Salmon-supported bears, seed dispersal, and extensive resource subsidies to granivores

In salmon-rich environments, which once spanned much of the Northern Hemisphere, bears occur at exceptionally high densities. Salmon, by growing bear populations, have the potential to exert wide-ranging effects on ecosystem processes. Salmon-supported bears provide seed dispersal services to plants, and bear scats containing thousands of seeds may then be efficient nutritional resources for granivorous small mammals that also function as secondary seed dispersers while hoarding seeds for winter. We taxonomically identified and enumerated seeds in individual bear scats to characterize patterns of bear frugivory. We then combined estimates of seed abundance and digestible energy content to quantify the energy available to granivorous small mammals, and we quantified the proportion of the mouse population that could be supported by locally abundant bear populations in lowland salmon systems. We additionally monitored seed-filled bear scats with remote cameras to quantify small mammal visitation rates, and live-trapped small mammals seasonally to determine whether rodents visited bear scats proportional to their densities or whether some species preferentially selected for bear scats, and to assess whether seasonal variation in scat visitation was driven by density or selection. Bears were an important initial dispersal agent for 12 species of fruit, particularly devil's club (Oplopanax horridus) and blueberry (Vaccinium spp.), which occurred in 80% (5839 seeds/scat) and 50% (10,719 seeds/scat) of scats, respectively. Seeds in bear scats were intensively utilized and dispersed by small mammals, primarily scatter-hoarding northwestern deer mice (Peromyscus keeni; 8.5 visits per day/4295 total visits) and larder-hoarding northern red-backed voles (Myodes rutilus; 2.2 visits per day/1099 total visits), with visitation rates proportional to the seasonal density of each species. Small mammals likely incurred significant nutritional benefits from seeds deposited in bear scats (kcal/scat, mean = 114, n = 71). In coastal Alaska riparian areas, bears are potentially capable of indirectly subsidizing the energy needs of 45-65% of local deer mouse populations. Thus, this work helps elucidate the role that salmon, by supporting abundant bears, plays in ecological communities via influencing seed dispersal and resource subsidies to the small mammals that compose the base of the food web.