To date, there is a lack of well-controlled field experiments that disentangle the effects of the intervening matrix from other landscape variables (e.g., patch geography or quality) that might influence animal dispersal among patches. We performed a field experiment to investigate how the movement of a delphacid planthopper (Prokelisia crocea) among discrete patches of prairie cordgrass (Spartina pectinata) is affected by the composition of the matrix (mudflat, native non-host grasses, and the invasive grass smooth brome [Bromus inermis]). Within each matrix type, marked planthoppers were released onto experimental cordgrass patches that were made identical in size, isolation and host-plant quality. We found that the emigration rate was 27% lower for patches embedded in mudflat than for patches in the two non-host grasses. The rate of immigration into patches isolated by 3 m was five times higher in the brome than in the mudflat matrix. Patches in the native grass matrix had intermediate immigration rates. Based on survey data, we found that matrix composition influenced both within- and among-patch distributions of planthoppers. Within-patches, individuals accumulated against mudflat edges (relative to patch interiors) but not against non-host grass edges. Among patches, planthopper incidence and density increased with the proportion of the matrix composed of open mud. The matrix was equal to that of patch geography (size and isolation) in its effect on planthopper distributions. We suggest that the low permeability of the mudflat relative to a non-host grass edge may explain these planthopper distributional patterns. Also, because mudflat-embedded patches are more nutrient rich than non-host grass-embedded patches, planthoppers are more likely to remain and build up densities on those patches. We predict that the displacement of native matrix types by invasive smooth brome will result in increased patch connectivity, greater spatial synchrony among patches, and a higher risk of regional extinction.
Keywords: connectivity; edge effects; edge permeability; emigration; immigration; matrix; patch quality, planthoppers; Prokelisia crocea; Spartina pectinata; tall-grass prairie; spatial distribution