New Study Illuminates the Bear-Salmon Synergy

by Jay Cooney

In the 1940’s, the state of Alaska targeted brown bears for culling under the premise that, if populations of these often piscivorous carnivores were reduced, a greater number of salmon would abound and substantially benefit the economy of fisheries (Gende & Quinn 2006). Since then, several remarkable studies have uncovered surprising synergistic relationships. Rather than merely depleting salmon stocks, bears assist with the spread of vital marine nutrients that, in turn, enrich the soil and nurture healthier streamside habitats. Bears selectively consume the portions of salmon densest in fat, thus often leaving 75 percent of a fish’s body unconsumed and left to decompose on the shore (Gende & Quinn 2006). The caloric energy remaining in the salmon carcass cycles throughout the ecosystem with dramatic influence, providing food for insects that are prey for larger insectivores, and increasing plant growth through the transfer of minerals like phosphorous that act as fertilizer. The spread of salmon-based tissue through bear feeding and feces is integral to the productivity of life in stream side forests, with approximately 70 percent of the nitrogen in foliage along British Columbian streams originating from decomposed salmon carcasses dispersed by black bears (Gende & Quinn 2006). These findings have been a game changer for biologists studying the riparian ecosystems where streams and forest meet in the Pacific Northwest, and have reinforced the notion that species exhibit an interdependence at all trophic levels (a species’ position in a food chain). A new study published in the journal Ecosphere documents this phenomenon, and holds implications for the visualization and conservation of British Columbia’s faunal food webs.

The paper “Intrapopulation diversity in isotopic niche over landscapes: Spatial patterns inform conservation of bear–salmon systems” is the result of research into the distribution of brown bears (Ursus arctos horribilis) and black bears (Ursus americanus) feeding on salmon (Onchorhynchus sp.) in British Columbia. For a large animal like a bear that lives and forages across a mosaic landscape, food sources are scattered in a form similar to patches. As the availability, location, and abundance of these resources varies across time and space due to factors like competition, animals exploit only a select portion of these patches encompassed within what is known as a realized niche. Determining the variety of individual animals’ realized dietary niches is critical for conservationists to make effective decisions on preserving areas of key ecological functioning under the pressures of limited funds and prioritized human activities. Through subjecting over 1,400 hair samples to stable isotopic analysis, which identifies chemical signatures indicating the consumption of certain animal protein sources, the authors constructed a visual isotopic landscape. This concrete dataset illustrates both trends and variation in the salmon consumption by 886 brown bear and black bear individuals differing in sex and spatial location. Across the board, brown bears were found to consume greater quantities of salmon than that eaten by black bears, with the latter species likely facing exclusion due to the intense aggression exhibited by brown bears towards competitors during the salmon run season (Adams et al. 2017). Females of both species were demonstrated to feed on salmon less frequently than do males, likely due to competitive pressures between sexes as well as the risk of infanticide if they encounter a male while accompanied by cubs (Adams et al. 2017). In contrast to the primarily coastal range of salmon-eating female brown bears, males with the same diet extended over 1,000 kilometers into the interior, a finding which has highlighted some biases in the management of marine fish stocks.

As stated by lead author Megan Adams, the visual isotopic landscape can potentially be utilized as a map for identifying hotspots of healthy biodiversity in British Columbia (Raincoast Conservation Foundation 2017). In the case of the bear-salmon relationship, the frequency of bears consuming salmon is an indicator of an individual predator’s fitness (its ability to successfully contribute genes to the next generation), as well as that of other species that reap the benefits of leftover nutrients. Adams et al. (2017) demonstrate that, while existing preserves in coastal areas do not optimize protection of salmon-eating bears any better than unprotected areas do, provincial preserves adequately integrated the ranges of this predator-prey affiliation (Adams et al. 2017). However, the dataset also demonstrates an extensive interior range of salmon predation by brown bears that was not considered by past management decisions. This finding serves as a reminder that we must not limit the scope of conservancies to the confines of human constructs and current understanding, such as our legislative boundary between land and sea. The famed luminary of sociobiology Edward O. Wilson (2015) described food web interactions as “near-bottomless in complexity,” and stressed the importance of intimately understanding all facets of an ecosystem before proper conservation can occur. A holistic approach to ecology also supports the call for management of the quota of harvested salmon to consider the critical relationship between these fish and other prominent predators such as orcas and eagles (Raincoast Conservation Foundation 2011). If we are to base our appreciation of wildlife on a standard of profit to our species, bears certainly benefit both the forests that we obtain wood from or seek mental refuge in, as well as the seafood on our plates. However, with an understanding of the delicate yet instrumental bear-salmon synergy, it is hard to not feel a strong sense of reverence for these charismatic animals beyond their conceived worth. On a personal note, perhaps my most profound feeling of biophilia was felt while observing a muscular black bear amble along a shoreline on Vancouver Island against a backdrop of lush temperate rainforest, habitat substantially fostered by the feeding of such influential mammals. If more observers of this breathtaking sight learn to see the forest through the trees, which in this case are invigorated by nitrogen from decomposed salmon, then a drive for conserving British Columbia’s pivotal carnivores may spread like wildfire.

References:

Adams, Megan S., Christina N. Service, Andrew Bateman, Mathieu Bourbonnais, Kyle A. Artelle, Trisalyn Nelson, Paul C. Paquet, Taal Levi, and Chris T. Darimont. “Intrapopulation Diversity in Isotopic Niche over Landscapes: Spatial Patterns Inform Conservation of Bear-salmon Systems.” Ecosphere 8.6 (2017): n. pag. Web. 2 July 2017.

Gende, Scott M., and Thomas P. Quinn. “The Fish and the Forest: Overview/Sea to Shore.” Scientific American 295 (2006): 84-89. Print.

“New Dietary Study Reveals Salmon Hotspots for Grizzly and Black Bears across 700,000 Square Kilometres.” Raincoast. Raincoast Conservation Foundation, 22 June 2017. Web. 13 July 2017. https://www.raincoast.org/2017/06/new-dietary-study-reveals-salmon-hotspots-for-grizzly-and-black-bears-across-700000-square-km/.

“Salmon Management Should Include Bears, Whales and Other Wildlife.” Raincoast. Raincoast Conservation Foundation, 6 June 2011. Web. 13 July 2017. https://www.raincoast.org/2011/01/salmon-management-should-include-bears-whales-and-other-wildlife/.

Wilson, Edward O. Half-earth: Our Planet’s Fight for Life. New York: Liveright Corporation, a Division of W.W. Norton, 2017. Print.

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