Classic models of intra-guild predation (IGP), consisting of one basal resource and two consumers where one (IG predator) feeds on the other (IG prey), predict that IGP will always decrease consumption of the basal resource. However, a subset of theoretical studies has suggested that weak-levels of IGP might actually enhance coexistence of the two consumers, in turn, generating increased consumption of the basal resource. These two contrasting predictions have yet to be tested empirically.
We performed a laboratory experiment to test how the strength of IGP impacts consumption of a basal resource. Using a well-studied biological system with two ciliate species engaged in IGP and three bacteria species consumed as the basal resource, we varied the contact rate between IG prey and IG predator to manipulate the strength of IGP. Data from the experiment were fit to standard models of IGP that were partially parameterized with literature values, and partially parameterized by data fitting to achieve a mechanistic interpretation of results.
Results/Conclusions
The experimental results showed that resource consumption first increased and then decreased as the strength of IGP increased. When IGP was weak to intermediate (40-60% of the IG prey is available to the IG predator), consumption of the bacteria increased by roughly 25% compared to controls with no IGP. Model results, which provided a reasonable fit to the data (R2 ~ 0.7), suggested that weak to intermediate IGP allowed the IG predator to attain higher density as a result of increased food availability (i.e. the IG prey). Higher IG predator density consequently increased consumption of bacteria. On the other hand, as the strength of IGP increased to higher levels, consumption of bacteria declined, with model results suggesting this occurred because the IG predator became satiated with higher availability of IG prey, thus releasing bacteria from consumption. Our results suggest that intra-guild predation can either increase or decrease consumption of basal prey depending on the absolute strength of IGP. Therefore, the role of IGP in biocontrol and the regulation of prey populations is potentially a bit more complex than previously expected.