A unified theory of biogeography and relative
species abundance and its application to
tropical rain forests and coral reefs
S. P. Hubbell
Department of Ecology and Evolutionary Biology, Princeton University, Princeton NJ 08544 USA
Smithsonian Tropical Research Institute, Box 2072, Balboa, Panama´
Coral Reefs (1997) 16, Suppl.: S9—S21
Abstract.
Theories of island biogeography and of relative species abundance are of central importance in biogeography and community ecology, yet these two bodies of theory heretofore have been largely unconnected. Incorporating speciation into the theory of island biogeography unexpectedly results in unification of these two theories. The unified theory predicts the existence of a fundamental biodiversity number h that controls not only species richness, but also relative species abundance in the source area metacommunity at equilibrium between speciation and extinction. With additional parameters for island size and migration rate, the theory also predicts relative species abundance on islands or local regions of continuous landscapes. Application of the theory to the biogeography and biodiversity of communities of tropical trees and reefbuilding corals are discussed. One important result is that only relatively modest migration rates are sufficient to dynamically couple the regional metacommunity and stabilize community structure on large spatiotemporal scales. Thus, regional, long-term compositional stasis in tropical rain forests and coral reefs can arise just as easily from the stabilizing effect of large numbers as from nicheassembly rules that limit species membership in communities. Because of the higher intrinsic vagility of corals, the theory predicts greater regional similarity in coral reef communities than in tropical tree communities.
Introduction
Biogeographers and community ecologists typically work on very different spatial and temporal scales, and therefore it is hardly surprising that their theories for how biotas and ecological communities are assembled should differ. Ecologists tend to focus on small-scale processes shaping the interactions of individuals and populations. They tend to be impressed by the strength and importance of species interactions and of niche differences in stabilizing species assemblages in particular locations. Biogeographers, on the other hand, focus on much larger scale processes and ask questions about migration and range and speciation and extinction in space and time. One might label these two perspectives of the organization of ecological communities as the ‘‘niche assembly’’ and ‘‘dispersal assembly’’ views, although these terms do not adequately capture the large differences in viewpoint that exist within each perspective. For example, theories in vicariance biogeography tend to downplay the role of dispersal in assembling regional biotas compared to theories in pan-biogeography.
The equilibrium theory of island biogeography (MacArthur and Wilson 1963, 1967) was a bold attempt to link these two very different scales and perspectives. MacArthur and Wilson proposed that biotas inhabiting islands or insular habitats are in diversity equilibrium but not in taxonomic equilibrium. They suggested that local communities experience a continual turnover of species through immigration and local extinction of species drawn from a large source area or metacommunity. The equilibrium diversity on isolated islands was expected to be lower than on a similar-sized piece of the continuous mainland primarily because of a reduction in immigration rates due to isolation and increased extinction rates due to smaller island population sizes. Various embellishments were later added to the theory. For example, Brown and Brown (1977) suggested that immigration would interact with extinction to produce a ‘‘rescue effect’’, further reducing local extinction rates in continuous landscapes.
However, for all its attempts to bridge the conceptual gulf between ecology and biogeography, the theory of island biogeography departs fundamentally from classical niche-assembly theory. Gone are niche differences among species. Species in the theory are treated as identical, subject to the same birth and death processes and the same probabilities of immigration and extinction. If species are not identical, then the theory’s simplification of the dynamics of island communities to enumerating species irrespective of taxa logically does not work. Thus, the theory is far closer to theories of pure dispersal assembly than to the niche assembly theories of classical ecology, which ironically, MacArthur also championed as the leading ecological theorist of his day (MacArthur 1972). Apart from whether one accepts the radical assumption of identical species, the theory of island biogeography is also conceptually incomplete in a number of important regards. From a biogeographer’s perspective, it is incomplete because it embodies no mechanism of speciation. Although species can appear and disappear from islands or habitats in the theory, this is a migrationand local extinction-driven phenomenon; no new species are allowed to originate in islands or in the source area.
From an ecologist’s perspective, the theory is incomplete in large part because it does not predict the abundances of species, only species richness. Relative abundance theory is briefly touched upon in MacArthur and Wilson’s (1967) monograph in relation to the species-area relationship. However, the expected equilibrium distribution of relative species abundance on islands was not derived from the first principles of the theory. Just a few years earlier, MacArthur (1957, 1960) published two papers on relative species abundance, but these papers were steeped in niche-assembly theory and did not readily lend themselves to a dispersal assembly theory. Later, May (1975) examined more fully the consequences of relative species abundance for species-area relationships, assuming that relative abundances were log-normally distributed (Preston 1948, 1962). However, this was a static sampling analysis, not a dynamical theory based on fundamental birth, death, and migration processes. Indeed, most of the existing models of relative species abundance are empirical statistical fits to observed distributions of abundance (Motomura 1932; Fisher et al. 1943; Preston 1948, 1962), or are based on static niche-assembly hypotheses (MacArthur 1957, 1960; Sugihara 1980) and are not grounded in a dynamical theory that can be related directly to the dynamical theory of island biogeography. The exceptions to this generalization are the theories of Casewell (1976), who proposed neutral models of community organization based on analogs in population genetics, Chesson and Warner (1981), who proposed that species abundances were determined by stochastic, frequency-dependent recruitment fluctuations, and Hughes (1984), a benthic ecologist who proposed a model similar to my own stochastic forest dynamics model (Hubbell 1979), which was a less general version of the theory discussed here.
Conclusions
The debate over whether ecological communities are dispersal assembled or niche assembled is long standing and is probably here to stay. This is likely because both perspectives are ‘‘correct’’ in some sense. However, the spatiotemporal scales on which they accurately depict the structure and dynamics of natural ‘communities are likely to remain fundamentally different. Recently ecologists have become increasingly interested in ‘‘macroecology’’ and are endeavoring to understand how many of their near and dear principles ‘‘scale up’’. In the end I suspect the answer will come back: ‘‘not many’’ in spite of some notable successes (e.g., Ricklefs and Schluter 1993; Brown 1995; Rosenzweig 1995). It is no accident, in my opinion, that the parameters of island biogeography theory are things like ‘‘immigration rate’’ and ‘‘island size’’, and not ‘‘resource supply ratio’’ or ‘‘prey handling time’’. The premise of this study is that MacArthur and Wilson (1963, 1967) were onto an important discovery, namely that a completely new set of rules and parameters govern metacommunity dynamics
I have endeavored to show that a relatively simple generalization of the theory of island biogeography with few assumptions is capable of describing macroscopic patterns of species richness and relative species abundance in some cases with quite high precision. I believe the keys to its success are the assumption of zero-sum community dynamics and the inclusion of a speciation mechanism. The theory then shows how species richness and relative species abundance will evolve and equilibrate over a metacommunity landscape. Remarkably this happens in a perfectly homogeneous environment inhabited by perfectly identical species in terms of per capita probabilities of birth, death and migration. Of course, real species are not identical, and they have niches. But the success of the present theory suggests that most of the detail about niche structure is lost or becomes ineffective at controlling community structure on large spatial and temporal scales. Dominance arises in the model metacommunity and local community by chance and not by competitive superiority. The theory also shows how migration can stabilize local and regional community structure for long periods and over wide areas with no assumption of niche assembly or limited membership communities
The dispersal assembly perspective is likely to be a lot more difficult to falsify qualitatively and quantitatively than might have been thought. If the predictions of the unified theory turn out to be even approximately correct for real ecological communities, then they have potentially profound implications for the contemporary theoretical paradigm in community ecology, for the organization of ecological communities, and for biogeography and conservation biology.
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