Effect of land cover, habitat fragmentation and ant colonies on the distribution and abundance of shrews in southern California

1. Because effects of habitat fragmentation and anthropogenic disturbance o n native animals have been relatively little studied in and areas and in in sectivores, we investigated the roles of different land covers, habitat fra gmentation and ant colonies on the distribution and abundance of shrews, No tiosorex crawfordi and Sorex ornatus, in southern California. 2. Notiosorex crawfordi was the numerically dominant species (trap-success rate 0.52) occurring in 21 of the 22 study sites in 85% of the 286 pitfall arrays used in this study. Sorex ornatus was captured in 14 of the sites, i n 52% of the arrays with a total trap-success rate of 0.2. Neither of the s pecies was found in one of the sites. 3. The population dynamics of the two shrew species were relatively synchro nous during the 4-5-year study; the peak densities usually occurred during the spring. Precipitation had a significant positive effect, and maximum te mperature a significant negative effect on the trap-success rate of S. orna tus. 4. Occurrence and abundance of shrews varied significantly between sites an d years but the size of the landscape or the study site had no effect on th e abundance of shrews. The amount of urban edge had no significant effect o n the captures of shrews but increased edge allows invasion of the Argentin e ants, which had a highly significant negative impact on the abundance of N crawfordi. 5. At the trap array level, the percentage of coastal sage scrub flora had a significant positive, and the percentage of other flora had a significant negative effect on the abundance of N. crawfordi. The mean canopy height a nd the abundance of N. crawfordi had a significant positive effect on the o ccurrence of S. ornatus. 6. Our study suggests that the loss of native coastal sage scrub flora and increasing presence of Argentine ant colonies may significantly effect the distribution and abundance of N. crawfordi. The very low overall population densities of both shrew species in most study sites make both species susc eptible to extinction in isolated habitat fragments due to environmental st ochasticity, and anthropogenic disturbance.