Ecology of Fragmented Landscapes

5. Fragment Context and Edge Effects

c h a p t e r f i v e ⠃⠄⠅⠂ Fragment Context and Edge E¤ects In landscapes and regions context is usually more important than content. That is, the surrounding mosaic has a greater e¤ect on patch functioning and change than do the present characteristics within the patch. —Richard T. T. Forman (1995) Many ecological studies of habitat loss and fragmentation have focused on how ecological patterns or processes are a¤ected by the characteristics of habitat fragments , such as size and isolation. For example, observational studies (see chapter 3) clearly show that larger patches support a higher diversity of species and that isolation may have detrimental e¤ects on species persistence. And most experiments (see chapter 4) have manipulated the size and isolation of habitat fragments , or fragmentation per se, to directly examine their e¤ects on ecological processes. This emphasis on characteristics such as size and isolation makes sense. After all, these features are easily measured, and there is a good deal of ecological theory to suggest that these two factors may largely explain variation in species richness among habitat islands or fragments. But as the title of Daniel Janzen’s 1983 paper graphically suggests, “no park [or fragment] is an island.” Ecologists have learned that there is often a lot of action at the interface where fragments end and the rest of the landscape begins. This chapter explores the dynamic changes that occur at the boundaries of fragments and examines how these changes are a¤ected by the characteristics of the surrounding landscape. It reviews an abundant quantity of literature on species responses to edges, and a growing body of research on varied landscape spatial arrangements, to suggest ways in which understanding the e¤ects of fragment spatial position in the landscape relates to prior studies of fragment size and isolation. This discussion emphasizes a major contribution of landscape ecology, which is to take an expanded view—to look beyond characteristics of local habitats or fragments and to consider the ecological dynamics of the broader landscape mosaic (e.g., Forman 1995). This expanded view stresses how landscape context influences ecological processes. Think of landscape context as a form of ecological peer pressure. In the painfully familiar form of peer pressure that most teenagers experience, opinions and actions are often shaped by the behaviors or habits of people in their immediate surroundings. The same thing can happen in habitat fragments that are embedded in complex landscapes. Like those teenage peers, the land uses and natural habitats that surround a fragment may exert strong influences on that patch. For example, a forest fragment surrounded by fertilized agricultural fields is likely to experience di¤erent pressures and inputs than one surrounded by urban development. Or a prairie dog colony surrounded by continuous grassland is likely to be quite di¤erent from one surrounded by heavily traªcked roads and industrial development (fig. 5.1; see the section on prairie dogs at the end of this chapter). Before delving into detail on the topic of fragment context, however, it is necessary to first mention edge e¤ects. The study of edges is intimately linked with landscape context, for it is at these boundaries between fragments and their surroundings where the most immediate imprint of landscape context is detectable. Put simply, edges are the borders between two habitat types, such as the edge between a river and its bank, or one between a meadow and a forest. Most conservation ecologists are concerned with edges that are created as a result of human activities, such as the edge between a forest and an agricultural field, or one between old-growth forest and a clear-cut. Accordingly, this discussion focuses primarily on the latter type, that is, on human-induced edges. Discussions of ecological edges often refer back to Aldo Leopold’s (1933) treatise on game management, where he noted that wildlife in the midwestern United States, such as deer, quail, and grouse, tended to be most abundant in landscapes rich with edges between deciduous forests and agricultural fields. Stimulated by this observation, wildlife managers spent the next few decades championing this “edges are good” paradigm, since edges were considered favorable for wildlife. 94 e c o l o g y o f f r a g m e n t e d l a n d s c a p e s Leopold’s concept began...