Edge-mediated Dispersal Behavior in a Prairie Butterfly

Animal responses to habitat boundaries will influence the effects of habitat fragmentation on population dynamics. Although this is an intuitive and often observed animal behavior, the influences of habitat boundaries have rarely been quantified in the field or considered in theoretical models of large scale processes. We quantified movement behavior of the Fender’s blue butterfly (Icaricia icarioides fenderi) as a function of distance from host-plant patches. We measured the butterfly’s tendency to move toward habitat patches (bias) and their tendency to continue to move in the direction they were already going (correlation). We found that butterflies significantly modify their behavior within 10– 22 m from the habitat boundary. We used these data to predict large scale patterns of residence time as a function of patch size, using three dispersal models: homogeneous response to habitat, heterogeneous response to habitat, and heterogeneous response to habitat with edge-mediated behavior. We simulated movement for males and females in eight patch sizes (0.1–8 ha) and asked how residence time varies among the models. We found that adding edge-mediated behavior significantly increases the residence of Fender’s blue butterflies in their natal patch. Only the model with edge-mediated behavior for females was consistent with independent mark– release–recapture (MRR) estimates of residence time; other models dramatically underestimated residence times, relative to MRR data.