Such observations typically generate a large number of questions. Indeed, a substantial fraction of the species sampled are singletons, that is, species which are sufficiently rare for only a single individual to have been sampled.

A substantial fraction of the species sampled are very rare.

The most common species accounts for a substantial fraction of the individuals sampled.

When abundance data on natural populations are collected, two observations are almost universal: Hubbell thus treats communities as being of a fixed number of individuals, usually denoted by J.Д®xceptions to the saturation principle include disturbed ecosystems such as the Serengeti, where saplings are trampled by elephants and Blue wildebeests or gardens, where certain species are systematically removed. Hubbell considers the fact that community sizes are constant and interprets it as a general principle: large landscapes are always biotically saturated with individuals. However, because the Unified Theory refers only to communities of trophically similar, competing species, it is unlikely that population density will vary too widely from one place to another. If a wide range of species are considered (say, giant sequoia trees and duckweed, two species that have very different saturation densities), then the assumption of constant community size might not be very good, because density would be higher if the smaller species were monodominant. There are unavoidable physical constraints on the total number of individuals that can be packed into a given space (although space per se isn't necessarily a resource, it is often a useful surrogate variable for a limiting resource that is distributed over the landscape examples would include sunlight or hosts, in the case of parasites).

2 Stochastic modelling of species abundances under the UNTBДЄlthough not strictly necessary for a neutral theory, many stochastic models of biodiversity assume a fixed, finite community size (total number of individual organisms).

The theory predicts the existence of a fundamental biodiversity constant, conventionally written Оё, that appears to govern species richness on a wide variety of spatial and temporal scales. The Unified Theory also makes predictions that have profound implications for the management of biodiversity, especially the management of rare species. Asymmetric phenomena such as parasitism and predation are ruled out by the terms of reference but cooperative strategies such as swarming, and negative interaction such as competing for limited food or light are allowed (so long as all individuals behave in the same way). Under the Unified Theory, complex ecological interactions are permitted among individuals of an ecological community (such as competition and cooperation), provided that all individuals obey the same rules. ДЄn ecological community is a group of trophically similar, sympatric species that actually or potentially compete in a local area for the same or similar resources. Hubbell built on earlier neutral models, including MacArthur & Wilson's theory of island biogeography and Gould's concepts of symmetry and null models. This can be considered a null hypothesis to niche theory.

Neutrality means that at a given trophic level in a food web, species are equivalent in birth rates, death rates, dispersal rates and speciation rates, when measured on a per-capita basis. The theory has sparked controversy, and some authors consider it a more complex version of other null models that fit the data better. This implies that niche differences do not influence abundance and the abundance of each species follows a random walk. Like other neutral theories of ecology, Hubbell assumes that the differences between members of an ecological community of trophically similar species are 'neutral', or irrelevant to their success.

The hypothesis aims to explain the diversity and relative abundance of species in ecological communities. The unified neutral theory of biodiversity and biogeography (here 'Unified Theory' or 'UNTB') is a theory and the title of a monograph by ecologist Stephen Hubbell.

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