Comparing occupancy and activity metrics for assessing temporal trends in vulnerable bat populations

Abstract

Appropriately monitoring vulnerable species is essential for resource management decision-making. We used two different metrics (occupancy and acoustic activity) to evaluate population trends of North American bats susceptible to a fungal disease and compared the outcomes of these two analyses. Our dataset consisted of passive acoustic survey data collected at nine U.S. national parks in the Great Lakes region between 2016 and 2020. Our focal species were little brown bat (Myotis lucifugus), northern long-eared bat (Myotis septentrionalis), tricolored bat (Perimyotis subflavus), and big brown bat (Eptesicus fuscus). Combining acoustic detection data with site-specific and environmental covariates, we developed park- and species-specific models of occupancy (use) and acoustic activity. For little brown, northern long-eared, and tricolored bats, probability of use declined 20 % and activity declined 66 %, on average. For big brown bat, probability of use decreased 9 % and activity increased 17 %. Our results showed that use of space and level of activity were not predicted by the same covariates but were positively related. We also found probability of use could remain high while activity decreased to relatively low levels. Although we observed precipitous declines in activity rates of three species, they were still using a high proportion of the landscape. We conclude that analyzing activity provides different but complementary information to analyzing species' use of space. The two metrics vary on different temporal and spatial scales and have different biases. Appropriate choice of which parameter to analyze is crucial, as different parameters may lead to different ecological conclusions, thus affecting how management decisions are made and how species conservation and recovery efforts are implemented.