Niche apportionment models

Niche apportionment models were developed because ecologists sought biological explanations for relative species abundance distributions.

These mechanistic models provide a useful starting point for describing the species composition of communities.

These models describe how species that draw from the same resource pool (e.g. a guild (ecology)) partition their niche.

Niche apportionment models have been used in the primary literature to explain, and describe changes in the relative abundance distributions of a diverse array of taxa including, freshwater insects, fish, bryophytes beetles, hymenopteran parasites, plankton assemblages and salt marsh grass.

One suggestion is that abundance measured as the numbers of individuals, may exhibit lower variances than those using biomass.

This model describes a situation where after initial colonization (or speciation) each new species pre-empts more than 50% of the smallest remaining niche.

In fact, the dominance pre-emption and geometric series models are conceptually similar and will produce the same relative abundance distribution when the proportion of the smaller niche filled is always 0.75.

Tokeshi (1993)[5] explained that this model, in many ways, is similar to Caswell's neutral theory of biodiversity, mainly because species appear to act independently of each other.

A similar model was developed by Sugihara in an attempt to provide a biological explanation for the log normal distribution of Preston (1948).

Tokeshi (1993)[5] argues that sequentially invading a resource pool is more biologically realistic than simultaneously breaking the niche space.

When the abundance of fish from all bio-regions in Texas were combined the distribution resembled the broken stick model of niche apportionment, suggesting a relatively even distribution of freshwater fish species in Texas.

However, one such community includes the relative abundance distribution of filter feeders in one site within the River Danube in Austria.

Researchers have used these models in many ways to investigate the temporal and geographic trends in species abundance.

[5] More recently statistical tests of the fit of niche apportionment models to empirical data have been developed.

[14][15] The later method (Etienne and Ollf 2005)[15] uses a Bayesian simulation of the models to test their fit to empirical data.

The former method, which is still commonly used, simulates the expected relative abundances, from a normal distribution, of each model given the same number of species as the empirical data.

Each model is simulated multiple times, and mean and standard deviation can be calculated to assign confidence intervals around each relative abundance distribution.

Figure 1. Whittaker plot of species Rank abundance for 7 most commonly considered mechanistic models for niche apportionment. The models with steeper slopes such as the Dominance preemption model represents less even communities where a few species are highly abundant and the remaining species abundances decrease sharply (see Species evenness ). Alternatively, shallow sloped models such as the Dominance Decay model are representative of more even communities where many species are equally abundant and relatively few species are rare.
Figure 2. The mechanisms with which a niche is broken. A resource pool is partitioned into niches of different sizes, which can be translated into the relative abundances of species in the total resource pool.