The Fragmented Forest

Area Sensitivity and Isolation Effects

Species whose occurrence or reproductive success is reduced in small habitat patches are referred to as area sensitive. For example, some species will only establish breeding territories in the interior of large forest tracts, far from an edge; these are known as forest-interior specialists. Area-sensitive species may attempt to nest in small forest patches but are often unable to raise young successfully because of high rates of nest predation or brood parasitism. For area-sensitive species, the size and shape of forest tracts are crucial factors for successful breeding (Whitcomb et al. 1981). Forest patches that are too small may not offer enough interior habitat to sustain breeding individuals. Similarly, a forest in the shape of a long corridor or peninsula does not offer as much interior habitat as a circular or square forest.

The degree of isolation, or the distance between a patch and the surrounding forest, may also be important in determining the long-term persistence of bird populations. For nonmigratory species, such as many woodpeckers, the ability of young birds to disperse and establish new territories is greatly reduced when the habitat is isolated. Forested corridors connecting isolated patches may be critical for these populations. Also, if fragments are separated by agriculture or regenerating forest, the possibility for dispersal may be greater than between fragments that are separated by residential or commercial development where movement is limited.