Unified Neutral Theory Of Biodiversity

OVERVIEW

''Neutrality'' is defined as per capita ecological equivalence among all
individuals of every species at a given Trophic Level in a Food Web ; "per capita equivalence" means that all species are held to behave (ie reproduce and die) in the same way as one another; and individuals of a particular species reproduce and die (behave) in the same way. Early neutral theories include the Broken Stick Hypothesis of Robert MacArthur and the Island Biogeography theories of
MacArthur and E. O. Wilson .

An ''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
2001). 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.
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).

The Unified Theory makes a large number of Falsifiable Hypotheses .
Differences between predictions of the Unified Theory and observations are of
very small magnitude. The Unified Theory also makes predictions that have
profound implications for the management of Biodiversity , especially
the management of rare species.

Non-neutral theories of biodiversity would include Niche Construction
and Dispersal Assembly . These theories are non-neutral because
they hold that different species behave in different ways from one
another. Other examples of non-neutral explanations would be to hold
that older organisms are fitter in the Darwinian sense.

Under Hubbell's theory, species drift is allowed to occur via speciation, which would occur with a specific probablity per birth. The neutrality of the Unified Theory implies that this probability would be independent of the parent's species (common species have a higher birth rate, and thus the UNTB predicts that speciation occurs more frequently for common species than rare species).

The theory predicts the existence of a fundamental biodiversity constant, conventionally written ''θ'', that appears to govern species richness on a wide variety of spatial and temporal scales.

THE UNIFIED THEORY AND SATURATION

Although not strictly necessary for a neutral theory, many Stochastic models of biodiversity assume a fixed, finite community size. 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).

If a wide range of species is 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.

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.

Hubbell considers the fact that population densities are constant and interprets it as a general principle: ''large landscapes are always biotically saturated with individuals''. Hubbell thus treats communities as being of a fixed number of individuals, usually denoted by ''J''.

Exceptions to the saturation principle include disturbed ecosystems such as
the Serengeti , where saplings are trampled by Elephant s; or Garden s, where certain species are systematically removed.

Species abundances

When abundance data on natural populations are collected, two
observations are almost universal:

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

A substantial fraction of the species sampled are very rare. 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.

Such observations typically generate a large number of questions. Why
are the rare species rare? Why is the most abundant species so much
more abundant than the median species abundance?

A non neutral explanation for the rarity of rare species might suggest
that rarity is a result of poor adaptation to local conditions. The
UNTB implies that such considerations may be neglected from the
perspective of population biology (because the explanation cited
implies that the rare species behaves differently from the abundant
species).

Species composition in any community will change randomly with time. However, any particular abundance structure will have an associated probability. The UNTB predicts that the probability of a community of ''J'' individuals composed of ''S'' distinct species with abundances

n_1

for species 1,

n_2

for species 2, and so on up to

n_S

for species ''S'' is given by

:

\int_{0}^{1}(Ix)_{n}(I(1-x))_{J-n}rac{(1-x)^{ heta -1}}{x}dx

m/(1-m) is a parameter that measures dispersal limitation (and resembles $\gamma$ above).

\langle \phi_n 
angle

is zero for ''n'' > ''J'', as there cannot be more species than individuals.

This formula is important because it allows a quick evaluation of the Unified Theory. It is not suitable for testing the theory. For this purpose, the approptiate likelihood function should be used. For the metacommunity this was given above. For the local community with dispersal limitation it is given by:

:

\sum_{A=S}^{J}K(\overrightarrow{D},A)rac{I^{A}}{(	heta) _{A}}

Here, the

K(\overrightarrow{D},A)

for

A=S,...,J

are coefficients fully
determined by the data, being defined as

:

{ M Prob}({ M Pop} I(t+1) N_i-1{ m Pop}_i(t)=N_i)=
{ M Prob}({ M Pop} I(t+1) N_i+1{ m Pop}_i(t)=N_i)=
{ M Prob}({ M Pop} I(t+1) N_i{ m Pop}_i(t)=N_i)=

Information About

Unified Neutral Theory Of Biodiversity