Rethinking edge effects: the unaccounted role of geometric constraints

Abstract

Edge effects strongly affect the abundance and distribution of organisms across landscapes, with wide-ranging implications in ecology and conservation biology. The extensive literature on the subject has traditionally considered that edge effects result from the active avoidance or preference of organisms for certain portions of the habitat patch, assuming that abundance is uniform across a patch when environmental conditions are uniform. We demonstrate that this assumption is incorrect due to the so-far ignored 'geometric edge effect' (GEE). In the absence of environmental gradients, abundance of any organism living in a bounded habitat patch will tend to be lower in areas located near the edges compared to areas in the centre of the patch, simply because the areas in the centre receive individuals from all directions, whereas areas near the edge do not receive individuals from outside the patch. This geometric effect was already known for species richness at large geographic scales, the mid-domain effect, but its importance in the literature of edge effects remained neglected so far. Using simulations, we show that the GEE tends to reduce population abundance and community richness near the edges of bounded habitat patches, and that apparently neutral or negative responses to the edge may occur even when habitat quality is higher near the edges. A published study that detected significant edge effects is reanalyzed, demonstrating that interpreting observed abundance patterns without taking the GEE into account -as traditionally done in the vast literature on edge effects -could provide misleading conclusions. The incorporation of the GEE into sampling and analytical protocols of future studies could advance substantially our ability to understand and predict edge effects in heterogeneous landscapes.

Urbanization Impacts on Mammals across Urban-Forest Edges and a Predictive Model of Edge Effects

“…In contrast, we found a neutral response with the proximity to the urban boundary, probably as a result of a lack of immigrants from the high-density housing development [58]; and a reduction in its abundance towards the interior of high-density housing developments. This response trajectory may be the result of a spillover of animals from forest to urban areas; with the strong reduction of the abundance with increasing distance to the forest representing dispersing animals across hard edges.…”

Dung beetle persistence in human-modified landscapes: Combining indicator species with anthropogenic land use and fragmentation-related effects

“…These statements are in accordance with the notion that dung beetle assemblages are sensitive to habitat loss (Klein, 1989) and the creation of forest edges, due to the fact that a considerable number of species are forest-dependent or forest-interior specialist species (Filgueiras et al, 2011). Many others are able to eventually exploit forest edges and the surrounding matrix for additional resources (Prevedello et al, 2012), while others are matrix or even edge-specialists (Peyras et al, 2013). Such ecological composition suggests that HMLs are able to retain a significant proportion of dung beetle fauna, although forest interior represents an irreplaceable habitat (Gardner et al, 2008) as shown by the indicator species analysis.…”

Response of non‐grassland avian guilds to adjacent herbaceous field buffers: testing the configuration of targeted conservation practices in agricultural landscapes

“…Last, softer edges may alleviate geometric constraints on richness near edges by altering perception of habitat boundaries (geometry hypothesis; sensu Prevedello et al . ).…”