Occupancy Modeling and Detection of Washington Ground Squirrels (Spermophilus washingtoni): Progress Report

Accurate assessment of the occurrence of animals that are difficult to detect is especially important for species that may be in decline. The Washington Ground Squirrel (Spermophilus washingtoni) is a candidate species for Threatened or Endangered status designation by the U.S. Fish and Wildlife Service and State of Washington. The species is difficult to survey because of its fossorial lifestyle and preference for native plant communities with features that enhance concealment. In 2008 we initiated a pilot study to better understand characteristics of occupancy and detection of this species in eastern Washington. Our interest was to determine the effects of environmental and survey covariates on occupancy and detection by modeling these characteristics gathered through systematic, repeated surveys. We identified four areas with populations of varied historic use by ground squirrels and conducted 150 detection surveys within each 1 km² study area during three periods in spring. We measured a suite of site and sampling covariates potentially affecting squirrel detection including weather conditions, survey timing, and site vegetation. We modeled squirrel occupancy using program PRESENCE. Study area, badger sign, burrow count, burrow visibility, time of survey, surveyor, and sky overcast entered the best model. Study area and burrow count were significant effects on occupancy, and surveyor and time of survey were significant effects on detection probability. Based on preliminary results, our experimental design appears to adequately capture factors necessary to model squirrel occupancy and we anticipate a second year of data collection in 2009 under a different set of conditions (study areas, surveyors, weather) to provide additional data.