Figure 6. - The geometry of a cascading trophic interaction with and
without intraguild predation. Trophic cascades are only possible if
the IGP link is not strong compared to the link between the top and
intermediate-level predators (from Polis and Holt, 1992).
Another level of complexity arises when species
may be in a position to compete for prey at one stage
of their ontogeny. Piscivorous species, for instance,
usually begin their life by feeding on zooplankton or
small invertebrates, in competition with species which
will serve as an important fraction of their prey when
they are large enough to become piscivorous.
Whereas many studies have analysed systems
in which predators and competitors are considered
as different species groups that interact, intraguild
predation which is another class of interactions,
combining predation and competition, has been less
well integrated into the conceptual framework of
ecology. Intraguild predation "is the killing and
eating of species that use similar resources and are
thus potential competitors" (Polis and Holt, 1992).
Intraguild predation often occurs among species that
eat the same food resource but differ in body size
such that the smaller species is a potential prey for
the larger. In particular, the young of many species
are vulnerable to species with whom the adults may
compete.
Intraguild predation interferes with trophic cascade
scenarios that involve webs having between three
and five links (e.g. plant-herbivore-intermediate level
predator-top predator). A change in one trophic level
could affect other levels. For instance, an increase
in a top predator could result in a decrease of
the intermediate level predator numbers, allowing
herbivore abundance to increase, and ultimately
increasing plant predation (Jig. 6). This scenario could
function in the absence of strong intraguild predation
links (i.e. top predators that also eat herbivores). If