Assessment of Nontarget Small Mammal Occupancy Using Broadly Designed Camera Arrays

The use of camera traps has become a mainstay method in ecological research and monitoring, particularly for rare, cryptic, or remote species. While many camera trap study designs focus on a single species, many nontarget species, or "bycatch," are captured as well. This bycatch can be leveraged to address questions beyond the scope of the original study designs. We aimed to assess the distributions and trends in the occurrence of nontarget species captured by an existing camera trap study initially designed to monitor mesocarnivores in Rhode Island. Using dynamic occupancy models in a Bayesian framework, we modeled the seasonal occupancy of eastern gray squirrels (Sciurus carolinensis), red squirrels (Tamiasciurus hudsonicus), eastern chipmunks (Tamias striatus), cottontails (Sylvilagus spp.), and small ground-dwelling rodents (spp. undistinguished) for each year from 2019 to 2023. We found that gray squirrel occupancy was mainly driven by annual variation and net primary productivity, a proxy for food availability. Red squirrel dynamics were influenced by the amount of coniferous forest around a site, seasonal variation, and human modification. Cottontail dynamics were positively related to the presence of young forests and negatively to human modification. Eastern chipmunk dynamics were related to seasonal timing, forest recovery after spongy moth (Lymantria dispar) damage, and precipitation. Small rodent dynamics were driven by forest structure, age, and local site features. We found that occupancy probabilities of gray squirrels, eastern chipmunks, cottontails, and small rodents increased from 2019 to 2023, and red squirrel occupancy probability showed a minor decline across Rhode Island. Our findings indicate that data on nontarget species captured via existing camera trap studies can be utilized to provide important insights about species that may not otherwise be assessed, ecosystem health, and provide potential predictors for use in analyses of target species.