Abstract Modeling the spatial distribution of wildlife abundance is paramount for management. In group‐forming species, group size and occurrence may be governed by different ecological processes. Hierarchical models can conveniently address group size and occurrence as separate processes when estimating abundance. Therefore, identifying factors influencing group size may improve models that the management of group‐forming species relies upon. We test this premise on Dall's sheep ( Ovis dalli ; henceforth sheep), a group‐forming ungulate for which spatial distance sampling models are used to inform management, but environmental features that affect the group size of sheep have yet to be explored within a spatial distance sampling framework. We first used multi‐level Bayesian models to test how spatially explicit indices of predation risk and food availability explained variation in sheep group size. We then demonstrated how including group size covariates within a spatial distance sampling model can improve model inference. Variation in sheep group size was associated with interactions between and among indices of predation risk and food availability. Larger predicted group sizes occurred in areas with higher indexed predation risk and in areas with lower risk but high indexed food availability (decreased competition). Groups tended to be smaller in steep terrain near topographical apexes and in areas with limited forage. Incorporation of these predictors of group size in our application demonstrated how our understanding of spatial patterns in abundance improved when we simultaneously modeled variation in both group occurrence and size. Our findings indicate that sheep are making complex trade‐offs between predation risk and food availability when deciding to aggregate with conspecifics. Explicitly modeling these ecological relationships within our spatial distance sampling model improved predictive performance, increased abundance estimates, and mechanistically linked ecological processes with population monitoring and management. Many wildlife species that form groups are of interest to active wildlife management and our grander understanding of wildlife ecology. Therefore, the concepts we developed here are broadly applicable across a wide range of group‐forming taxa.
Delisle et al. (Mon,) studied this question.