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Key Words ecological boundary, ecotone, edge effect, effective area model, core area model, habitat fragmentation
Abstract Edge effects have been studied for decades because they are a key component to understanding how landscape structure influences habitat quality. However, making sense of the diverse patterns and extensive variability reported in the literature has been difficult because there has been no unifying conceptual framework to guide research. In this review, we identify four fundamental mechanisms that cause edge responses: ecological flows, access to spatially separated resources, resource mapping, and species interactions. We present a conceptual framework that identifies the pathways through which these four mechanisms can influence distributions, ultimately leading to new ecological communities near habitat edges. Next, we examine a predictive model of edge responses and show how it can explain much of the variation reported in the literature. Using this model, we show that, when observed, edge responses are largely predictable and consistent. When edge responses are variable for the same species at the same edge type, observed responses are rarely in opposite directions. We then show how remaining variability may be understood within our conceptual frameworks. Finally, we suggest that, despite all the research in this area, the development of tools to extrapolate edge responses to landscapes has been slow, restricting our ability to use this information for conservation and management.
INTRODUCTION
The edges between habitat patches are often ecologically distinct from patch interiors, and understanding how ecological patterns change near edges is key to understanding landscape-level dynamics such as the impacts of fragmentation. Landscapes are often viewed as patches of habitat and nonhabitat (Figure 1a, see color insert), and thus "edge effects" have often been conceptualized as an ecological change that is due to moving away from the "core" area of a patch and not directly linked to landscape context. In fact, landscapes exist as mosaics of several different patch types (Figure Ib), so understanding the ecology of habitat edges requires understanding the complex influences that each different adjacent patch has on a focal patch. As patches become smaller and more irregularly shaped, they become increasingly dominated by edge habitat. Therefore, understanding the ecology of habitat edges is critical both for landscape ecology and for large-scale conservation and management decisions.
Because...