Forest Fragmentation

Forest fragmentation is an aspect of deforestation. It refers to the conversion of relatively continuous forests into a system of isolated forest patches separated by agricultural, urban, or other human land use. The forest remnants may represent areas unsuitable for conversion for economic, physical, political, or cultural reasons (e.g., because of steep terrain or for nature preserve creation). Forest fragmentation is pervasive worldwide. Large-scale forest clearance and fragmentation began in parts of Europe and Asia that had long histories of dense human settlement. It spread to North America during the European settlement about 200 to 300 years ago, beginning with the eastern and midwestern forests (Figure 1). While the abandonment of marginal farmland later facilitated reforestation of some areas, many areas remain where the forest cover represents less than 30% of the landscape. Reforestation has not necessarily reduced fragmentation. In some parts of Southern Ontario, for example, fragmentation has increased in recent decades even while the total forested area has increased. The tropics are the locale of the most widespread and rapid current or recent deforestation. This is ecologically significant because although tropical forests constitute less than 10% of Earth's land surface, they house more than half of its biological diversity. Much of Central and South America (Brazil) and southeast Asia (e.g., Malaysia, Indonesia, and Papua New Guinea) have rapid current rates of forest loss. Deforestation has slowed in areas such as West Africa, Thailand, the Philippines, and Madagascar, but these places had already lost most of their natural forest cover by the last third of the 20th century.

Forest fragmentation represents a huge change in the biogeographical context of forested systems to which plant and animal species have become adapted. Not only are the configurations of the communities changed and population sizes reduced, they are surrounded by a human landscape that may be inhospitable and intrusive. Forest fragmentation involves changes in (a) the spatial population dynamics of species in fragmented systems and (b) the biophysical environments of fragments.

Changes in Population Dynamics

In many landscapes, including North America and the tropics, fragmentation represents a relatively recent phenomenon in relation to the life spans of tree species, which constitute the bulk of forest biomass. It may take centuries for their populations to adjust to biogeographic changes, and so forest response cannot be deduced from simple measures of species composition in these systems. Theoretical constructs such as the equilibrium theory of island biogeography and other, subsequent, theoretical and computer simulation models of the behavior of disjunct populations or “metapopulations” predict that fragmentation will lead to species extinctions because dispersal among forest “islands” will be rare and insufficient to counteract ongoing local extinctions within fragments. This prediction assumes two characteristics of fragmented populations. First, population sizes in fragments are smaller and more extinction-prone than in continuous forests. Second, the long distances and potentially hostile matrix separating populations will hinder [Page 1160]interfragment dispersal and reestablishment. The combined effect is regional extinctions.

Figure 1 Deforestation and fragmentation between 1831 and 1950 in Cadiz Township, Wisconsin

Source: Burgess, K. L., & Sharpe, D. M. (Eds.). (1981). Forest island dynamics in man-dominated landscapes (p. 3). New York: Springer-Verlag. Reprinted by permission of Springer Science and Business Media.

Note: Dark areas are forest.

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