Cryptic indirect effects of exurban edges on a woodland community
Exurban development (e.g., second homes) in woodlands spreads urban land use impacts beyond suburbs, but because exurban developments often retain many components of original ecosystem structure—such as a forest canopy rather than open lawn—their ecological impacts may be underestimated. Changes in seed-dispersing ant behavior prompted by exurban land use, such as edge avoidance, may pose deleterious impacts on the woodland plants (myrmecochores) they disperse, and hence the floristic diversity of exurban forests. We examined the effects of exurbanization on seed-dispersing ant nesting and foraging, seed retrieval, dispersal direction and subsequent impact on myrmecochores. We used a matrix of forested and exurbanized habitats to test whether (1) exurban edges decrease ant nest colonization and seed foraging, (2) ants disperse seeds away from exurban edges, and (3) consequently, there is lower ant-dispersed plant abundance nearer exurban edges. We found that exurban development poses little impact on keystone seed-dispersing ants because they foraged, colonized and thrived in fragmented woodland habitats as well as they did in intact forests. Exurban edges changed ant behavior, however, so that they generally moved seeds toward forest interiors, and, hence, away from edges. Corresponding to this behavioral change, we found that ant-dispersed plants declined with proximity to edges, whereas other woodland herbs with dispersal modes other than ants were unaffected. Exurbanization poses little threat to seed-dispersing ant viability, but, by changing foraging patterns (specifically, limiting the directionality of dispersal), it indirectly threatens the plants they disperse. Edge effects on biota commonly are associated with cascades through abiotic resources, but we show a deleterious biotic cascade between exurban edge, keystone ants and herbaceous plants. Species-mediated services, such as seed dispersal and pollination, are key resources, and assessing the full consequences of land use change therefore necessitates evaluation of impacts on biotic interactions.