Quantifying edge effects: the role of habitat contrast and species specialization

Abstract

Edge effect is a key process influencing populations and communities, particularly in fragmented landscapes. A general analytical framework has been proposed to quantify the strength of the edge effects (extent and magnitude); however, factors determining the later remain poorly explored. Using a continuous approach we explore the response of dung beetle species and assemblages to ecotones which differ in environmental dissimilarity in the Southern Atlantic forest of Argentina. Using baited pitfall traps and automatic sensors, we estimated dung beetle abundance, microclimatic conditions and vegetation structure along five different forest-plantations transects. At the assemblages level, the majority of species showed either edge avoidance or preference; however, the response depended on the environmental dissimilarity between habitats (plantation and native forest) and varied from a neutral response on mature plantations (low contrast ecotone) to edge avoidance on recent ones (high contrast ecotone). At the species level, the degree of habitat specialization explains the differential response of species to edge effects; more specialized species showed stronger edge response while generalist species showed softer or neutral responses. Environmental dissimilarity between confronted habitats and species specialization explain the quantitative component of edge effects on species and assemblages. Functional groups (rollers and tunnellers) often showed opposite responses to edge effects. At the landscape level, functional connectivity of forest fragments is probably drastically reduced by high contrasts matrices (such as recent plantations) for native forest species, whereas soft ecotones (such as native forest-mature plantations) maintained functional connectivity. These results are particularly relevant on highly fragmented landscapes, such as the Atlantic forest, where edge effect is probably one the most important mechanisms affecting native species and communities.