Coupled effects of forest growth and climate change on small mammal abundance and body weight: results of a 39-year field study

In rapidly changing environments, the combined effects of climate change and forest stand changes—such as growth or regeneration—are altering the availability of resources, particularly in systems with pulsed resources like seed-masting. These environmental shifts can have cascading impacts on animal populations, ultimately reshaping ecosystem structure and function. However, relevant studies are rare as they require long-term monitoring of both seed supply and animal populations. We investigated how temporal changes in resource availability influence the demographics and physical traits of white-footed mice (Peromyscus leucopus) using a 39-year dataset from Maine, USA, which includes: mouse abundance and body weight, red oak (Quercus rubra) tree size and acorn production, and seasonal temperatures. Our analysis of over 5,000 individual mice revealed a significant increase in both abundance (by 67%) and average body weight (by 15%) over four decades. We found that oaks produced more acorns as they grew, while warmer spring temperatures led to larger crops. This indicates that both forest growth and climate change have driven the increase in resource availability. The increase in acorn production was translated into higher mouse abundance and body weight. Notably, heavier mice also showed a higher probability of survival. These results demonstrate that changes in food supply, driven by the combined effects of forest growth and climate change, have significant effects on animal population dynamics. Furthermore, given the important role of white-footed mice as seed predators and dispersers and disease vectors, these shifts have far-reaching implications for the ecosystem.