Species Competition and Predation - Ecological Systems - page 224

between consumers as a result of consumption of shared resources. Turning that on
its head, when the resources are prey, they affect each other indirectly by providing
food for common predators, potential increasing danger from those predator spe-
cies. This indirect interaction between prey species is known to have analogous
effects to competition, and has thus been termed apparent competition [ 18 ].
Although first suggested in a single-species context many years ago [ 19 ], it has
only recently been generally understood, and so has received far less direct study
than competition, but it is also likely that studies of competition have inadvertently
included the effects of apparent competition: protocols to cleanly separate the two
effects have yet to be developed.
Conceptual difficulties have plagued understanding of how competition and
predation structure communities owing to the intrinsic complexity of the subject.
The theory, however, has been making steady progress, and a much more compre-
hensive theoretical framework is available now than even a few years ago. The
body of this essay explains the fundamentals of how competition and predation are
hypothesized to structure communities in the light of these recent advances.
Included are the intricacies introduced by the complex behavior organisms, and
the often conceptually difficult area of how predictions about competition and
predation can be made in the face of temporal and spatial variation in the physical
environment, and in the presence of fluctuations in the populations of the organisms
themselves. Applications to a theory of invasive species are then presented.
Invasions of alien species represent perturbations to natural systems that can lead
to a process of reassembly of communities of organisms. Competition and preda-
tion are believed to have large roles in this reassembly process, and explaining it is
a critical challenge in community ecology. Because of the impacts of alien species
on native communities, there are major implications for conservation biology also.
Finally, challenges in the study of competition and predation, and promising future
directions, are presented.
Feedback Loops
The fundamentals of competition and predation can be best understood in terms of
feedback loops within a food web. Figure 13.1 shows a simplified food web, which
should be considered as part of a food web rather than being any reasonably
complete web that one might find in nature. It shows three trophic levels allowing
understanding of how the middle trophic level is affected by the trophic levels
above and below, which represent, respectively, predators and resources of that
middle trophic level. Species in the same trophic level, by sharing trophic position,
have strong similarities in their ecology. There may be other ecological similarities,
and also differences within a trophic level, in the way the species relate to other
trophic levels and to other elements in their environment. Species with similar
ecology are commonly referred to as a guild and here the middle trophic level is
the focus of discussion and is referred to as the focal guild, or just “the guild.”