Species Competition and Predation - Ecological Systems - page 247

density in presence of unmanipulated competitors. More developments along these
lines are needed to test other specific mechanisms.
New theoretical understanding has also revealed relatively basic deficiencies in
empirical studies. For instance, frequency-dependent predation shows much prom-
ise as a strong coexistence mechanism, yet there are few rigorous field studies of
this phenomenon [ 50 ]. It seems that the dominance of predation-competition trade-
offs, including keystone predation ideas, has led to the neglect of frequency-
dependent predation as a coexistence mechanism in empirical studies. However,
it is also true that studies of keystone predation assumed that the mechanism was
powerful when acting alone, and this impression seems likely to have inhibited the
search for the stabilizing mechanisms that must be present also for keystone
predation to be effective in promoting multispecies coexistence.
Naturally, the theory, though rich and not rigorously tested, still has some glaring
deficiencies. One issue that complicates the Lotka-Volterra theory presented above is
that as parameters are changed gradually, species in the resource and predator guilds
that interact with the focal guild may not be supportable [ 27 ]. This possibility leads to
abrupt changes in the coefficients of density dependence and related parameters [ 26 ].
It is also true that depressing a given member of the focal guild to low density may
mean that only a subset of the resource species or predator species is present [ 17 ].
These facts, however, do not alter the invasion criteria presented here provided they
are based on the resources and predators actually present in a given invasion scenario.
They do, however, complicate interpretation of the criteria as parameters are changed.
It is not known how important these issues are, and a comprehensive theory of their
effects is needed.
The integrated understanding of the roles of predation and competition in species
coexistence and exclusion presented here also raises the critical question of what
factors control density dependence through these two processes. Of potentially
major significance are other species in food webs beyond those directly linked to
the focal guild. For instance, if the major natural enemies of the focal guild are
themselves subject to strong density dependence from their natural enemies, they
will be limited in their ability to change in abundance in response changes in the
density of the focal guild. This means that density-dependent feedback to the focal
guild through their natural enemies would be limited to behavioral responses of the
natural enemies to focal guild densities. The idea of trophic cascades has long
postulated how various density-dependent effects permeate from one part of a food
web to another [ 112 - 114 ]. Integration of trophic cascade research with species
coexistence research has the potential to make important advances with major
implications for understanding the broader impacts that humans are having on the
planet through widespread disruption of food webs, especially the destruction of
many large carnivorous species [ 115 - 117 ].
Most of the discussion here has been purely ecological focused on interactions
between a few species. However, natural populations and communities are shaped
by evolution and community assembly processes, of which the considerations
discussed above form just a small part. It is now known that evolutionary change
can be fast to the extent that ecological dynamics and evolutionary dynamics cannot