Stable isotope analyses document intraguild predation in wolf spiders (Araneae: Lycosidae) and underline beneficial effects of alternative prey and microhabitat structure on intraguild prey survival

Intraguild predation (IGP) is common among generalist predators and an important issue in food web theory, because IGP may destabilise communities by increasing extinction of species. Also, IGP may interfere with the effectiveness of generalist predators as biological control agents. In general, occurrence of IGP in laboratory or field studies is inferred from abundance data or direct observations only. We investigated if tracing stable isotopes allows distinction between different types of predation and confirmation of IGP. Wolf spiders were chosen as model organisms for generalist predators; IGP of third instar A. cuneata on second instar P. palustris was investigated in a laboratory experiment. The availability of alternative prey and the complexity of the microhabitat were manipulated, since both factors are thought to facilitate coexistence of predators. Stable isotope analysis documented predation of A. cuneata on P. palustris and predation on alternative prey by both juveniles. Both the presence of alternative prey and the availability of shelter reduced mortality of juvenile P. palustris during the first week. During the second week mortality increased in complex structure without alternative prey presumably due to enhanced activity and cannibalism among starving P. palustris. Thus, microhabitat complexity and prey abundance may foster coexistence of wolf spiders in the field. In conclusion, stable isotope analysis was proven a powerful tool to investigate animal behaviour without direct observation. The method allowed disentangling predator feeding behaviour when more than one type of prey was present.