Species Competition and Predation - Ecological Systems - page 246

same guild on a regional scale suggests that spatial partitioning, perhaps coupled
with temporal partitioning, is sufficiently strong for natives to persist in a region
even if they are eliminated or driven to low abundances locally [ 92 , 107 ].
Invaders naturally interact with species outside their own guild also. As
predators and diseases they have been more frequently the cause of extinctions
of native species [ 44 , 96 ]. However, they can also be the agents that enhance
predators or diseases, damaging native species through apparent competition [ 43 ,
108 ]. Ways of controlling such invasives is therefore of substantial interest in
conservation biology [ 43 , 108 ].
Future Directions
The picture of community organization driven by competition and predation
presented here has been developing for decades but it is relatively recently gelling
on a new synthesis. It is now able to integrate competition, predation, temporal
variation, and spatial structure in one common framework to gain understanding
of not just individual mechanisms but their interactions as well [ 17 , 51 ], as
exemplified by the simple comparison presented above of how the strength of
stabilization of coexistence changes with the circumstances. This picture now
leads to comprehensive theoretical understanding that greatly clarifies a confusing
picture of numerous potential mechanisms of a few years ago [ 109 ]. The most
glaring lack now is in rigorous empirical study of mechanisms by which
communities are structured, especially stable coexistence mechanisms [ 14 ].
Many empirical studies focus on features of mechanisms without truly testing
whether they have a role in stabilizing diversity [ 13 , 14 ]. Patterns of morphology of
animals [ 11 ] and plants [ 110 ] that are related to how they gather resources or avoid
predation can provide strong circumstantial evidence, but still leave open numerous
possibilities as to the actual mechanisms [ 110 ]. One problem has been that the sort
of data necessary to test mechanisms has not been clear because the mechanisms
have not been truly understood, but that issue is now much less significant. One
recent development is a theory of testing mechanisms [ 13 ] aimed to guide empirical
studies. The key problem is to determine ways of testing whether density feedback
loops do indeed concentrate intraspecific density dependence relative to interspe-
cific density dependence in the manner proposed by a specific mechanism. In
general, this is not an easy proposition, but it is also not infeasible for well-
resourced project, given the right approach. Developing these approaches is the
aim of the theory of testing mechanisms [ 13 ]. For example, to test for stabilization
by temporal or spatial resource partitioning, the concept of covariance between
environment and competition has been developed, which specifically measures how
competition is linked to environmental factors [ 111 ]. Testing whether coexistence
is stabilized by partitioning environmental conditions involves showing that covari-
ance between environment and competition weakens as a species is reduced to low