The Primacy of Mammals as Seed Dispersers and Predators in Salmon-Bearing Ecosystems Public Deposited

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  • Plants often encase seeds in a nutritional reward to incentivize seed dispersal by birds and mammals, but these seeds may also be removed and destroyed by seed predators. Although birds are typically thought to be the primary seed dispersers of berries in temperate systems, in southeast Alaska and other salmon-bearing ecosystems, where partially frugivorous bears (Ursus arctos and U. americanus) are especially abundant, mammalian seed dispersal pathways may be uniquely important. Salmonbear ecosystems that have historically existed through most of the temperate and boreal regions of planet earth have shrunk to only exist in the North Pacific. Research on salmon-bear interactions has focused on the direct flow of marine-derived nutrients, but little attention has been paid to the indirect effects that salmon have on ecosystems by supporting high densities of bears. Brown and black bears are known seed dispersers of fleshy-fruited shrubs in southeast Alaska, where brown bears are supported in remarkably high densities by anadromous salmon. Salmon, through brown bears, could impact the understory plant community of this ecosystem if bears provide key seed dispersal services that are not redundant with those provided by birds. We used a combination of motion-triggered camera traps and eDNA to quantify the relative roles of thrushes, brown bears, and black bears as seed dispersers of devil’s club (Oplopanax horridus) berries during the summers of 2014 and 2015. We found that overall, brown bears are the dominant seed dispersers of devil’s club, followed by black bears, and that avian seed dispersers accounted for only a small fraction of the total berries harvested. This is the first record of a temperate plant being primarily dispersed by mammalian endozoochory. Additionally, we identified that red squirrels (Tamiasciurus hudsonicus) are the dominant, previously unidentified, seed predators of devil’s club. This research demonstrates that bears serve disproportionately important roles as seed dispersers, and suggests that plant community structure may be influenced by the abundance of salmon-supported bears.
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