Is intra-guild predation common?

Background/Question/Methods

Intra guild predation (IGP) is a relationship in which species that share prey also prey upon one another. Although predictions about IGP suggest that coexistence of IG predators and prey should be rare and that the presence of multiple IG predators should reduce suppression of prey, observations in nature suggest that neither of these predictions is consistently met. One proposed explanation for this is that IG predation is actually rare in nature. For example, lady beetles act as fierce IG predators in laboratory studies, but in natural settings, lady beetles may be able to limit IGP by avoiding contact with other lady beetles. We evaluated the hypothesis that lady beetles rarely come into close contact with heterospecifics by surveying lady beetles each week in maize and soybean over four summer seasons (2008-2011). Using data on which species and stages of lady beetles were present on each plant we calculated how often potential IG predators occurred on the same plant at the same time. We compared these observed contacts with the contacts predicted assuming lady beetles were randomly distributed among plants to evaluate the extent to which potential IG predators avoided each other in time and space. 

Results/Conclusions

We found that about two thirds of the time, IG predators shared a plant less often than predicted based on the total frequency of observations of each species/stage. However, we cannot conclude that the predators we studied were successfully able to avoid IGP or that IGP was rare among these predators in maize and soybean. Although adults were in less frequent contact than predicted, larvae, which are much more likely to fall victim to IGP, often faced high levels of contact with IG predators. This contact was the same or greater than predicted by chance and suggests that IGP may be very important to the population dynamics of some species. Further, because observed contacts among IG predators do not include those contacts that resulted in the death of one of the predators, they may underestimate encounters. These survey methods are valuable in identifying species and life stages for which IG predation may be important to population dynamics.