Environmental Engineering Reference
In-Depth Information
Chapter 1
Alternative to Neutral Theory
C. Patrick Doncaster
INTRODUCTION
Hubbell's 2001 neutral theory (HNT) unifies biodiversity and biogeography by modeling
steady-state distributions of species richness and abundances across spatio-temporal
scales. Accurate predictions have issued from its core premise that all species have
identical vital rates. Yet no ecologist believes that species are identical in reality. Here
I explain this paradox in terms of the ecological equivalence that species must achieve
at their co-existence equilibrium, defined by zero net fitness for all regardless of in-
trinsic differences between them. I show that the distinction of realized from intrinsic
vital rates is crucial to evaluating community resilience.
An analysis of competitive interactions reveals how zero-sum patterns of abun-
dance emerge for species with contrasting life-history traits as for identical species. I
develop a stochastic model to simulate community assembly from a random drift of
invasions sustaining the dynamics of recruitment following deaths and extinctions.
Species are allocated identical intrinsic vital rates for neutral dynamics, or random
intrinsic vital rates and competitive abilities for niche dynamics either on a continuous
scale or between dominant-fugitive extremes. Resulting communities have steady-
state distributions of the same type for more or less extremely differentiated species
as for identical species. All produce negatively skewed log-normal distributions of
species abundance, zero-sum relationships of total abundance to area, and Arrhenius
relationships of species to area. Intrinsically identical species nevertheless support
fewer total individuals, because their densities impact as strongly on each other as on
themselves. Truly neutral communities have measurably lower abundance/area and
higher species/abundance ratios.
Neutral scenarios can be parameterized as null hypotheses for testing competitive
release, which is a sure signal of niche dynamics. Ignoring the true strength of interac-
tions between and within species risks a substantial misrepresentation of community
resilience to habitat loss.
The HNT unifi es the disciplines of biodiversity and biogeography by modeling
steady-state distributions of species richness and relative species abundance across
spatio-temporal scales [1]. Surprisingly accurate predictions have issued from its core
premise that all species are exactly identical in their vital rates. As a null hypothesis
to explain what should be observed if all species were perfectly equal with respect to
all ecologically relevant properties, it has proved hard to refute [2]. Yet no ecologist,
including Hubbell, believes that species are equivalent in reality [3, 4]. The challenge
