: 1) multi-causal, or probabilistic, in nature; and 2) potentially of reversible character. As a result, there is an important element missing in attempts to view evolutionary processes in spatial terms solely from a phylogenies-based perspective: that which cannot be attributed to an unambiguous co-spatial and co-temporal causal factor cannot be understood (see the related complaints of Endler, 1982a; Craw, 1982, 1983; Hilborn and Stearns, 1982; Thompson, 1983).
Vicariance biogeographers argue, of course, that the solidity of this correlation between place and speciation is actually a strength of their approach, and to the extent that we consider it useful to be able to link the history of speciation events with the history of the immediate forces resulting in these, it certainly is. However, in the sense that this forces us to associate process with form in idiographic fashion (in essence, an "exceptionalist" position not unlike that defended by Hartshorne, 1939), it is also a great weakness. Vicariance biogeography is, in fact, a new but more sophisticated version of environmental determinism. Its major innovation lies in its re-directioning of attention to the controls on a process, speciation. The points made by earlier determinists (e.g., E.D. Cope, J A. Allen, F. Ratzel, C.H. Merriam, E. Huntington, R.D. Ward, and E.C. Semple) rested largely on correlations they attempted to make between structure and environmental conditions. These could not be translated into useful causal models; vicariance approaches, however, can. But even this advance contains biases that might lead, for example, to the fallacious reasoning that that which is causally simpler to specify is more important to an understanding of evolutionary process. This is well illustrated by the long-term interest in the study of "centers of creation" [[p. 26]] (Kinch, 1980; Nelson, 1978), now more commonly referred to as "centers of endemism" or "centers of evolution." Nelson (1978) and Cracraft (1982b) have claimed that these have formed the main subject of discussion in zoogeography since the eighteenth century. Apart from the fact that the historical accuracy of this assessment is debatable, the narrowing of attention to events of speciation in these areas is to be deplored as much as earlier overemphasis on dispersal processes (Craw (1984) has stated similar objections). To begin with, the mere fact that such areas may well have been, quantitatively, places where evolutionary causal factors produced the largest numbers of species is not necessarily an argument in favor of the idea that these are, qualitatively, the most important evolutionary centers. The latter, for example, might be better interpreted as those places where populations having the highest potential for eventually yielding radically new yet evolutionarily successful forms tend to locate. (The relevance of this remark becomes more apparent on consideration of the recent findings of Jablonski et al. (1983) concerning relative species turnover rates of onshore versus offshore marine invertebrate communities and resulting evolutionary trends.) Highly specialized forms such as those often characterizing centers of endemism in the tropics certainly cannot be thought of in these terms, as their peculiar adaptations (often involving complex mimicry, camouflage, and behavioral devices) lock them into time and place-specific associations. Areas dominated by such forms might even be better characterized as "centers of devolution" (note in this context Alberch and Alberch (1981) on the relation of truncated development to adaptation in tropical American salamanders, and the well known fact that tropical faunas contain many relict forms). It seems that a more worthwhile understanding of the spatial expression of the evolutionary process might be [[p. 27]] gained through the study of the types and rates of interaction occurring among locales (for example, centers of endemism and dispersal-dominated settings) than within them. Inasmuch as such interaction must be contextualized in spatial relationships, it is better considered on probabilistic than deterministic grounds. Vicariance biogeography methods, prescribing the analysis of individual and location-specific events that are not relatable to one another beyond means of historical narrative, can be of only limited aid to such study.
This argument, incidentally, is exactly parallel to that made by geomorphologists that it is usually more important in the study of regional landscape evolution to emphasize the roles of spatial/environmental factors than it is to dwell on the varying characteristics of the parent matter that provides the raw material for landform development--in this regard consider the general position of the climatic geomorphology school as exemplified in Budel, 1981; Tricart and Cailleux, 1972; Derbyshire, 1973. An analogous set of conclusions signalled the end of the school of anthropological evolutionism in the late nineteenth century (Hays, 1958; Lowie, 1937; Smith, 1980.
Nelson (1983) has attempted to put the vicariance biogeography movement into perspective by expressing his opinion (p. 489) that "the problem of biogeographical classification is due to the failure to recognize the fundamentals of biological classification." He goes on to conclude that now that we have discovered a truly "natural" means (i.e., cladistics) to the latter, vicariance biogeography can provide the route to a final replacement of the "artificial" regional systemizations of the past (for example, that of Sclater/Wallace). This may be a reasonable assessment if our goals center on the reconstruction of the history of distribution patterns and the [[p. 28]] placement of speciation events. If, on the other hand, we are more interested in how evolution reduces to a set of interactions that can be expressed in spatial/ecological terms, instances of vicariance can only be viewed as one kind of outcome in a more general, and wholly continuous, ecogeographic process (see the comments of Craw, 1982, 1983, 1984).
A Summary of Past Perspectives
An underlying dichotomy of positions regarding the nature of zoogeographic inquiry can be discerned in the sum of preceding words. Where historical studies are involved, workers tend to portray existing patterns of distribution as having come about in association with a continuing process of speciation, diffusion, and extinction that has yielded as a main outcome phylogenetic patterns and only as a by-product spatial ones. Where the object of study is ecological interaction, the present distribution of animals is explained by appealing to the selective influence of spatially-varying ecological controls. While neither position can be attacked as promoting an internally inconsistent understanding of animal distribution, each has its strengths and weaknesses bearing on related discussions:
Position one: strengths: The strongest elements in favor of this approach are its reliance on an internally-consistent, vast, spatial/temporal empirical base--the paleontologic record--and a historically successfully-applied evolutionary causal model that provides a longitudinal structure for that base.
Position two: strengths: Inasmuch as it must be true that successful range change cannot take place unless the population's individual components remain within their range of sum environmental tolerances throughout that [[p. 29]] change, the act of range change must be constrained by those tolerances within any given time period. Such tolerances can, at least in principle, be measured, and for any organism. Moreover, so too can many of the barriers that affect direction and rate of range change. As a result, formal modelling involving the specified effects of constraints influencing range change is made easier.
Position one: weaknesses: Where conditions are multi-causal, it is very difficult to frame testable hypotheses regarding the outcome of process within a purely historical approach, for at least two very different reasons. First, and in a zoogeographic sense practically speaking, each individual case of range change occurs only once, and involves the intersection in space and time of a nearly infinite number of variables whose relative influences have no possibility of comparison to a standard. More fundamentally, history-focused evolution models such as natural selection are at their core virtually untestable, or more exactly, unfalsifyable (Caplan, 1978). In the absence of testable propositions, the discussion of historical events is reduced to narrative (Goudge, 1961). In fact, the historical approach to zoogeography has been a classic example of inferring process from pattern all along: note, for instance, the studies of Wallace, Matthew, and Croizat. Geographers worry a great deal over the potential dangers of this method of inquiry (Getis and Boots, 1978; Harvey, 1969; Amedeo and Golledge, 1975; Abler, Adams, and Gould, 1971; Haggett, 1975), though biologists in general have seemed less concerned about the problem (see, however, the comments of Eldredge, 1981; Ball, 1983).
Position two: weaknesses: Despite its appeal in linking all present elements of the picture, the ecological approach is not flexible enough to suggest other than single cross-sectional correlations between present [[p. 30]] distributions and present environmental conditions. Past conditions-both of environment and the organisms themselves-can never be reconstructed to an extent approaching knowledge of present conditions; moreover, the limiting factors bias precludes explicit treatment of change in the system over time.
It is somewhat curious that no one has been able to construct a normative model of zoogeographic regionalization processes that explicitly views the characteristics of distribution as having arisen from an interplay of system potentials and constraints. The MacArthur and Wilson approach comes closest to this ideal with explicit delineation of area/remoteness constraints and a demography-based argument for an equilibrium turnover state, but the theory is too simple: the vector of colonization is chance dispersal, and the movement of propagules is essentially in one direction only. Moreover, islands are typically evolutionary sinks more than they are evolutionary sources (the "taxon cycle" notion of Wilson, 1961). These latter qualifications do not apply to continental conditions of evolution and dispersal. Rather, evolution and range change on the continents occur within/across a complex set of environmental gradients that sponsor non-equilibrium conditions. I believe it should be the job of zoogeographers attempting to model the overall regionalization process to show how this set of constraints acts upon intrinsic biological potential to yield the range change events whose ultimate result has been the distribution of faunas we now observe.
A survey of the more recent literature provides an indication that others and also thinking in a synthesis-oriented mode. Evolutionary ecologists, following the initial efforts of MacArthur (1955), MacArthur and Wilson (1963, 1967), and MacArthur (1969), have proceeded along aspatial [[p. 31]] modelling routes to analyses of speciation/extinction equilibrium relationships in an effort to understand global diversity patterns (see, for example, Rosenzweig, 1975; and Cody, 1975). But these efforts have done little to improve our understanding of spatial changes in distribution and the relevance of such to the overall pattern of evolution. Zoogeographers have increasingly called for attempts to develop pattern analysis techniques relevant to attribute study. Specifically, there have been calls for the erection of testable propositions regarding speciation mechanisms and the adaptational characters these would result in in present population distributions (Ball, 1975; McDowall, 1978; Nelson and Platnick, 1981; Platnick and Nelson, 1978; Nelson and Rosen, 1981; Simberloff et al., 1981; Endler, 1982b). But again, this mode of thinking is organism-oriented and cannot suggest how the recursive relations between organism and environment propel the whole process. In the case of dispersal-based speciation models, populations are assumed to be on the move, but for no clearly compelling reason. In the case of vicariance-based models, populations are assumed to diverge under deterministic circumstances that can be inferred to produce change, but that cannot specify the ecological dynamics of that change nor how it influences change at later times in other places (Craw, 1982, 1983, 1984).
The Present Study
In this work I propose a structural model consisting of theoretical statements linking environmental control factors to a dynamic equilibrium interpretation of distribution change. It therefore focuses on properties of spatial interaction among populations rather than the historical/evolutionary associations of these; this is in keeping with [[p. 32]] criticisms presented earlier. The goal is to provide an understanding of the spatial structure of organismal distribution patterns that: 1) is based on a reasonable interpretation of the immediate ecological controls on distribution; and 2) can be linked to an evolutionary perspective through its portrayal of the changes in spatial interaction that lead to such events as speciation. The stress concept mentioned in the first chapter is used to anchor the discussion on the ecological control aspect. The community level emphasis also mentioned in the first chapter is necessary to the development of a normative model of distributional change in the face of the individualistic hypothesis (Gleason, 1926; Cain, 1947; Whittaker, 1953) usually employed to understand the nature of the controls on individual populations. (The emphasis on community-level controls is not, however, an altogether radical move since the notion of community-level evolution has been a subject of recurring interest over the last century--see Wallace, 1876; Kropotkin, 1902; Allee, 1931; Wynne-Edwards, 1962; Van Valen, 1971; Wilson, 1976, 1980; Lewontin, 1970; Aarssen and Turkington, 1983; and Dunbar, 1960.)
The model can be summarized as follows. Populations of organisms are viewed here as systems that in sum contribute to a general environmental dynamic equilibrium involving the phenomenon of distributional change. As populations' distributional ranges change over time they enter into associations with other populations to form communities that are in essence ad hoc structures. The rate at which they enter into such associations is seen as determined by two independent factors: 1) an aspatial biological factor peculiar to each particular gene pool; and 2) a spatial factor which affects all populations. The first factor explains variation among populations with regard to the rate at which range change occurs. The [[p. 33]] second factor, spatial variation in the physical environment's potential to provide vital resources at rates and in amounts necessary to community-level function, controls across all populations relative rates of range change in different spatial directions. In this context, a potential surface (Sheppard, 1979; Baker, 1982; Rich, 1980) can be envisioned across which diffusing populations move; the rate at which movement in a particular direction at a given location takes place is viewed as dependent on the "topographic" characteristics of that portion of the overall surface. Where steep gradients in the potential surface exist, for example, population range boundaries will in theory be expected to extend more slowly, because the gradient is viewed here as mirroring degree of spatial variation in community structure. The conditions associated with "high" and "low" portions of the surface will also have biological implications. Where, for example, environmental conditions (as will be defined) are very suboptimal, within-community spatial interaction will become highly ordered; i.e., interaction among forms will be highly programmed, member populations being forced into highly specialized existences.
These notions, and their presumed meaning in terms of the pattern of spatial distribution of organisms, will be developed in the next three chapters. The chapter following them will be concerned with the framing of empirical tests of the set of ideas introduced earlier.
[[p. 34]] III. MODEL DERIVATION: SYSTEM FRAMEWORK
Energy and Mass Flow Through the Earth's Surface System
In this chapter a deductive style of argument is used to build a model of physical-biological system interaction that lends itself to the study of the distribution of organisms. The discussion developed leads to conclusions that are in some ways surprising and counter-intuitive, yet philosophically attractive and subject to empirical test. Development of the argument begins with the introduction of a simple model of the general flow of energy and mass through the earth's surface system.
Figure 1 gives one means of portraying the relevant relationships; this framework is taken as given in all the discussion following. Through this model, energy and/or materials on the earth's surface are viewed as continually circulating through two delimitable sectors, the "biotic" and "abiotic", and across two interfaces (between the abiotic sector and the extra-planetary environment, and between the biotic sector and the abiotic sector). The term "biotic sector" is meant to refer to the world sum of that which is living organism. All of that on the earth's surface which is not living organism--including organic wastes, carrion, not-yet-assimilated ingested foodstuffs, etc.--is assigned to the "abiotic sector."
Two interpretations of the system depicted in Figure 1 are possible: as a state-space, and as a recursive process. The state-space view conveys an ecological, or cross-sectional, interpretation in which the system is understood as: 1) open with respect to energy flow and closed with respect to material flow, and 2) operating under steady-state conditions. Analysis of state-space infrastructure must proceed under the assumption that there
[[p. 35]]
Figure 1. A general representation of mass and energy flow through the earth's surface system, with the latter envisioned as divided into two subsectors. See text for discussion.
[[p. 36]] is no progressive change in the components making up the system over time (Schrodinger, 1945; Maruyama, 1960, 1963). Under such uniformitarian conditions, when subsystems of limited lifespan reach the end of their usual terms of existence, they are replaced by like entities.
Cross-sectional studies tend to emphasize the means through which systems maintain equilibrium under ranges of conditions imposed by external forces. In general, subsystemization is viewed in such instances as contributing to system "invariability" (Weiss, 1971). This perspective leads to a view of organismal function dominated by a "deviation-from-norm" kind of thinking; i.e., that the functions of particular biological subsystems can be stated in terms of ranges of input to, and output from, the unit (Wiener, 1949; Ashby, 1956; Conrad, 1983). To one degree or another, therefore, studies linking the state of organisms to the immediate state of their environment are implicitly applications of the theory of limiting factors (Trudinger et al., 1979).
When the investigator wishes to study processes involving irreversible change, the cross-sectional approach depicted above proves too confining. In such work, a more critical consideration of the way intrasystem feedback controls develop becomes necessary. As Carson (1969: 76) states:
"A system may achieve equilibrium between form and process (assuming that the external variables which control the processes do not change) almost immediately in some cases; in other instances, the system may proceed so slowly towards equilibrium that an evolutionary approach is necessary to understand the nature of the system at any one point in time. In the situations where a system rapidly achieves equilibrium between form and process, an evolutionary model is unnecessary and a complete understanding of the nature of the system is furnished by a knowledge of the way in which the equilibrium pattern depends upon the external variables. An exception occurs when the outside variables themselves change through time in a systematic manner: although it is still possible to understand the nature of the system at any one point in time by references to the current state of the controlling variables, a more complete explanation is afforded by setting the system in a historical framework."
[[p. 37]] In the above passage Carson suggests two things of importance: 1) that evolutionary (irreversible) change in a system may be linked to controls set by exogenous variables, and 2) that irrespective of such change, the current state of the system can still be understood in terms of those same variables. Maruyama (1960, 1963) and Zadeh (1969) have introduced similar ideas. The notion that exogenous factors control organic evolution is not a new one, of course, but the tendency has been to dwell on the way such factors exert influences on the development of particular populations or phylogenies. In Chapter II I suggested that this strategy invariably leads to little more than the identification of correlations between adaptive responses and environment. This route will not be followed here. Instead, we will begin with the idea that the biotic sector as a whole evolves in response to constraints set by the abiotic sector. This argument will be made independently of any particulars regarding what we normally consider the "characteristics" of biological evolution (i.e., the temporal unfolding of phyletic lineages and appearance of associated adaptational innovations).
System Controls and Exchanges
It is relatively easy to state the conditions of existence of a system in steady-state with its environment. First, thermodynamic equilibrium is maintained, a simple consequence of the law of conservation of energy. Second, it is assumed under steady-state conditions that the amount of negentropy imported to the system remains equal to the entropy generated by it (H.T. Odum, 1971; Conrad, 1983; Huggett, 1980). This constraint limits the kinds of change possible within the system to uniformitarian kinds of adjustment; i.e., to the aforementioned maintenance process [[p. 38]] characterized by replacement of "worn-out" subsystems by subsystems of like structure.
The description of the state of a system changing in an ordered fashion through time is more complicated, since change must be explained in the face of ambient ecological equilibrium. As Huggett (1980) points out, the very word "equilibrium" implies absence of change, yet at some level of organization every system is undergoing change. The earth as a whole, for example, operates under very nearly steady-state conditions with respect to total energy throughput; nonetheless, its surface, at least, has undergone a continual process of evolution since it came into being. We must conclude from this historical fact that steady-state conditions have not actually been reached in the earth's surface system; that is, that negentropy import slightly exceeds total entropy produced. The first effect of this apparent paradox is to leave us with a problem regarding terminology. Some geomorphologists (see discussion in Huggett, 1980) have attempted to resolve this difficulty by viewing systems whose input-output balance changes only very slowly with time as being in a state of dynamic equilibrium, and this will be the solution adopted here. Thus, for purposes of cross-sectional study and depiction of the earth-level energy balance, the earth represents a steady-state system that is in dynamic equilibrium. With respect to the evolutionary development of its component subsystems, however, it is in continuous dis-equilibrium. In Carson's terms, form and process are not in equilibrium. (That they are not is all the more reason for avoiding historical models linking particular processes to particular forms, because the relationships involved are likely to be transient ones that cannot be spatially generalized.)
We will assume for the purposes of remaining discussion, and in concert [[p. 39]] with the opinion of others (e.g., Prigogine, 1947, 1961; Wiley and Brooks, 1982; Nicolis and Prigogine, 1977; Chorley and Kennedy, 1971; Iberall, 1976) that the earth's surface constitutes a nonequilibrium system describing a movement toward steady-state energy conditions and a dynamic equilibrium material turnover state. Given this framework, a model of the dis-equilibrium attending this movement will now be used as the base for making predictions about the way organisms should be distributed in space. We need first give attention to the general evolutionary setting of the biotic sector.
Though the emphasis here is on the evolution of the biotic sector, it is apparent that the conditions underlying change in it and the abiotic sector are mutually causal (in the sense of Maruyama, 1963): both energy and material resources move through each and back and forth from one to the other. As a result, intra-sector processes in each may be viewed as exogenous variables with respect to the operation of the other. Nonetheless, the two differ in that the abiotic sector as defined represents the only set of exogenous influences on biotic sector organization (whereas input to the abiotic sector originates in both the biotic sector and extra-system sources, especially the sun). This fact makes it easier to establish a simple causal model of biotic sector evolution. To maintain high levels of order in a living system, negentropy must be imported to it (Schrodinger, 1945; Maruyama, 1963; Kuppers, 1983). It follows from initial definitions that all such import to the biotic sector must pass through the interfaces between the latter and the abiotic sector. Across these move the resources that are necessary to the maintenance of biological activity; these have been "made available" to the biotic sector through the operation of return pathways that have evolved within the abiotic sector (e. g., [[p. 40]] biogeochemical cycles in the more obvious sense, and organismal death--which is often directly followed by ingestion by other organisms--in a less obvious sense).
Regardless of whether the abiotic sector can "make available" the resources necessary to life, negentropy import can only be accomplished when two conditions are met: 1) when organisms capable of assimilating resources exist, and 2) when the latter are present when and where the resources are available. If we are to define a state-space involving the biotic sector and its abiotic environment, therefore, we must grant that evolution has produced organisms capable of both finding and processing the resources necessary to their individual maintenance as steady-state systems. This is taken here as given.
A second fundamental notion is that obtaining and assimilating resources requires energy. This investment leads to an immediate net increase in entropy levels within the biotic sector as chemical energy is converted to heat. The increase is then balanced, however, by the negentropy gained (imported) as the ultimate result of assimilation of foodstuffs.
The important thing to note in the straightforward description given above is that it suggests two different views of the meaning of the energy budget of organisms (singularly or in the collective sense). When assimilation processes are viewed in terms of the energy budget supporting them, steady-state conditions are implicitly recognized and the meaning of the energy expended cannot be extended to an evolutionary understanding. On the other hand, when the energy budget of the organism is seen as committed to spatial interaction with the elements of its environment, a more dynamic view is possible. Specifically, the energy expended by the organism can [[p. 41]] potentially be understood as contributing to change within that environment. Where its activities result in a change that ultimately leads to a net reduction in the amount of energy that need be expended to obtain and re-assimilate a given resource, it follows that evolution within the overall biotic sector has occurred: the same amount of negentropy has been imported at a lower cost in entropy production. We might thus understand biological evolution to proceed as a nonequilibrium process in which organismal activity contributes to a continually more efficient resource turnover process.
The idea that refinement in the turnover characteristics of environmental resources is related to community development toward steady-state conditions is fundamental to the theory of ecological succession. E.P. Odum (1971: 256-257), for example, states:
"An important trend in successional development is the closing or 'tightening' of the biogeochemical cycling of major nutrients, such as nitrogen, phosphorous, and calcium...Mature systems, as compared to developing ones, have a greater capacity to entrap and hold nutrients, for cycling within the system."
The recursive view of change expressed in the above seems at first encounter similar to the systems view of surface processes evolution being developed here. Moreover, inherent in the notion of succession is the idea that physical environmental changes have an important effect on where and when community change will occur. Nonetheless, ecological succession must be viewed as a uniformitarian process when placed within its context in system evolution in general, and probably lacks the flexibility to be applied to the study of the evolution of populations. This is most forcefully evident in the difficulties attending use of the term "climax" (E.P. Odum, 1971; Oosting, 1956; Whittaker, 1953). The ecological climax is a steady-state condition virtually by definition, a philosophical constraint that conflicts [[p. 42]] with the knowledge that population-level evolution must be proceeding even as ecological equilibrium is reached. Perhaps the best that can be done under these circumstances is to suggest that the "tightening" of resource cycles must work at two levels: one involving the short-term integration of populations into stable, self-reproducing community structures, and another involving a kind of change in organisms that leads in the long-term sense to ever more efficient climax structures. Our interest is more in the latter process, so succession will not be dealt with further here.
The recursive nature of development of resource cycles is also treated in the literature on biogeochemical succession (note, for example, Trudinger et al., 1979; Hutchinson, 1964; Cloud, 1976; Van Valen, 1971b; Windley, 1975). The orientation of related discussions, however, tends to be similar to that concerning ecological succession. Again, this introduces a degree of inflexibility that is difficult to overcome when evolutionary explanations are sought. A more adaptable understanding can be obtained by more directly relating the turnover of resources to population-level change.
Every organism (or population) acts as a mediator in the general turnover of resources in its encompassing environment. Obtaining resources requires an expenditure of energy, and, as suggested earlier, if there should occur from one turnover cycle to the next a general reduction in the amount of energy expended by a population to obtain a given resource, all other things remaining equal it follows that the overall system has undergone evolution (note, however, that we cannot conclude that evolution has occurred within that particular population). This will be true most fundamentally because there will have been a net reduction in the amount of entropy produced by the system over time. Before we can suggest how this idea is related to the evolution of individual populations, however, we must [[p. 43]] return for a while to discussion of biotic-abiotic sector interaction.
Complex systems are often characterized in terms of feedback relationships. Huggett (1980: 91) describes these as follows:
"....The interplay of positive- and negative-feedback relations in a system can be subtle. Paradoxically, both types of relation can operate simultaneously to maintain the over-all stability of a system....homeostasis is that group of system-stabilizing relations which are characterized by negative feedback; homeorhesis is that group of system-stabilizing relations which are characterized by positive feedback. Homeostasis may be thought of as all those relations which act to preserve a system by keeping it in steady state during its existence. Homeorhesis may be thought of as all those relations which act to preserve not a steady state but a flow process...."
In the general system described by Figure 1, the biotic sector is characterized by the interaction of single positive and negative feedback processes. Positive feedback from the abiotic sector enters the biotic sector as a flow of (potential) energy and materials which fuels both life-sustaining and system-changing processes. How should we characterize this flow in terms of the phenomenon of adaptation? In one sense, adaptations can be viewed as homeostatic devices. Again, when the energy budget of an organism is considered in terms of its own thermodynamic equilibrium, steady-state conditions are envisioned; the structural ends to such self-maintenance may be termed adaptations. On the other hand, when its energy budget is viewed as being committed to a routine of activity that contributes to spatial interaction, the adaptational suite of the organism can be construed as serving a homeorhesic purpose. Potential nonequilibrium conditions are created when the organism returns energy and materials to the abiotic sector in amounts essentially equal to those received, but at different locations. All we need assume is that there is spatial interaction among the locations of input and output; i. e., that the organism's operation within its behavioral space contributes to a non-random [[p. 44]] change in the manner in which energy and materials are later made available again.
The biotic sector evolution model initiated earlier can now be completed in outline by suggesting that it is the process of adaptation that signifies increasing levels of negentropy in the system; that is, that mediates evolutionary change by making possible increasingly efficient resource turnover. This leads us in the direction of spatial interaction modelling. Thus far, I have suggested that general system evolution occurs as abiotic-biotic sector equilibrium is lost, or, more precisely, as adaptation produces the means whereby organismal activity results in a net reduction in the amount of energy required to return vital resources to the same stage in a given cycle type. I have not yet specified the mechanism that actually initiates the process of adaptation that keeps the members of each population system in continuing thermodynamic equilibrium as the abiotic sector changes in response to historical inertia. This, I maintain, is movement through space and the spatial interaction that is part and parcel of that movement. A primary characteristic of living things is their ability to change location within their frame of reference. Such movement may be restricted to certain portions of a life cycle, but there is surely nothing alive which lacks this ability. Changes in location serve the immediate purpose of bringing an individual organism into physical reach of those items necessary to life maintenance; in the longer term sense, moreover, population-level locational adjustments may be viewed as acts which are necessary to/inherent in the continuation of steady-state conditions in the face of the reality of ever-changing environmental conditions. However, the steady-state approached at any cross-sectional instant can never be maintained. The particular suite of adaptations [[p. 45]] developed in response to one set of conditions will never be quite appropriate in dealing with any later conditions. The biological process representing the on-going resolution of this disequilibrium is competition; the biological result of the process is the development of new responses to the sets of conditions encountered; i.e., new adaptations.
The above line of reasoning is not particularly remarkable and requires little further comment here. What I should like to explore in the remainder of this work is the possibility that the net direction of organismal locational adjustment on the terms presented may be predictable. If this is so and the main underlying factors can be isolated, we will then have a model which can be applied to more than just a consideration of net biotic sector evolutionary trends.
It is relatively easy to use the ideas presented so far in this chapter to develop a model leading to the prediction of direction of range change by populations. Simply, we should expect those areas where vital resources are being returned to availability at the most ideal rates to be those toward which ranges will most likely extend. Where resources are returned at suboptimal rates (note that the term "rate" here is used to denote a pattern of return involving both spatial and temporal components) it will be necessary for organisms to evolve special adaptations (behavioral and/or morphological) enabling them to maintain thermodynamic equilibrium while functionally being "at the right place at the right time" to collect and process these resources. (A good example of this kind of adjustment is offered by the irregular conditions of water supply in deserts and the many special strategies that desert plants have invoked to ensure successful reproduction when precipitation does occur). The process of selection underpinning such change will consume both energy and time; as a result, [[p. 46]] suboptimal habitats will be occupied more slowly, all other things being equal.
It can be argued that earlier statements being true, the above must be true as well. Were range adjustments directionally random in nature, we would expect to derive no net change in the overall state of the system--that is, evolution would not be occurring, because all instances of relative gain in system information levels would be balanced by relative losses elsewhere. Directional channelling of range change is thus the only way that consistent gains can be made.
The focus of this discussion must now begin to change from deduction to the identification of means whereby the ideas can be tested. This will first involve relating the dynamics of population range change to the supposed spatial variation in return rate characteristics of vital resources.
Spatial Interaction and Evolution
The most obvious spatial characteristic of a population is its distributional range. This will change through time as the population responds to various influences; in fact, distributional range is a direct correlate to the evolutionary history of a given population, since it will appear as a delimitable pattern with the initial divergence of the form and disappear with its extinction. This correlation between range and habitat has frequently been interpreted as a causal relationship. Limiting factors concepts better referred to individual-environment interaction have been used time and time again to justify the idea that species X appears limited to a certain areal distribution. Whether this extension of individual level thermodynamics to the population level is legitimate is debatable, but use [[p. 47]] of the device at least provides a reasonably straightforward portrayal of microstate dynamics. The state-space picture that emerges is of a direct coupling between a positive feedback flow (materials and energy made available to the biotic sector as a function of their transmission through the abiotic sector) and a negative feedback response (behavior and morphology).
Recall that the steady-state recognized above is, however, a fiction in the longer-term sense. We know that the system, and its component populations, change irreversibly over time. I have already suggested how we might view this change at the level of biotic sector functions. To apply these notions to the modelling of individual population change necessitates re-assessing the notion of environmental "control." Specifically, the action of environment must be interpreted as forcing change rather than constancy. For the first step in this re-assessment we are indebted to the work of Maruyama (1963).
Maruyama (1963) made an important contribution to General Systems Theory with his delineation of the concepts of "deviation-amplifying positive feedback" and "deviation-countering negative feedback." The following quotation from Greer-Wooten (1972: 17-18) is useful at this point:
"The fact that the second law of thermodynamics holds for an open system plus its environment, but not for the system itself, does not appear to have been sufficiently appreciated earlier. The 'steady state' is defined by the approach of minimum entropy production, and in fact at that time entropy is maximized--subject to the conditions in which the steady state was attained. There is thus a continual tendency to the development of maximum entropy, given a certain structure and set of input and export relationships for the open system. Any changes in the environment will result in disequilibrium and the beginning of a new cycle.Chorley describes such developments with changed imports as the open system tending towards higher levels of organization, becoming more ordered and heterogeneous. Such changes appear to be contrary to the second law, but Maruyama accounted for them by [[p. 48]] deriving important notions to describe open system change in general. If the exchanges of the system with its environment do not change markedly, then entropy production continues to be maximized up to the steady state. Such imports have been labelled negative entropy (or negentropy) and are also called 'information' and 'order.' However, the reactions of the system components to imports from the environment are also important in describing system change. Such system 'feedbacks' can be of two types: negative feedbacks (deviation-countering processes) maintain equilibrium situations, conditions that Maruyama called 'morphostasis;' in comparison, positive feedbacks (deviation-amplifying) influence system change, whether it be towards greater or less order....
One important conclusion that can be drawn from the discussion above is that in analyzing the dynamics of systems, the researcher should place more emphasis on flows (of energy, materials or information) between components of the system, and the system and its environment, than on changed attributes of the elements."
In the above passage there is a point made that is especially important to the present discussion. This is the notion that a system may respond to imports from its environment through either deviation-countering or deviation-amplifying processes. It has already been suggested that adaptation can be viewed as a deviation-countering process; accepting this idea allows us to maintain the classical physio-ecological truism that an organism must be adapted to the conditions imposed upon it by its environment to persist there. As earlier stated, however, range change and the history of the evolution of adaptations must be interpreted as indicative of a system that is not in equilibrium; i.e., that a deviation-amplifying process is also in operation., The ongoing development of new adaptations can be viewed as no more than a continuous change in: 1) the means by which equilibrium conditions are approached (homeostatic view); or 2) the potential for effecting change (homeorhetic view). The process of change itself is the movement of the overall biotic sector away from equilibrium and in the direction of higher levels of order, and in this adaptation plays only an indirect role.
[[p. 49]] If we accept the above a number of important results fall out. Most importantly, we are led to question (as have Gould and Lewontin, 1979) whether evolutionary change is best viewed in terms of adaptational structures. Neither the homeostatic nor the homeorhetic view of the function of adaptations casts the latter as actual evolution; rather, at most they seem to represent a physical correlate to the interaction state that more aptly describes it (a view that can be extended to the process of adaptation as well: Wiebes (1982: 243) has appropriatedly dubbed this "the historical narrative of evolution"). This interaction state is based in spatial relationships; that is, in the way the spatial structure of the biotic and abiotic sectors influences the turnover characteristics of resources. To a certain extent one might argue that the implied relationship of adaptation and spatial interaction to evolution reduces to a "chicken or egg" circularity, but there are important reasons why this should not be considered the end of the matter:
1) To begin with, the new causal relationship posed between adaptation and spatial interaction lends its way to both state-space and process interpretations. In this understanding, Carson's (1969) "outside variables" (abiotic sector provision of vital resources) can be linked to a system response that may be stated as either ecological state-maintenance (equilibrium) or evolutionary dynamism (disequilibrium). As part of a discussion of Wiley and Brooks (1982) Whitten (1983: 442) has recently remarked:
"....internal ordering depends on a system's ability to export entropy to its environment .... The virtue of the thermodynamic approach to evolution is its ability to connect life ecologically to the rest of nature through shared matter and energy flows; denying the ecological component of evolution, or the influence of ecology on development, badly weakens (their) thermodynamic base."
[[p. 50]] Wiley and Brooks (1982) introduced a nonequilibrium approach to evolution that has been strongly criticized on a number of counts (especially note Lovtrup, 1983; Bookstein, 1983). Whitten (1983) is of the opinion that, though the model itself is "quite flawed" (p. 438), its "spirit" is welcome. Here, I present a model of evolutionary change which, though "in the spirit of" the Wiley and Brooks effort, differs from it by stating nonequilibrium conditions in spatial terms rather than biological ones. This causal structure provides a convenient basis for testing possible models involving evolutionary change, since as discussed in Chapter II one of the greatest strengths of the ecological approach to biogeography lies in its methods of dealing with identifiable constraints on range change.
2) Moreover, this understanding of the entirely separable--but still complementary--roles of spatial interaction (and its implicit result: distributional change) and adaptation solves outright the philosophical dilemma attending the view that evolution involves a process (adaptation) that yields structures (adaptation) of non-independent definition (Brookfield, 1982; Ghiselin, 1966; Grene, 1971; Gould and Lewontin, 1979). As Lewontin (1984: 237-238) has put it, "The process is adaptation and the end result is the state of being adapted...The problem is how species can be at all times both adapting and adapted." When evolution is understood as the disequilibrium characteristics of biotic sector/abiotic sector spatial interaction (and not "the process of adaptation"), the homeostatic, "ecological," role of adaptation can be accented to provide a straightforward causal picture devoid of circularity and attending logical difficulties. In this view, adaptations are regarded simply as the structural attributes that mediate energy degradation, or that, as Wicken (1983: 440) puts it, "provide a means by which potential energy can be [[p. 51]] converted to thermal entropy and released to space." The problem that Wicken (1983), Wiley and Brooks (1982), Lovtrup (1983), and Bookstein (1983) have not been able to resolve is precisely how progressively greater amounts of this just-referred-to potential energy are converted over time; i.e., why evolution and not equilibrium? Wicken (1983; 440) edges nearest to an answer by suggesting that it is natural selection "which selectively preserves certain sequences (i.e., adaptations) while rejecting others." Again, however, a label has been attached here to a spatial interaction process that is not specified as such, and it thus remains a fuzzy concept.
3) Further, re-interpreting evolution as a spatial interaction process provides a response to the complaint that the study of the "evolution" of adaptations (i.e., phylogenetic studies) reduces to narrative. It still must be accepted that particular adaptations arise as responses to one-of-a-kind combinations of environmental and biological circumstances. It must also be accepted that this understanding resists any ordered interpretation beyond the identification of when and where each arose (and the sequencing of this information with all other such information). But when the homeorhetic function of adaptations is recognized, these criticisms are rendered moot. When adaptations are viewed as structures underpinning spatial interaction, they can also be implicitly understood to promote system "flow" toward steady-state conditions. Whereas the homeostatic function of adaptations is simply to mediate the conversion of potential energy to thermal entropy (per discussion in the last paragraph), their homeorhetic function may be viewed as the mediation of spatial interaction. As resource re-cycling capabilities of the system tighten as a general function of the latter, less thermal entropy needs be generated per potential energy imported (the result being system evolution). We therefore [[p. 52]] needn't think of evolution as leading to the unique structures we call adaptations, but instead to standing interaction processes interpretable on normative grounds: in the biological sense, for example, as competition/natural selection, and in the spatial sense as distribution patterns. In this view, it is more the properties of spatial interaction that evolve than the organisms themselves.
4) Lastly, the causal structure posed between spatial interaction and adaptations provides a framework within which long-standing issues in evolutionary theory might be examined in new ways. An example is provided by the "saltationist-incrementalist" debate; that is, whether evolution proceeds in sudden starts and stops or as a gradualistic process (see Eldredge and Gould (1972) for representative discussion). This discussion must be separated into two components when examined in the light of ideas set out here. There is first the one that has existed all along--that the fossil record does not suggest a gradualistic kind of evolutionary progression despite the fact that this is what classical (as well as Neo-Darwinian) selection theory logically demands. But the fossil record can only document changes in structure; i.e., in mode of adaptation. I have just argued that bodily form/behavior can be held to represent either the results of evolution, or a kind of potential for evolution, but not the interaction process constituting evolution itself. Importantly, regardless of whether saltation can describe the manner of sequential unfolding of adaptive assemblages, it may or may not describe the way changes in the spatial interaction structure of the biotic sector take place. This realization helps us to identify a fundamental problem in the way saltationist/gradualist discussions have developed. The gradualism point of view (e.g., as held by Darwin, Mayr, Simpson, and Wallace) is [[p. 53]] fundamentally an externalist's approach to evolution; through it natural selection becomes largely a function of environment (whether the latter be specified in physical or biological terms). Those who take an incrementalist stance, on the other hand, implicitly fall into an internalist's mode of thinking: that evolution is regulated by potentials and constraints that are acted upon by the environment (note the arguments of Gould and Lewontin, 1979; Gould and Vrba, 1982). It is pointless to ask which of these is the actual mode of evolution. While the nature of spatial interaction changes with time, this is almost certainly a gradualistic process, since it must be a function of a collective biotic sector change process mediated by changes in the abiotic sector. It has been the mistake of the gradualists to synonymize evolution with observable changes in form of organisms over time; this view leads to the "adaptation yields adaptation" circularity philosophically, and to the empirical contradictions of the fossil record. Arguments denying gradualism, on the other hand, are equally short-sighted. It can hardly be believed that environment-originated selection factors do not have important influence on the way characters are selected, regardless of whether population-specific constraints are also involved, or, for that matter, whether the fossil record can be proven to indicate saltational patterns of change. What is actually being "selected" is not a suite of adaptations, but instead a means of turning over material resources. In short, the central issue to be resolved is exactly how gradualistic change in spatial interaction patterns yields saltation in the development of individual phylogenies, and not whether one or the other is "correct." Despite the fact that his analysis focuses on internal factors in evolution, Waesberghe (1982: 26) comes to similar conclusions: "To an alternative model evolution is the saltatory [[p. 54]] origin of new taxa, prepared by a gradually improved ecosystem of interdependent external and internal factors."
(For the sake of internal consistency it should be pointed out that the comments of the last paragraphs apply equally well to a consideration of abiotic sector evolution. It is the erosion and transport of rock materials from various locations and their eventual deposition together in new ones that leads to the production of ever-more diversely constituted rock units. Orogeny in this sense is a process directly analogous to adaptation; in both cases the structural outcomes: 1) are time- and place-specific; 2) are novel and irreversible; 3) are impermanent; 4) are products of local conditions ultimately set by the unfolding of larger scale/longer term geological/climatological processes; and 5) fall out regardless of the particulars of history and composition (i.e., rock type and organism type) associated with them.)
The view held here of the dynamic interrelationships among distributional changes in populations, their adaptive capabilities, and environmental input might be stated in slightly more general terms at this point with the aid of the following quotation from Boulding (1956: 13):
"Another phenomenon of almost universal significance for all disciplines is that of the interaction of an "individual" of some kind with its environment....each of these individuals exhibits 'behavior,' action, or change, and this behavior is considered to be related in some way to the environment of the individual--that is, with some other individuals with which it comes into contact or into some relationship....The 'behavior' of each individual is 'explained'....by certain principles of equilibrium or homeostasis according to which certain 'states' of the individual are 'preferred.' Behavior is described in terms of the restoration of these preferred states when they are disturbed by changes in the environment."
In the present context the "individual" is a population, and "behavior" is synonymously range (spatial) change and adaptation. The latter two are [[p. 55]] viewed here as complementary understandings of a single process in which each population contributes to a slow system-wide progression toward steady-state conditions. In this work I concentrate on the spatial interpretation of the "behavior" concept portrayed above. This allows us to understand the "preferred states" of organisms/populations as being evolutionarily transient and individually unique in a biological sense (adaptation), yet still subject to normative interpretation (as distribution patterns belieing non-random states of spatial interaction among populations and their environment).
Spatial Interaction in the Community Context
We have now approached a means through which to translate an aspatial and cross-sectional understanding of abiotic-biotic relationships (e.g., the first interpretation given of Figure 1) into a spatial and process-oriented version of the same that can be related to population-level change. There is one more matter to consider in this chapter before we can turn to the formal modelling considerations leading to hypothesis tests, however. This is also the most difficult to approach, for we must now bring together all that has preceded into a single comprehensive view that is conducive to such modelling. To accomplish this, we must consider the relationship of range change, as interpreted through the present approach, to changing community structure.
While range change is usually examined from the perspective of within-population processes (as in Parsons, 1983; Van Valen, 1971a), it is clear that it does not take place in the larger sense in a void: changes in population distribution characteristics also reflect the constraints set by local community organization properties (Brown, 1983, Chapter Four). There [[p. 56]] is, moreover, a real advantage to looking at zoogeographic change patterns from the community level: flexibility. From an analytical standpoint (and per earlier discussion), it is difficult to view a species population as other than an ecological or a historical entity, because the species is a relatively individual and "irreversible" entity in both space and time (Ruse, 1973; Ghiselin, 1980. The historical element implicit in the notion of species is its genetic relation to other species (that is, its relative position in the life hierarchy), not to the history of the ecological system continuously sustaining it. Little information regarding the location of the particular community in which an organism happens to find itself is stored in the form of adaptations (one immediate reason why physiological ecologists have had little luck with evolutionary modelling). Communities, on the other hand, are neither rigidly delimited--or even, perhaps, delimitable--in space and time nor express much of anything about their "genetic" relationship to communities elsewhere. Nonetheless, like species, their characteristics can easily be linked to ambient environmental conditions, and they do exhibit a historical side: the pattern, over time, of assimilation of species populations into them.
This is an important association. It affords a means through which historical process can be viewed in terms of spatial interaction instead of the irreversible evolutionary outcomes that are the product of phylogenies: both subtraction and addition of forms characterize change in a community through time. With the switch to the spatial interaction approach emerges the possibility of normative modelling of distribution patterns. To begin with, all populations are members of communities ipso facto and contribute to the non-population-specific resource turnover processes mediated by community structures. The important consideration, however, is whether the [[p. 57]] reverse can be shown as likely: that turnover characteristics in space and time of some one (or more) vital resource lead to the assimilation characteristics in space and time of populations into communities. Biologically, these "characteristics of assimilation" will include a bewildering array of adaptive changes and strategies dependent on population-specific histories, and will be difficult to link, across all populations, to individual abiotic sector forcing functions. The spatial character of assimilation, however, will be directly evident in changing distribution patterns, which might be interpreted more easily: for example, as a function of spatial variation in one or more fundamentally vital abiotic sector-mediated variable.
Supposing that it is useless to attempt a definition of community grounded in the evolutionary histories of the species populations making up communities, there can likewise be no prior meaning attached to the specific suite of phenotypic expressions associated with them. This is not to say that general classes of adaptational strategies (adaptation to extremes of cold, heat, moisture, etc.) cannot be linked to particular kinds of habitats or community structures, but instead to again point out that the genetic means to such ends are in a historical/phylogenetic sense idiosyncratic; that is, taxon-specific (a re-statement of the individualistic hypothesis). Our only other option, it seems, is to believe that particular lineages and strategy types come together necessarily to generate the community characteristics we witness, a teleological viewpoint that can be reconciled neither with the geographic understanding that interaction in space is stochastic nor with empirical evidence presented by supporters of the individualistic hypothesis. Neither does it seem reasonable to argue that the vagaries of change in the physical environment [[p. 58]] can somehow be directly foreseen by organisms, unless we are willing to accept a Creationist stance.
A conservative way to treat community assimilation processes is thus to start with the hypothesis that communities are accidental structures that evolve as a simple function of the particular populations that happen to arrive and become integrated into them. This view, in fact, is little different from the current framing of "community" as a "concept" rather than a prior reality (see related discussions in Gleason, 1926, 1939; Cain, 1947; Saarinen, 1982; Whittaker, 1962, 1973; McIntosh, 1967; Pielou, 1974). The individualistic view is well summed up by Whittaker (1973: 327):
"Species are distributed 'individualistically,' each according to its own way of relating to environment....Species do not fit naturally into groupings that correspond to community-types and are discontinuous with other such groupings....Community-types are not natural but arbitrary units in the sense that their extensional definitions are strongly influenced if not wholly determined by phytosociologists' choices of the characteristics by which communities are to be classified....further....not only species but also groupings of species....show relative independence of one another, and may be differently combined into particular communities."
It is thus argued that, in terms of those populations coming together, community structure will develop as a non-predetermined function of their collective adaptive flexibility. This point of view seemingly casts long-term community-level change as a kind of chance drift phenomenon analogous to the one now thought to take place at the genetic level (Kimura, 1983a, 1983b; King and Jukes, 1969). Were communities entirely self-contained, we might be able to accept this notion as a means of depicting the associations that emerge among community members; clearly, however, communities are not isolated systems. Communities can be viewed as geographically-limited portions of the biotic sector. Like the biotic sector in general, they depend on the abiotic sector to provide the means of [[p. 59]] turnover of resources. This exogenous influence must effect controls on their operation and development (H.T. Odum, 1967, 1971). Attention must be drawn to how this exogenous influence might