Evaluating livetrapping and camera-based indices of small-mammal density

Abstract

Density estimates are integral to wildlife management, but they can be costly to obtain. Indices of density may provide efficient alternatives, but calibration is needed to ensure the indices accurately reflect density. We evaluated several indices of small-mammal density using livetrapping and motion-activated cameras in the Cascade Mountains of Washington (USA). We used linear regression to compare spatially explicit capture–recapture density estimates of mice (genus Peromyscus Gloger, 1841), voles (genera Microtus Schrank, 1798 and Myodes Pallas, 1811), and chipmunks (genus Neotamias A.H. Howell, 1929) with four indices. Two indices were based on livetrapping (minimum number alive (MNA) and number of captures per 100 trap-nights) and two indices were based on photos from motion-activated cameras (proportion of cameras detecting a species and number of independent detections). We evaluated how the accuracy of trap-based indices increased with trapping effort using subsets of the full dataset (n = 7 capture occasions per site). Most indices provided reliable indicators of small-mammal density, and livetrapping indices (R2 = 0.64–0.98) outperformed camera-based indices (R2 = 0.24–0.86). All indices performed better for more abundant species. The effort required to estimate each index varied and indices that required more effort performed better. These findings should help managers, conservation practitioners, and researchers select small-mammal monitoring methods that best fit their needs.