Density-dependent behaviors shape spatial redistribution of mountain ungulate abundance after a population die-off

Abstract Context Density dependence can affect patterns of animal abundance and interact with environmental disturbances. Resource restrictions from environmental disturbances may increase competition to cause population die-offs, after which competition may decrease. For group-forming prey species, theory suggests reduced competition may cause changes in density-dependent group formation and habitat selection. Because resources and environmental disturbances are heterogeneously dispersed in space, subsequent density-dependent behavioral shifts may be heterogeneous and drive spatial patterns of populations after die-offs. Objectives We sought to: (1) assess density-dependent behavioral changes in group size and habitat selection before and after a die-off, and (2) explore the role of objective 1 on the spatial redistribution of population abundance after a die-off. Methods We used a 15-year dataset on Dall’s sheep ( Ovis dalli ) within the Brooks Range in Alaska, USA. We fitted a joint hierarchical distance sampling model with posterior sampling via Markov chain Monte Carlo to track changes in group size, habitat selection, and spatial abundance. Results After a die-off (> 50% population reduction), we documented 62% and 33% increases in sizes of nursery and adult-only groups, respectively. Nursery-group occurrence was concentrated at higher elevations after the die-off, as predicted, and declined in areas with prolonged snow cover. Sheep abundance within nursery groups declined in areas with late snowmelt, while declines in sheep within adult groups were more homogeneous. Conclusions Spatial heterogeneity of resources and environmental disturbances can interact in a density-dependent fashion to affect behavioral changes that shape spatial redistributions of populations after die-offs. Our findings support interactions between density-dependent theories on habitat selection and group formation and highlight a smaller core area that appears critical for sheep conservation after die-offs.