The influence of intraguild predation on prey suppression and prey release: A meta-analysis

Intraguild predation (IGP) occurs when one predator species consumes another predator species with whom it also competes for shared prey.  One question of interest to ecologists is whether multiple predator species suppress prey populations more than a single predator species, and whether this result varies with the presence of IGP.  We conducted a meta-analysis to examine this question, and others, regarding the effects of IGP on prey suppression.  We found that assemblages with mutual IGP (i.e., both predators potentially consume one another) have greater prey suppression with single predator species than with multiple predator species, but this result was not found for assemblages with unidirectional or no IGP.  When unidirectional IGP was present, we found that intermediate predators were generally more effective at suppressing the shared prey than the top predator, in agreement with IGP theory.  Adding a top predator to an assemblage generally caused prey to be released from predation, while adding an intermediate predator caused prey populations to be suppressed.  However, the effects of adding a top or intermediate predator depended on the effectiveness of these predators when they were alone.  Considerable variation exists in the effects of IGP across different ecosystems, with most of the strongest patterns being driven by terrestrial invertebrate ecosystems.  The ability to make conclusions about IGP theory (which is based on equilibrium conditions) using data from short-term experiments is discussed.