OCCURRENCE PATTERNS OF BIRD SPECIES IN HABITAT FRAGMENTS - SAMPLING, EXTINCTION, AND NESTED SPECIES SUBSETS

Comparison of the species-area relationship in unfragmented chaparral habitat with that in urban chaparral fragments confirmed that rapid population extinction of resident bird species has occurred in these fragments. A strong positive correlation between the relative persistence ability of a species and its density remains even after correcting for the sampling effect of area. We conclude that this pattern is due to differences between species in extinction vulnerability attributable to density; the more abundant species persist longer in fragments. This differential vulnerability to extinction produces a pattern in which the bird species present in species-poor fragments are nested subsets of those in species-rich fragments.