Edge Effects and Conservation of Biotic Diversity

For three-quarters of a century the edge or zone of transition between different ecological associations has intrigued ecologists. The high diversity of plants and animals associated with edges and ecotones quickly became known as the edge effect principle, which is widely referred to as a fundamental concept of ecology (e.g., Wiens 1976). For the 55 years since enunciation as a game management principle, wildlife biologists have considered that "the potential density of game of low radius requiring two or more types is, within ordinary limits, proportional to the sum of the type peripheries" (Leopold 1933:132). Standard habitat management guides include the prescription to "create as much 'edge' as possible because wildlife is a product of the places where two habitats meet" (Yoakum & Dasmann 1971). But increasing emphasis on plant and nongame wildlife conservation during the last two decades has revealed many characteristics of edges and ecotones that are now considered undesirable. The term "ecotone" derives from the Greek root tonus, referring to tension. As originally coined, "ecotone" referred to the zone of tension between ecological communities. As it turns out, the term was a harbinger of tension that now exists between scientific communities. Debate about whether land managers should or should not continue to prescribe the creation of edges in the name of wildlife conservation now simmers in the wildlife profession and several land management agencies. This subject was the topic of a special session at the 1987 annual meeting of the Society for Conservation Biology. This introduction and three of the five papers presented in that session provided the historical and much of the contemporary basis for evaluating such land management decisions. As originally formulated, "The line that connects the points of accumulated or abrupt change ... is a stress line or ecotone" (Clements 1907:297). This as well as other early formulations clearly alluded to a onedimensional concept, and ornithologists such as Kendeigh (1944) and Johnston (1947) went so far as to conclude that bird densities should be reported as pairs per 100 acres for forest interior species, but pairs per mile of edge for edge species. Numerous indices aimed at assessing habitat interspersion and/or landscape diversity-for example, the shoreline index-are based on this one-dimensional aspect of edge (Welch 1948; Baxter & Wolfe 1972; Scheurholtz 1974; Patten 1974; Fried 1975; Ghiselin 1977; Taylor 1977; Thomas, Maser, & Rodiek 1979; Asherin, Short, & Roelle 1979; Hays, Summers, & Seite 1981). But confusion surrounding the nature of edges and ecotones emerged shortly after the concept was coined. Shelford (1913) clearly referred to a two-dimensional phenomenon when he stated "The forest margin, as we have seen, possesses in addition to the characteristic species . .. . " and Deventer (1936) treated swamp edge, forest edge, and fence row as though they were discrete habitat types. Johnston (1947) concluded her classic study by stating "The forest edge is considered to be a distinct community because it is inhabited by a characteristic set of species...." Beecher's (1942) analysis nearly foretold the current forest management quandary when he observed that forest margins only function as edge habitat when their extent is greater than some "minimum threshold acreage." Richard Yahner has published extensively on the topic of edge effects as they pertain to various groups of wildlife and his paper presented here reviews these various issues as they are understood by practicing wildlife ecologists and managers. Swynnerton was perhaps the first to be confronted with intrinsic contradictions of managing for edge ef-