Area-sensitive distributions counteract negative effects of habitat fragmentation on breeding birds

We used a modeling approach to determine the conditions under which fragmen tation 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 se asons. The model combined: (I) the frequency distribution of patch sizes in the landscape, (2) the distribution of individuals across the range of pat ches in the landscape, and (3) the fecundity of individuals as a function o f patch size in the landscape. Simulated landscapes varied in patch size di stribution while holding total forest area constant. Three distribution pat terns were modeled: the classic "area-sensitive" pattern in which density i ncreased as patch size increased, an area-insensitive pattern in which dens ity was independent of patch size, and an "inverse-area-sensitive" pattern in which density increased as patch size decreased. We used this model to e xamine 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. Wh en landscapes consisted of both large and small patches for breeding, popul ations were either above or below replacement, depending on how individuals were distributed among patches: Classic area-sensitive distributions produ ced growth rates above replacement levels, whereas inverse-area-sensitive d istributions produced growth rates below replacement levels. The results of the model suggest that reduced fecundity associated with habitat fragmenta tion could lead to population declines when (1) landscapes are highly fragm ented, or (2) landscapes offer both large and small patches for breeding, b ut distribution patterns place a high proportion of the breeding population on small patches that are less favorable for breeding.