Response of rodents to habitat fragmentation in coastal southern California

We employed an island biogeographic approach to determine whether small fragments of the shrub habitats coastal sage scrub and chaparral, isolated by urbanization, are capable of supporting viable populations of native rodent species. The distribution of native rodents in 25 urban habitat fragments was assessed by live-trapping. Over half of the fragments surveyed (13 of 25) did not support populations of native rodents. Fragments supported fewer species than equivalently sized plots in large expanses of unfragmented habitat, and older fragments (fragments that had been isolated for a longer period of time) supported fewer species. Both results implied that local extinctions occurred in the fragments following insularization. Stepwise multiple polychotomous logistic regression was used to determine which biogeographic variables were the best predictors of species number across fragments. The area of shrub habitat in each fragment was the most significant predictor of species diversity; age of a fragment was also significant and was negatively correlated with species number, but the isolation distance of a fragment had no relationship to species diversity. We found a negative relationship between extinction vulnerability of native rodent species and relative abundance: species that were more abundant in unfragmented habitat persisted in more habitat fragments. Random environmental and demographic fluctuations (island effects) and edge effects associated with fragmentation are proposed as causes of these local extinctions.