Recolonizing wolves indirectly alter seed survival
Apex predators are capable of inducing trophic cascades that alter patterns of abundance across food webs and drive the community structure of many ecosystems worldwide. Top predators, such as the gray wolf (Canis lupus), can influence plant communities by altering herbivore abundance and activity. However, we hypothesize that wolves may also have previously undocumented effects on granivores and the seeds they consume. Wolves competitively exclude coyotes (Canis latrans), thereby releasing foxes (Vulpes vulpes) from competition with coyotes. Because foxes are superior to coyotes in their ability to capture small mammal prey, an increase in fox abundance results in decreased rodent abundance. Through these cascading effects, wolf recolonization has potential to reduce seed predation via reduced rodent abundance and altered patterns of seed predation resulting from rodent predator avoidance (e.g. use of dense vegetative cover as refuge). By placing seed depots that manipulate small mammal access under either high or low vegetative cover, we were able to characterize patterns of tree seed survival in plots with high and low wolf occupancy. This allowed us to identify changes in the intensity and spatial distribution of seed predation influenced by wolf recolonization.
Our results support the hypothesis that wolves have indirect effects on granivory by rodents. Large seed (Acer saccharum and Acer rubrum) survival increased with high wolf occupancy, but small seed (Betula alleghaniensis and Tsuga canadensis) survival was not significantly altered by wolf occupancy. In areas with high wolf occupancy, seed survival was 30.4% higher for A. saccharum and 21.8% higher for A. rubrum, compared to survival in low wolf areas. Excluding small mammals from depots decreased removal of A. rubrum by 93.3%, A. saccharum by 72.8%, B. alleghaniensis by 38.0%, and T. canadensis by 66.2%. These results indicate that rodents are major seed predators within our study system with a preference for larger seeds. Predation of seeds by rodents was not influenced by vegetative cover, suggesting that the effects of wolf occupancy on seed predation are mediated by altered rodent abundance, not foraging behavior. Differences in seed survival have the potential to influence recruitment and change plant community composition. Knowledge of the indirect effects of wolves on seed survival may contribute to the prediction and management of changes that occur as wolves continue to recolonize their historical range.