Salmonid species diversity predicts salmon consumption by terrestrial wildlife

Abstract

Resource waves-spatial variation in resource phenology that extends feeding opportunities for mobile consumers-can affect the behaviour and productivity of recipient populations. Interspecific diversity among Pacific salmon species (Oncorhynchus spp.) creates staggered spawning events across space and time, thereby prolonging availability to terrestrial wildlife. We sought to understand how such variation might influence consumption by terrestrial predators compared with resource abundance and intra- and interspecific competition. Using stable isotope analysis, we investigated how the proportion of salmon in the annual diet of male black bears (Ursus americanus; n = 405) varies with species diversity and density of spawning salmon biomass, while also accounting for competition with sympatric black and grizzly bears (U. arctos horribilis), in coastal British Columbia, Canada. We found that the proportion of salmon in the annual diet of black bears was ≈40% higher in the absence of grizzly bears, but detected little effect of relative black bear density and salmon biomass density. Rather, salmon diversity had the largest positive effect on consumption. On average, increasing diversity from one salmon species to ~four (with equal biomass contributions) approximately triples the proportion of salmon in diet. Given the importance of salmon to bear life histories, this work provides early empirical support for how resource waves may increase the productivity of consumers at population and landscape scales. Accordingly, terrestrial wildlife management might consider maintaining not only salmon abundance but also diversity.

Keywords: Ursus; black bear; competition; foraging; grizzly bear; resource waves; salmon; stable isotope analysis.

Figure 1

Study area in coastal British Columbia, Canada (2009–2014; 22,000 km²)

Figure 2

Annual proportion of salmon (Oncorhynchus spp.) in diets of male black bears (Ursus americanus) in coastal British Columbia, Canada as a function of (a) relative black bear density, (b) spawning salmon biomass density, and (c) salmon‐species diversity (Shannon–Weaver index) in the presence and absence of grizzly bears (U. arctos horribilis). Points show median dietary estimates for unique bear‐year combinations (2009–2014; n = 157). Curves represent model‐averaged predictions from top candidate Generalized Linear Mixed Models (GLMMs) (≥0.95 cumulative model weight), incorporating the effects of competition and salmon, with beta error structure (marginal R ² = 0.21). Shaded regions represent model‐averaged 95% prediction confidence; pink shading representing the model predictions for grizzly bear presence, and blue representing the model prediction for grizzly bear absence. Grey shaded regions represent model prediction overlap between the grizzly presence and absence

Figure 3

Temporal and spatial foraging opportunities afforded by (a) low (Shannon–Weaver diversity = 0.58; two species) vs. (b) high (1.20; four species) species diversity of spawning salmon (Oncorhynchus spp.). Actual diversity values from a watershed on the central coast of British Columbia, Canada shown in (b) are predicted to lead to 40% more salmon consumption by black bears (Ursus americanus) compared to the artificially reduced diversity shown in (a). Whereas this illustration of increased diversity increased salmon availability by 28% more days and 62% more stream length, total salmon biomass density increased by only 3%