The Fragmented Forest

The Landscape Context

If all lands were managed to enhance local diversity by creating edges, diversity at a larger regional scale might actually decline because area-sensitive species would disappear from the larger landscape (Figure 5, below). As long as large habitat blocks continue to be managed in this manner, forest-interior species will continue to decline, leaving only edge specialists; those birds that can tolerate disturbance from predators, or those that reject Brown-headed Cowbird eggs. One goal of current conservation planning is to reach a balance between supporting desirable edge species locally, while protecting the regional and global populations of the more vulnerable forest-interior birds. This is best accomplished by considering the landscape context of local forest patches.

A. Regional-sized forest (10,000+ acres) with some natural fragmentation: Local diversity is low because few edge species are present. Regional diversity is high because region supports many area-sensitive species. B. Regional forest becomes more fragmented: Local diversity increases because edge species are added to fragmented areas. Regional diversity remains nearly constant as gains in edge species offset losses of some area-sensitive species. C. Regional forest becomes extensively fragmented: Local diversity remains constant, but edge species are distributed over a wider geographical area. Regional diversity declines because no new edge species are added, but many area-sensitive species are lost.

Figure 5. Thumbnail sketches of regional forest show how increasing fragmentation leads to more edge habitat and lowered, regional avian diversity. Illustrations by Keila Sydenstricker.

More and more, land managers and conservationists are taking into account the importance of managing habitat at large landscape scales (Freemark et al. 1995). For example, the value of an individual 100-acre (40 ha) woodlot to birds varies greatly, depending on whether it is part of an extensively forested landscape or the only woodlot for miles. Similarly, decisions to fragment large forested areas may be influenced by the importance of these areas to regional bird populations. By managing habitat at the landscape scale, managers can contribute to the health of regional populations through their own local actions on the ground. The forest-management guidelines we provide below explicitly consider how landscape features such as amount of surrounding forest, degree of isolation, and amount of edge affect the suitability of mature forest habitats for one high-profile focal species, the Scarlet Tanager.