In the late 1990s, Stephen Hubbell and Graham Bell independently rediscovered and developed the framework of neutral theory in ecology further. They emphasized that neutral theory could describe not only species richness in communities, but also relative species abundance. Relative species abundance could be incorporated provided the neutrality assumption was recast at the biological level of organization of individuals rather than at the level of species, which had been the assumption in the theory of island biogeography. With this change, the theory could describe species undergoing random changes in abundance due to drift and random dispersal. These random fluctuations in populations cause some species to increase in abundance, and others to decrease in abundance, purely by chance differences in birth, death, and migration. The second most important advance of neutral theory was to incorporate a speciation process. With these changes, neutral theory was able to unite two previously disconnected bodies of theory in ecology into a single theory: the theories of island biogeography and of relative species abundance. The resulting theory became known as the UNT of biodiversity and biogeography.
Hubbell's and Bell's version of the UNT differed in some respects. Hubbell's version emphasized dispersal limitation in the assembly of communities by building his theory explicitly on the theory of island biogeography, whereas Bell's version did not. Also, unlike Bell's version, Hubbell's model distinguished between two biogeo-graphic scales: the large-scale metacommunity, which replaced the mainland source area of island biogeography theory, and the local community. The metacommunity is the evolutionary biogeographic unit in which species in the regional species pool spend their entire evolutionary lifetimes. The metacommunity is the biogeographic unit in which local communities are embedded and from which they receive immigrant individuals and species. Another difference was that in Bell's version, species in communities did not interact, whereas in Hubbell's version, species competed diffusely and the dynamics of the community were governed by a zero-sum game. The zero-sum assumption means that no species can increase in abundance without a matching decrease in the collective abundance of all other species in the community. Hubbell's version applied to communities of trophically similar species that actually or potentially compete for the same limiting resources. Species in Bell's community, however, were not subject to a zero-sum rule, and need not be trophically similar, but nevertheless had symmetric demography (equal per capita probabilities of birth and death). More recent theoretical work formally proves that these two versions of neutral theory, zero-sum and non-interacting, are asymptotically identical as the size of the community increases.
The UNT treats space either explicitly or implicitly. The implicit-space version characterizes the classical problem posed by the mainland-island dichotomy in the theory of island biogeography. Most of the analytical results have been obtained for the implicit version. The explicit-space version of the UNT is usually simulated on a lattice, but fewer analytical results are available in this version.
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