When edges meet: Interacting edge effects in an African savanna

1.Ecological edges (zones separating ecosystems or land cover types) can function as active boundaries, unique habitats and dynamic transition zones. Abiotic factors, species and species interactions exhibit strong responses to edges, and these responses - edge effects - can profoundly impact ecosystem structure and function. 2.Edge effects may be altered by the presence or proximity of other nearby edges. This phenomenon - edge interaction - is poorly understood, though its importance is increasingly recognized. Edge interactions are likely in fragmented or patchy landscapes that contain many edges. In such landscapes, understanding how nearby edges interact may be critical for effective conservation and management. 3.I examined edge interactions in an East African savanna. In this landscape, abandoned cattle corrals develop into treeless, nutrient-rich 'glades' that persist as preferentially grazed areas for decades to centuries. Glades represent important sources of structural and functional landscape heterogeneity and have major impacts on distributional patterns of plant and animals. 4.I used existing variation in inter-glade distance to investigate the importance and strength of glade edge interactions for plants, Acacia ants and large herbivores. Specifically, I compared response patterns obtained from transects that extended outward from isolated glades (>250m from another glade) and non-isolated glades (<150m from another glade). 5.Edge effect patterns between nearby glades differed significantly from patterns around isolated glades. When compared to areas outside isolated glades, areas between glades had almost twice the density of trees, half as much large herbivore use, reduced cover of glade-dominant grasses, and different Acacia ant communities. Many of the edge effects observed between non-isolated glades could not be inferred from effects around isolated glades. 6. Synthesis. These findings suggest that edge interactions can alter plant and animal distributions in patchy landscapes. Edge effects near multiple edges can be stronger, weaker or qualitatively different from those near isolated edges. Such edge interactions can increase or decrease structural and functional continuity between nearby patches. Appropriate extrapolation of local edge effects in complex and fragmented landscapes will require greater understanding of edge interactions. © 2011 The Author. Journal of Ecology © 2011 British Ecological Society.