In temperate coastal ecosystems throughout much of the world, anadromous fish historically supported large omnivorous bear populations. In Alaska and British Columbia, where anadromous fish are still prolific, bears distribute salmon derived nutrients to plants, insects and vertebrate scavengers, yet little is understood about the how salmon, by supporting large bear populations, indirectly contribute to ecosystem processes. As large omnivores, bears may serve as keystone hubs that fulfill many roles in ecosystems. For example, it is currently unknown whether bears rather than birds are the primary seed dispersers of the fleshy fruited plants that dominate the understory in coastal AK and BC, whether their seed-filled bears scats influence the dynamics of small mammal populations, and whether secondary seed dispersal from bear scats by small mammals improves seedling recruitment and plant community dynamics. During 2014 and 2015 we addressed these questions by (1) quantifying the number of seeds in brown and black bear scats, (2) by monitoring small mammal activity at bear scats, (3) by using cameras and eDNA from residual saliva to monitor the rate of fruit consumption by different vertebrate seed dispersers and seed predators, and (4) by monitoring the response of small mammal populations to experimental bear scat addition. We discuss the results of this research and the implications for what has changed in the Pacific Northwest in the absence of a strong bear-salmon interaction.
Results/Conclusions
We found that bears, not birds, were the primary seed dispersers of devil’s club (Oplopanax horridus) fruit. Further, brown bears (Ursus arctos) dispersed more seeds than black bears (Ursus americanus) and female bears more than male bears. Bears dispersed the seeds of at least 12 fleshy-fruited plants. Individual bear scats contained tens to hundreds of thousands of seeds, including as many as 150,000 blueberry seeds (Vaccinium spp.) and 80,000 devil’s club seeds. The digestible energy content in individual bear scats was as high as 5000 kcal, enough of a nutritional subsidy to meet the energetic needs of a mouse for over one year. We observed extensive foraging activity at bear scats by six species of vertebrates, but northwestern deer mice and northern red-backed voles were by far the most common visitors. Some of this visitation presumably led to secondary seed dispersal through scatter hoarding. We additionally observed dozens of battles between small mammals in defense of bear scats.