Patterns and mechanisms of spatial variation in patch use by egg-foraging social wasps, egg-mass-infesting flies, and eucoiline fly-parasitoids

Background/Question/Methods

Organisms often exhibit strong habitat preferences, but the relative importance of habitat preference for predicting species’ distributions can be strongly dependent upon the spatial configuration of patches in the landscape. Patch clustering can lead to increased colonization of both preferred and non-preferred patches in clusters and decreased colonization of preferred isolated patches. These patterns may arise because of dispersal or detection limitations for isolated clutches and because organisms have difficulty distinguishing between patches of different quality when they are in close proximity. Few studies have examined the relative importance of patch spatial configuration and habitat preferences for explaining the spatial distribution of organisms in a landscape. We assessed how the spatial arrangement of terrestrially-deposited frog (Agalychnis callidryas) egg masses influences their use by three insects: predatory wasps (Polybia rejecta), phorid flies (Megaselia sp. nov.), and the fly’s parasitoid (Figitidae). We (1) monitored natural patterns of insect presence relative to egg mass spatial distribution at a pond, (2) experimentally rearranged egg masses into clusters and isolates and assessed patterns of wasp attack and fly and parasitoid colonization, and (3) tested if fly colonization of non-preferred egg masses without wasp damage increases when they are near preferred masses with dead, broken eggs.

Results/Conclusions

Variation in the spatial distribution of frog egg masses had cascading effects on all three insects in both field and experimental studies. Specifically, wasps foraged most heavily in clusters, leaving carrion behind which in turn attracted scavenging phorid flies which in turn resulted in increased numbers of parasitoid wasps. Flies were more likely to colonize non-preferred egg masses if they were near preferred egg masses with dead eggs. Importantly, parasitoids colonized isolated egg masses less frequently than flies, and parasitoids failed to colonize egg masses not preferred by flies, even those near preferred ones. The spatial distribution of habitat patches can have important effects on habitat selection behaviors that drive community structure and species interactions within patches. The spatial clustering of egg masses caused insects to aggregate in clusters and use less preferred habitat over suitable, but isolated, habitat patches. However, flies and parasitoids showed small differences in their propensity to use patches within clusters, which could have important implications for long-term community dynamics. Specifically, differential responses of hosts and parasitoids to the spacing and condition of habitat patches contributed to the formation of spatial refuges from parasitoids that may be important in maintaining host populations.