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A grizzly bear eats a salmon in Mussel Inlet on British Columbia's Central Coast. The region is one of the watersheds in a recently published study that shows the impact of human disturbance on bears' ability to access their preferred food source.Alex Harris/Handout

Imagine a supermarket that has everything required to feed a community but is located behind multiple roadblocks and hazards that make shopping there a scary proposition.

That, in effect, is what grizzly bears may be facing when trying to access salmon-rich waters in British Columbia. But for the bears, it’s people that create the roadblock, according to a new study. The research aims to quantify the effect by using hair samples to chart changes in the grizzly bears’ diet.

“What we find matters the most is the amount of human activity. That’s the main predictor of how much fish bears are going to access in a given watershed,” said Megan Adams, an ecologist with the Raincoast Conservation Foundation and lead author on the study.

Dr. Adams say the consequences go beyond undernourished bears. When grizzlies are impeded in their ability to consume salmon, the ecosystem is deprived of the resources that are made available when bears scatter the remnants of the fish they eat across the landscape.

Grizzly bears are apex predators that once ranged from Alaska to Mexico and as far east as Manitoba. They are also omnivores that can have a varied diet. But in the British Columbia Interior, salmon migrating from the Pacific Ocean can make up a substantial share of a grizzly bear’s food supply.

By eating salmon, bears facilitate a massive transfer of nutrients from the marine environment onto land, which is reflected across multiple species, including insects and plants that thrive on the minerals and other biological resources the salmon bring. These enriched environments in turn can help to nurture newly hatched salmon.

In two of the province’s largest watersheds, the cycle has been shut down by various forms of human intervention, Dr. Adams said. For example, while salmon are present in the Fraser River, much of the lower portion of watershed is densely populated with human communities and infrastructure that keep bears away. The converse is true in the Columbia River, where multiple dams have blocked salmon from reaching areas where bears are present.

In their study, published this week in the journal Ecology and Evolution, Dr. Adams and her colleagues focused on 22 watersheds in the Interior of British Columbia where the bear-salmon interaction is still functioning. They were able to obtain data about how much salmon bears were eating through an analysis of hair samples collected by the province from 1995 to 2014. In general, the presence of various carbon and nitrogen isotopes in the samples can be used to determine how much of the bears’ diet was derived from an ocean source compared to a terrestrial source at the time the hair was growing.

The researchers were also able to look at the variables at play across the different watersheds, including the availability of fish relative to other food sources and the changing human footprint on the landscape over the same 20-year interval based on satellite images.

Once other variables were taken into account, the team had a measure of how human activity has altered the bears’ diet. The results include a reduction of up to 59 per cent in salmon consumption for female bears with increasing human disturbance.

While the study did not make specific recommendations, Dr. Adams said the results suggest that even modest amounts of disturbance within a kilometre of a fishing area can have an effect on bear diets. She added that wider buffer zones may be needed to improve the bears’ access to salmon without disturbance from logging or other forms of human activity.

But other experts caution that the study does not show a direct cause-and-effect relationship between human presence and bear diets, only a correlation.

“It is important to note that salmon was still found in the diet of bears” that were analyzed, said Scott Nielsen, a professor of conservation biology at the University of Alberta, who was not involved in the study. This implies the bears were not completely avoiding salmon streams because of an increased human footprint. Instead, the human presence may have caused them to exploit the landscape differently.

Sarah Sells, an ecologist with the University of Montana, agreed there could be other ways to interpret the results but added that the study “provides compelling evidence that humans influence how grizzly bears use salmon resources.”

Dr. Adams said her conclusions are supported by the modelling portion of the study, which holds other variables constant to discern the human impact, and by other research that shows how bears behave around human-altered areas.

She added that the study signalled a need for more detailed follow-up in the face of rapid changes across the province.

“We know that places where bears and fish still overlap are increasingly rare,” she said. “We know that human development in British Columbia is ever-expanding. So, what are we going to do about it?”

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