Ecosphere (Jan 2018)
The primacy of bears as seed dispersers in salmon‐bearing ecosystems
Abstract
Abstract Salmon–bear ecosystems that historically existed throughout most of the northern temperate and boreal regions of planet earth now persist only in the North Pacific. Extensive research on salmon–bear interactions has focused on the role that bears (Ursus arctos and U. americanus) play in provisioning terrestrial systems with marine‐derived nutrients, but little attention has been paid to how the much higher bear population densities in salmon‐bearing ecosystems then affect ecological communities. Salmon‐supported brown bears secondarily consume large quantities of fruit and may thus serve as important seed dispersers, but the relative seed dispersal services provided by bears and birds are unknown. We sought to (1) quantify the number of seeds dispersed by bears relative to birds, and (2) by brown bears relative to black bears, and to (3) assess whether the two sympatric bears temporally partition berry resources as a result of competitively dominant brown bears switching to feed on salmon, thus opening niche space for black bears. We used a combination of motion‐triggered camera traps and environmental DNA (eDNA) from residual saliva to quantify the roles of birds, black bears, and brown bears as seed dispersers of devil's club (Oplopanax horridus), the dominant understory shrub at our field site in northern southeast Alaska. Brown bears were the numerically dominant seed dispersers, particularly before salmon became widely available, after which black bears became common seed dispersers, likely due to alleviation of interference competition. Birds accounted for only a small fraction of seed dispersal. This is the first demonstration of a temperate plant being primarily dispersed by mammalian gut passage. Our results suggest that bears are uniquely important seed dispersers, which may influence plant community composition in salmon‐bearing ecosystems.
Keywords
