HABITAT LOSS AND POPULATION DECLINE - A METAANALYSIS OF THE PATCH SIZE EFFECT

We evaluated the conditions under which patch size effects are importa nt determinants of local population density for animals living in patc hy landscapes. This information was used to predict when patch size ef fects will be expected to occur following habitat loss and fragmentati on. Using meta-analysis, we quantitatively reviewed the results of 25 published studies that tested for a relationship between patch size an d population density. Patch size effects were strong for edge and inte rior species (negative and positive patch size effects, respectively), but negligible for generalist species that use both edge and interior habitat. We found significant differences in mean patch size effects between migratory and residential species, between herbivores and carn ivores, and among taxonomic groups. We found no evidence that patch si ze effects were related to landscape characteristics such as the propo rtion of landscape covered by habitat, median patch size, or the scale at which a study was conducted. However, species in the Western Hemis phere tended to have larger absolute effect sizes, and eastern species tended to be more variable in their response. For landscapes undergoi ng habitat loss and fragmentation, our results predict the following: (1) among generalist species that use both the edge and the interior o f a habitat patch, the decline in population size associated with habi tat destruction should be accounted for by pure habitat loss alone; (2 ) for interior species, the decline in population size associated with habitat fragmentation per se will be greater than that predicted from pure habitat loss alone; (3) for edge species, the decline in populat ion size will be less than that predicted by pure habitat loss alone; (4) these relative effects will not be influenced by the extent of hab itat loss, but they will be affected by the pattern of habitat when la rge or small patches are preferentially removed; and (5) as loss and f ragmentation increase within a landscape, migratory species will gener ally suffer less of a decline in population size than resident species .