Edge effects are defined as changes in the density of an organism near a boundary between habitat elements. These effects are often thought to be generated by changes in movement behavior in the vicinity of the edge, but in most cases the mechanism is unknown. We quantified the movement behavior of a minute parasitic wasp, Anagrus columbi, in relation to edges in its habitat. This wasp attacks eggs of the planthopper Prokelisia crocea, which inhabits a wet prairie ecosystem composed of patches of its host plant Spartina pectinata interspersed within a matrix of mudflat, smooth brome, and native grasses. Two edge types are common in this system, cordgrass-mudflat and cordgrass-brome. We conducted mark-recapture experiments in which wasps were released at the cordgrass-matrix edge and 50 cm within cordgrass or matrix, for both edge types. The marked wasps were then recaptured using a grid of sticky-traps. We fitted an advection-diffusion model to these data, yielding estimates of the diffusion rate and advection coefficient for cordgrass and matrix, for each release position and edge type.
The advection-diffusion model yielded positive, zero, and negative estimates for the advection coefficient for matrix, edge, and cordgrass releases, respectively. The overall pattern suggests that marked wasps strongly biased their movements toward the edge when released in matrix, and to a lesser extent when released in cordgrass, while edge releases showed little bias. The advection coefficients were similar for the two edge types. There was no evidence for differences in diffusion rate for the three substrates (cordgrass, brome, mudflat), and the diffusive and advective components of movement were of comparable magnitude. Our results suggest the wasps may be attracted to cordgrass patches across short distances, and that bias in their movements may concentrate them at the patch edge and also help retain them within the patch. We also contrast our results with the edge behavior observed in the planthopper. Our methodology could be readily adapted to other systems where direct observations of movement are difficult but mark-recapture studies are feasible.