Endangered Species Recovery Program
Unpublished ReportCalifornia Department of Parks and Recreation
Daniel F. Williams
Population sizes of riparian brush rabbits (Sylvilagus bachmani riparius) and riparian woodrats (Neotoma fuscipes riparia) in Caswell Memorial State Park (MSP) were estimated using capture-recapture models. Three areas of the Park were trapped separately for 7 to 9 day periods in January 1993. Estimated naive densities were: 3.0 ± 1.1 brush rabbits/ha, for 241 brush rabbits in 81 ha of the Park (95% confidence interval 170-608), and 4.8 ± 3.7 woodrats/ha, for 437 woodrats in 91 ha of the Park (95% confidence interval 218-2011). The density of woodrat houses was 8.3/ha. Thirty-one sites with three groups of four or more fresh rabbits pellets (deposited on top of recently-fallen leaves) were permanently marked in areas throughout the Park. These sites can be assessed in January of future years and compared with the data for 1993 to calculate an activity index for population monitoring. Several assumptions of this monitoring protocol are untested and it is recommended that a 7 to 10 day livetrapping program be used with or in lieu of the pellet assessment. Livetrapping would provide a better brush rabbit population index, allow detection of pathologies, test the efficacy of the pellet assessment method, and simultaneously assess the woodrat population. Recommendations for management actions, should population levels fall to threshold values below the 1993 level, include quarterly or monthly population monitoring by livetrapping, habitat assessment, testing for pathogens, and developing provisions for temporary removal of the populations to captivity, depending upon the threshold level surpassed. Also recommended are genetic studies to assess the levels of inbreeding and genetic diversity and to determine the relationships of the Caswell MSP populations to the geographically closest populations of brush rabbits and dusky-footed woodrats.
PDF Copy: esrp_1993_rbr-popcensus-csmp.pdf