Species Competition and Predation - Ecological Systems - page 234

from use of the various different resources involved. There is also a body of thought
leading to the neutral theory of community ecology which says that highly diverse
natural communities, such as tropical forests, behave as if all average fitnesses are
equal, and stabilizing mechanisms are absent [ 32 ]. In terms of the theory here, this
would mean that the average fitnesses, k , are the same for all species and r = 1 for
every species. Coexistence cannot be stabilized in these circumstances: invader
growth rates are zero, and in the Lotka-Volterra model, the total abundance of all
species is stabilized, but relative abundances are not. They are instead predicted to
drift as a consequence of the chance processes of individual birth and death.
However, there is no general reason to expect this extreme situation to arise in
nature. The patterns that agree with those in nature can also be predicted by models
with stabilizing mechanisms present [ 33 ], and fitness differences in systems without
stabilizing mechanisms quickly lead to the collapse of diversity [ 34 ].
Competition-Based and Predation-Based Coexistence
Mechanisms
Competition is often thought of as a major factor that limits species diversity,
without a clear distinction being made between interspecific and intraspecific
competition. That thought quickly leads to the idea that lower population densities
will lessen the magnitude of competition and therefore promote the maintenance of
species diversity [ 16 ]. In particular, that line of reasoning leads to the idea that high
mortality rates, or harsh and stressful conditions, might generally promote species
diversity [ 35 ]. However, as emphasized above, it is the ratio of interspecific
competition to intraspecific competition that is critical to species coexistence and
not the absolute value. Thus, lowering densities, and indeed lowering the intensity
of competition, need not have any effect on the ratio of interspecific to intraspecific
competition, and therefore need have not have any effect on species coexistence. In
fact, competition can be important for stable coexistence. If competition is the only
form of density dependence, then it is essential for stable coexistence. The chal-
lenge for species coexistence is not competition per se but interspecific competition
that is strong relative to intraspecific competition. A pattern of species interactions
that intensifies intraspecific competition relative to interspecific competition is a
competition-based species coexistence mechanism [ 17 ].
Recent appreciation that density dependence from predation can play a similar
role to competition leads to the idea of predation-based coexistence mechanisms.
These are mechanisms that concentrate intraspecific apparent competition relative
to interspecific apparent competition [ 17 ]. What then are the joint effects of
competition and predation on species coexistence? Both natural enemy attack and
resource competition are likely to be important in most guilds of coexisting species
simply because it is difficult to avoid these phenomena. In discussions of species
coexistence, predation has sometimes been viewed as important primarily as