Area-Sensitive Distributions Counteract Negative Effects of Habitat Fragmentation on Breeding Birds

We used a modeling approach to determine the conditions under which fragmentation of breeding habitat can cause landscape-scale population declines in songbirds. The simulated species resided in a system of forest patches (a landscape) and could potentially disperse among patches between breeding seasons. The model combined: (1) the frequency distribution of patch sizes in the landscape, (2) the distribution of individuals across the range of patches in the landscape, and (3) the fecundity of individuals as a function of patch size in the landscape. Simulated landscapes varied in patch size distribution while holding total forest area constant. Three distribution patterns were modeled: the classic "area-sensitive" pattern in which density increased as patch size increased, an area-insensitive pattern in which density was independent of patch size, and an "inverse-area-sensitive" pattern in which density increased as patch size decreased. We used this model to examine population growth under various landscape, distribution, fecundity, and survival scenarios. In the "fragmentation hypothesis" simulations, in which fecundity decreased as patch size decreased within the landscape, population growth rates were highly sensitive to distribution patterns and to landscape composition. When landscapes consisted of both large and small patches for breeding, populations were either above or below replacement, depending on how individuals were distributed among patches: Classic area-sensitive distributions produced growth rates above replacement levels, whereas inverse-area-sensitive distributions produced growth rates below replacement levels. The results of the model suggest that reduced fecundity associated with habitat fragmentation could lead to population declines when (1) landscapes are highly fragmented, or (2) landscapes offer both large and small patches for breeding, but distribution patterns place a high proportion of the breeding population on small patches that are less favorable for breeding.