  Community ecology has traditionally relied on the competitive exclusion
principle, a piece of common wisdom in conceptual frameworks developed to
describe species assemblages. Key concepts in community ecology, such as
limiting similarity and niche partitioning, are based on competitive exclusion.
However, this classical paradigm in ecology relies on implications derived from
simple, deterministic models. Here we show how the predictions of a symmetric,
deterministic model about the way extinctions proceed can be utterly different
from the results derived from the same model when ecological drift (demographic
stochasticity) is explicitly considered. Using analytical approximations to the
steady-state conditional probabilities for assemblages with two and three
species, we demonstrate that stochastic competitive exclusion leads to a
cascade of extinctions, whereas the symmetric, deterministic model predicts a
multiple collapse of species. To test the robustness of our results, we have
studied the effect of environmental stochasticity and relaxed the species
symmetry assumption. Our conclusions highlight the crucial role of
stochasticity when deriving reliable theoretical predictions for species
community assembly.
