Variation in Local-Scale Edge Effects: Mechanisms and Landscape Context

Abstract

Ecological processes near habitat edges often differ from processes away from edges. Yet, the generality of “edge effects” has been hotly debated because results vary tremendously. To understand the factors responsible for this variation, we described nest predation and cowbird distribution patterns in forest edge and forest core habitats on 36 randomly selected plots in three states in the midwestern United States. We tested four hypotheses that may explain the variation and mechanisms responsible for edge effects among the 36 plots: (1) the landscape context, (2) the local predator community, (3) the local bird (host–prey) community, and (4) the nest site microhabitat structure. We used artificial nests baited with quail and clay eggs to determine nest predation patterns and predators and used point count surveys to determine cowbird and host abundance in forest edge and forest core habitats. Raccoons, opossums, canids, and birds accounted for most predation of artificial nests. Neither local host abundance nor mean nest concealment of artificial nests significantly influenced nest predation rates in habitat edge or in habitat core. Nest predation was significantly greater in highly fragmented landscapes than in unfragmented landscapes and was significantly higher in edge habitats than in core habitats. However, detection of edge effects varied, depending upon landscape type. Higher predation rates in edge habitats were detected in highly and moderately fragmented landscapes, but not in unfragmented landscapes. Both mammalian and avian predator groups contributed to higher predation rates along edges in highly and moderately fragmented landscapes. Cowbird abundance was significantly related to host abundance, but the effect of hosts varied depending upon habitat type. In edge habitats, cowbird abundance was negatively associated with host abundance in all three landscapes studied. By contrast, cowbird abundance was positively associated with host abundance in core habitats. Once the effects of host abundance were removed, cowbird abundance in core habitat was greater in highly fragmented landscapes than in moderately and unfragmented landscapes, but did not differ between the latter two. In edge habitat, cowbird abundance did not differ between landscapes, but abundance in edges tended to be highest in the highly fragmented landscape and lowest in the unfragmented landscape. Cowbird abundance did not vary between edge and core habitat in any of the landscapes studied. We suggest that the first approximation to predicting the impact of agricultural or permanently managed edges on forest songbird reproductive success is to assess habitat characteristics at the landscape scale. Given geographic location, local factors such as host abundance and predator composition should be assessed.

Testing mechanistic explanations for mammalian predator responses to habitat edges

“…This result is partly in support of the null hypothesis (i.e., no response of predators to transitions between two habitats). However, a contrasting pattern of predator distribution was found in the subsequent year when our data indicated, as have many previous studies (e.g., Dijak and Thompson 2000;Donovan et al 1997), the increased activity of predators specifically along habitat edges. Moreover, in 2007, predator activity within grassland and forest interiors was significantly lower than in edge structures and did not differ between these habitat types.…”

“…Although many studies have detected increased predation pressure along the edges of various habitat types (e.g., Bayne and Hobson 1997;Dijak and Thompson 2000;Donovan et al 1997;Malt and Lank 2007), only a few were able to explicitly test the competing mechanistic explanations of the edge effect as outlined above. This might be due to drawbacks associated with experimental designs.…”

“…First, predation risk and/or the presence of predators often were not found throughout the entire edge gradient (i.e., in the habitat edge and both adjacent habitats; Albrecht 2004;Chalfoun et al 2002;Heske 1995), which means that it was not possible to determine if the edge effect arose as a consequence of predator penetration into the adjacent habitat or due to specifically enhanced activity of edge specialist predator species along the habitat edge. Second, since most studies did not collect data on prey distribution (e.g., Dijak and Thompson 2000;Donovan et al 1997;Hilty and Merenlender 2004), habitat qualities could not be sufficiently evaluated. Third, most studies failed to reliably determine particular predator species (e.g., Huhta et al 1998;Pasitschniak-Arts and Messier 1995;Storch et al 2005).…”

Fungal Assemblages Associated with Roots of Halophytic and Non-halophytic Plant Species Vary Differentially Along a Salinity Gradient

“…Regarding host plants, endophyte communities of I. crithmoides and I. viscosa were only similar in the intermediate zone of the gradient (zone B). This was also the zone with a larger species richness, representing a buffer area with mixed species from both communities, plus a number of highly adaptable other species ("edge effect"; [35]). The halophytic I. crithmoides appeared to harbor a plastic community depending on its position in the dune slack and adapted to the specific site conditions.…”

Raccoon (Procyon lotor) Activity is Better Predicted by Water Availability than Land Cover in a Moderately Fragmented Landscape

“…Procyon lotor L. (Raccoon), for example, exhibits greater abundance in heterogeneous than in homogeneous landscapes (Oehler andLitvaitis 1996, Pedlar et al 1997). Greater abundance of Raccoons in fragmented landscapes is often assumed to cause negative effects for other species, such as greater rates of nest predation for songbirds (Donovan et al 1997, Schmidt 2003 and depredation of threatened species (e.g., turtles or their eggs; Feinberg andBurke 2003, Mitchell andKlemens 2000).…”