The fitness outcomes of species interactions can shift in sign and magnitude as environmental conditions change. For example, the occurrence and benefits of spatial aggregations in prey species may be influenced by temporal variation in predation risk. Among species that face death from shared predators, forming aggregations may prove mutually beneficial through risk dilution, earlier detection and more efficient deterrence of predators. Alternatively, prey species may better avoid shared predators by becoming more dispersed and less conspicuous to predators. However, the benefits of aggregation (or dispersion) may only become clear when predation risk is especially high. Furthermore, costs due to competitive interactions among prey may become more important when predation risk is low. Yet, variation in predation risk and its effects on species interactions are often poorly understood in natural systems. Cavity nesting birds may help mitigate predation risk when they nest simultaneously in the same tree, but interspecific competition for nest sites and low average predation rates should favor more dispersed nests on different trees. We examined how variation in nest predation risk across 15 years influenced the occurrence and fitness effects of sharing nest trees in cavity nesting birds.
Shared nest trees were more likely to be surrounded by other suitable nesting trees than random points, suggesting breeding pairs were actively choosing to nest close together. Sharing nest trees was more common in years with higher nest predation risk. Nests in shared trees experience less nest failure than solitary nests, but only in years with higher nest predation risk. Together, our results suggest that high levels of predation risk across years cause interspecific breeding aggregations to have more positive fitness effects. More generally, they show that consideration of environmental variation per se can be critical for understanding how species interactions affect fitness.