Salmonid species diversity predicts salmon consumption by terrestrial wildlife

Salmon species diversity is more important than the total number of salmon when it comes to how much salmon bears eat. Increasing the diversity of salmon has a significant benefit for bears.

A collage of images and graphs from a published peer reviewed article on salmonid species diversity and bear health: Hakai, Raincoast, University of Victoria, and Spirit Bear Foundation logos at the bottom.

Research by scientists at Spirit Bear Research Foundation, Raincoast Conservation Foundation, and the University of Victoria, led by Christina Service, shows that salmon species diversity – the number of spawning salmon species available – is far more important and positively related to salmon consumption in coastal black bears than biomass abundance.

We used stable isotope analysis on hair samples from black bears to estimate their salmon consumption, a measure that strongly relates to population productivity and health. We wanted to understand how consumption patterns were affected by biomass, salmon diversity, and competition with other bears, both black and grizzly. The landscape level study (22,000 km2) used non-invasively sampled hair from 379 black bears and 122 grizzly bears in collaboration with the Wuikinuxv, Nuxalk, Heiltsuk, and Kitasoo/Xai’xais First Nations in the Great Bear Rainforest of coastal British Columbia.

Our results were published in the Journal of Animal Ecology. Read the full open access paper, “Salmonid species diversity predicts salmon consumption by terrestrial wildlife.”

Open access article

Key findings

The study found that salmon biomass did not affect salmon consumption, at least in the relatively productive watersheds in this region. Rather, salmon species diversity had a large and positive effect. For example, the same amount of salmon biomass across four species allowed black bears to consume approximately 3 times more salmon each year compared to a scenario in which the same biomass comprises one large run of a single species.

Grizzly bears also played a role in salmon consumption by black bears. Black bears consumed approximately 40% less salmon when they co-occurred in watersheds with grizzly bears.

Application

Given the importance of salmon to bear population health, this work provides early empirical support for how resource waves (spatial variation in resource availability that can extend feeding opportunities, such as those provided by salmon) 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. These results provide insights to managers of both terrestrial wildlife and fisheries. Should managers want to maintain opportunities for bears to eat high levels of salmon, maintaining the diversity of species present in watersheds will be key. This requires safeguarding smaller, less commercially important salmon species, often neglected in salmon management.

Citation

Service C. N., Bateman, A. W., Adams, M. S., Artelle, K. A., Reimchen, T. E., Paquet, P. C. & Darimont, C. T. (2018). Salmonid species diversity predicts salmon consumption by terrestrial wildlife.

Journal of Animal Ecology. 2018;00:1–14. https://doi.org/10.1111/1365-2656.12932

Open access article 

Abstract

  1. 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.
  2. We sought to understand how such variation might influence consumption by terrestrial predators compared with resource abundance and intra- and interspecific competition.
  3. 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.
  4. 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.
  5. 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.

Study system

Our remote study area on the central coast of British Columbia is composed of mainland valleys, ocean fjords, and an assemblage of islands (<1 km2 to >2,220 km2) separated by tidal waters (Service et al., 2014). Here, black bears are present across the entire landscape. By contrast, grizzly bears are prevalent in mainland watersheds, but are absent from many island watersheds (Service et al., 2014).

Select figure

FIGURE 2 Annual proportion of salmon(Oncorhynchusspp.) 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).
Figure 2. Annual proportion of salmon (Oncorhynchusspp.) 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
R2 = 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

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