Are black bears (Ursus americanus) effective seed dispersal agents? With a little help from their friends.

Abstract

Black bears (Ursus americanus) are generally considered effective seed dispersal agents for fleshy-fruited plants because they can consume hundreds of fruits at once and have large home ranges. Seedlings can emerge from fecal piles, but if any seeds are removed from feces by rodents, it is often considered seed predation. In theory, removal of seeds from bear feces by seed-caching rodents could represent a second phase of dispersal that benefits fleshy-fruited plants, yet this idea has never been tested. I tested four hypotheses regarding the idea that a second phase of seed dispersal by seed-caching rodents is beneficial to fleshy-fruited plants that are initially dispersed by black bears in the Sierra Nevada. Using Trail Master infrared cameras to photograph animals and scandium-46, a gamma-emitting radionuclide, to track seeds, I determined that deer mice (Peromyscus maniculatus) removed seeds from bear feces and cached them in soil. These seeds escaped several sources of mortality by being moved to relatively safe locations. A field germination study confirmed that caching can benefit seedling emergence. In addition, rodents discovered seeds in bear feces more quickly than those in bird feces, suggesting that a bear-rodent tandem could be the most effective seed dispersal syndrome for some fleshy-fruited plants. Results were not consistent across years, due to either alternative food sources or high overwinter mortality of deer mice. With further study, the twophase seed dispersal syndrome presented here could help elucidate patterns of species diversity and distribution of fleshy-fruited plants.