Scale dependence in occupancy models: implications for estimating bear den distribution and abundance

Monitoring programs are typically designed to identify long-term trends in animal abundance, however estimating abundance at a relevant scale can be logistically prohibitive. This is particularly true for species that occur at low densities or those with large home ranges. In such cases, occupancy surveys are often employed in place of more expensive abundance estimation techniques such as mark-recapture because precise estimation of occupancy probability generally requires fewer data. Although choice of plot size is a critical design element of occupancy monitoring, relatively little effort has been expended to develop or test plot size recommendations. Animal movement between surveys can complicate efforts to obtain an optimal plot size, but surveys of fixed objects, such as nests, dens, or burrows can provide insight about scale effects because the population exposed to sampling does not change during the duration of the survey. We used repeated aerial occupancy surveys to obtain estimates of brown bear (Ursus arctos) den distribution and abundance in a portion of Katmai National Park and Preserve in Alaska. We then used these data to assess the importance of plot size selection and highlight the effects of spatial grain on the resulting inference and utility for monitoring. Scale effects in estimates of mean den-based site occupancy, but not total den abundance demonstrated that careful selection of sample unit size is important if estimating occupancy probability is a primary monitoring objective. We expect occupancy surveys based on important structures such as nests or dens could have wide applicability for many species.