Butterfly responses to habitat edges in the highly fragmented prairies of Central Iowa

1. The behaviour of two butterfly species, a habitat specialist (Speyeria idalia) and a habitat generalist (Danaus plexippus), was tracked at four prairie edges to determine the extent to which edges act as a barrier to emigration. The four edge types studied were crop, road, field and treeline. The edges differed in structure ranging from high-contrast (treeline) to low-contrast (field). 2. S. idalia, the habitat specialist, responded strongly to all edges, even those with low structural contrast. However, S. idalia's response was strongly affected by conspecific density at crop and field edges; individuals were less likely to exit from high density plots, S. idalia responded to edges both by turning to avoid crossing them, and returning to the plot if they had crossed. 3. D. plexippus responded strongly only to treeline edges. Wind direction and time of year were important factors influencing behaviour at edges for this species. Conspecific density was not a significant factor affecting their behaviour. D. plexippus responded to edges by not crossing them, but rarely returned once they had crossed. 4. In highly fragmented landscapes, such as the one in which this study occurred, butterflies which show little or no response to edges may exhibit high emigration rates because of the high probability of encountering an edge in small habitat patches. Butterflies may respond strongly to even subtle habitat boundaries, but those responses may be modified by the edge structure, local environment or other conditions. Therefore, modifying edge structure may be a way to influence emigration rates, making it a useful tool for conservation.