Community Ecology and Conservation of Bear-Salmon Ecosystems

Taal Levi,Morgan D Hocking,Kevin S White,Sophie L Gilbert,Megan S Adams,Jonathan B Armstrong,Rachel E Wheat,Christopher C Wilmers,Christopher T Darimont,William Deacy,William J Ripple,Grant V Hilderbrand,Thomas P Quinn,Anthony P Crupi,Yasaman N Shakeri

doi:10.3389/fevo.2020.513304

Abstract

Apex predators play keystone roles in ecosystems through top-down control, but the effects of apex omnivores on ecosystems could be more varied because changes in the resource base alter their densities and reverberate through ecosystems in complex ways. In coastal temperate ecosystems throughout much of the Northern Hemisphere, anadromous salmon once supported abundant bear populations, but both taxa have declined or been extirpated from large parts of their former ranges with limited research on the consequences of diminished or absent interactions among species. Here we review the biogeography of bear-salmon interactions and the role of salmon-subsidized bears in (1) resource provisioning to plants and scavengers through the distribution of salmon carcasses, (2) competition among bears and other large carnivores, (3) predation of ungulate neonates, (4) seed dispersal, and (5) resource subsidies to rodents with seed-filled scats. In addition to our review of the literature, we present original data to demonstrate two community-level patterns that are currently unexplained. First, deer densities appear to be consistently higher on islands with abundant brown bears than adjacent islands with black bears and wolves, and moose calf survival is higher at low bear densities (&lt;∼25 bears per 100 km2) but is constant across the vast majority of bear densities found in the wild (i.e., ∼&gt;25 bears per 100 km2). Our review and empirical data highlight key knowledge gaps and research opportunities to understand the complex ecosystem effects related to bear-salmon interactions.

Highlights

Given the enormous geographic range of large-bodied, omnivorous bears, and their current, historic, and potential future overlap with salmon [which are experiencing rapid range shifts due to climate change that are expected to intensify (Battin et al, 2007)], it is remarkable how little attention has been paid to the community ecology of the bear-salmon system
Most importantly for management, how do terrestrial prey, such as ungulates, co-exist with hyper-abundant salmon despite abundant shared predators that benefit from salmon? We have reviewed some potential mechanisms for coexistence, but a number of complex bear-salmon-ungulate interactions are plausible
No research has examined the impact of black bear exclusion by brown bears on the predation rates of ungulate neonates, an important shared prey (Figure 3), but the observed impacts of mesopredator release on prey species in diverse systems worldwide (Ritchie and Johnson, 2009) suggest that it is plausible to expect that reduced densities of brown bears in salmon systems could unintentionally increase predation rates on ungulate neonates by releasing black bears, which can attain higher population densities than larger-bodied brown bears (Peacock et al, 2011) and are themselves important predators of ungulate neonates (Supplementary Table 1)

Summary

Introduction

We consider how salmon-supported bears can affect (1) resource subsidies to plants and vertebrate and invertebrate scavengers via the distribution of salmon carcasses, (2) food web structure via competition with other large carnivores, (3) population dynamics of ungulates via predation on neonates, (4) seed dispersal of berry-producing shrubs, and (5) food availability to small mammals in the form of seed-filled bear scats. Brown and black bears can be important predators of ungulate neonates (Zager and Beecham, 2006; Supplementary Table 1), but little is known about how co-occurring salmon influence this dynamic Salmon could support higher densities of bears, and to an extent wolves (Adams et al, 2010), leading to increased predation on ungulates.

Results

Conclusion