As human populations get larger, we occupy more space in the world, which leads to increasing contact with wild animals. However, the effect of human presence on wild animals is not well understood. American black bears (Ursus americanus) are abundant and widespread across North America, yet little is known about the influence of humans on their consumption of salmon. Thomas Shardlow and colleagues from the University of British Columbia Okanagan and Ministry of Forests, Canada, set out to determine how salmon stocks and the presence of humans influences the number of salmon in the diet of American black bears in British Columbia, Canada.
Between 2008 and 2011, Shardlow and colleagues surveyed the salmon stocks in various streams and rivers – from Vancouver Island and Princess Royal Island to Gill Island – by foot and snorkel during the fish's migration and spawning periods. The researchers also collected berry and leaf samples from bear trails parallel to the study streams along with other food that the animals might consume, such as shore crabs, prickleback, woodlice and ants. In addition, the team collected bear hair samples to test and see what type of food the bears were eating. To do so, the researchers set up barbed wire hair snag sites above a scent lure 5–30 m from the waterways, retrieving the hair samples every 21 days during the summer and every 10 days during the autumn. Lastly, to measure the effects of humans on the bears’ diet, the team classified human presence in two separate ways; on a scale of 0–4 (0 being no human presence, and 4 as frequent human activity and bear hunting) and categorized the human presence as either ‘high’ or ‘low’ based on the range of human activities.
Back in the lab, after following the bears and their prey in the wilderness, the researchers measured the carbon and nitrogen levels in the hair and food samples. Additionally, they obtained previously published values for the carbon and nitrogen content of black-tailed deer, before comparing the values to identify whether the bears were consuming plants, salmon, other forms of meat from the ocean (such as fish or crustaceans) or land-based meat such as ants and deer. The team also sent the bears’ hair samples to Wildlife Genetics International to determine the sex of the bears.
The researchers found salmon populations ranging from 800 to 13,800 fish in the study waterways. Yet, despite the availability of salmon, the bears’ diet mainly consisted of leaves and berries; and all bears consumed less salmon in areas with high levels of human presence. However, the team was also surprised that the bears shunned a salmon diet even when they rarely saw humans. They suggest that even a small amount of human presence may have a considerable impact on the foraging behaviour of bears. In addition, the researchers found that female bears ate less salmon than males. As female bears need to fatten before heading to dens during winter where they give birth, a decrease in the number of salmon in their diet may reduce their ability to build-up sufficient fat stores for winter to support their growing cubs. The diet of all bears contained greater quantities of ants and deer than salmon or crustaceans, which led the researchers to suspect that bears eat more land animals to obtain sufficient protein from their diet.
Shardlow and colleagues have provided evidence that the presence of humans in the wild – no matter how minimal – can lead to a dietary shift in bears. As humans encroach more on wild spaces, disturbing the residents, the need to understand the dynamics of ecosystems is vital. Importantly, wildlife and ecosystem management need to consider the roles that humans play, no matter how insignificant.
