Species Competition and Predation - Ecological Systems - page 240

represents in the diet of a predator is proportional to the abundance of that prey
species. This makes intuitive sense as a prey species might be expected to be
encountered at a rate proportional to its abundance. However, if a predator's
efficiency at finding a prey species, or capturing it, increases with the rate at
which it is encountered, then the actual rate of capture is super-proportional to
the abundance of the prey species. The effect of this on the prey mortality rates is to
make them increasing functions of intraspecific prey density, and possibly decreas-
ing functions of the densities of other species. This phenomenon is known as
frequency-dependent predation [ 50 ]. An important outcome is the intensification of
intraspecific apparent competition relative to interspecific apparent competition. In
models, frequency-dependent predation has been shown to be a powerful species
coexistence mechanism. At a qualitative level, the outcome is similar to predator
partitioning (or natural enemy partitioning) where each prey species has its own
specialist natural enemy [ 50 , 51 ]. Thus, frequency-dependent predation achieves
with one natural enemy the same effect as several specialist natural enemies that do
not have frequency-dependent behavior.
Frequency-dependent predation, which is also called “switching” in the ecolog-
ical literature [ 52 , 53 ] and “apostatic selection” in the evolutionary literature [ 54 ],
has considerable empirical support to the extent that it has been studied [ 50 ]. Most
studies reporting this phenomenon have been laboratory studies, with few prey
species, and much less is known empirically from field studies. Moreover, the
theoretical studies have only demonstrated multispecies coexistence from symmet-
rically acting forms of frequency-dependent predation: those that might be expected
from increasing prey encounter and capture efficiency with increasing prey abun-
dance [ 51 ]. The theory of optimal diet selection, however, leads to an asymmetrical
form of frequency-dependent predation where the per capita predation rate on prey
of low value to a predator depends on the absolute abundance of higher ranked prey,
and not directly on the abundance of that prey itself [ 55 ]. Although coexistence can
be promoted also by this form of behavior [ 56 , 57 ], it is much less well investigated
than symmetrically acting frequency dependence, and its performance in multispe-
cies situations is not understood.
Environmental Variation and Population Fluctuations
Environmental fluctuations are often intuitively expected to undermine equilibrium
perspectives such as those developed on the basis of Lotka-Volterra models [ 58 ].
In fact, however, rather than undermine them, they provide new contexts in which
the critical phenomena considered here are realized. Fundamentally, they allow
resource partitioning and predator partitioning to occur on large scales of time even
though they do not occur on small scales [ 51 , 59 ]. This is possible because different
species in the same guild may have different responses to the physical environment,
such as weather. Annual plant species, for instance, often have species-specific