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On the Trails: Spatial overlap among bears and between bears and humans

Studies: Female bears will forage away from males, even if that means being closer to humans

Posted: April 27, 2012 - 12:00am
In spring, black bears often come to the Visitor Center area to feed on cottonwood catkins, often breaking many branches. Most of the visiting bears are females or young bears, apparently finding both food and a refuge from bigger bears in an area with many humans.  Photo courtesy of Laurie Craig
Photo courtesy of Laurie Craig
In spring, black bears often come to the Visitor Center area to feed on cottonwood catkins, often breaking many branches. Most of the visiting bears are females or young bears, apparently finding both food and a refuge from bigger bears in an area with many humans.

Coastal Alaska supports both black and brown bears, both of which inhabit the rainforest and potentially eat much the same kinds of food.

With both species on the prowl for resources, where do they occur together and how do they get along?

In Southeast, both species occur in many areas, and one can watch both species fishing for salmon in places such as Anan Creek. Here, the black bears tend to avoid the brown bears, but there are usually so many salmon that both kinds of bears feed well.

But, that is not always the case.

Brown bears achieve larger body size than black bears of the same age and gender. Larger body size allows brown bears to be dominant over black bears and potentially to exclude them from choice feeding areas. Salmon spawning runs are prime foraging places and brown bears are often capable of near-monopolies there. One study, near Denali, found that brown bears ate much more salmon than black bears, in general, and when salmon runs were poor, black bears got no salmon at all because brown bears prevented access to the streams. So, in years of small salmon runs, black bears had poor body condition and poor reproduction.

Another study, on the Kenai Peninsula, also found that brown bears ate far more salmon than black bears. Occasional male black bears visited salmon runs, but obtained few fish. No female black bears had access to the stream. Both species of bear ate berries extensively, but fruits made up a higher proportion of energy intake for black bears. Part of the Kenai Peninsula apparently lacks brown bears, for some reason, and in that area, black bears normally consume lots of salmon.

Although both species of bear occupy much of Southeast, there are broad areas where only one kind of bear has well-established populations. On Admiralty, Chichagof, and Baranof islands, only brown bears occur. But why, given that they co-occur with black bears in other areas?

On the other hand, on the islands south of Frederick Sound, including Haida Gwaii, only black bears are found. However, fossil remains show that brown bears formerly occurred there, and brown bears are seen there upon occasion even now, but they do not establish breeding populations.

Why are the browns absent in these regions? Selective hunting? Some subtle habitat change? An interesting puzzle.

Humans now occupy significant chunks of coastal Alaska, largely displacing bears from most urban and agricultural areas. Rather than deal here with the obvious issue of habitat usurpation by humans, I’d like to focus on the possible effects of hunting and bear-viewing activities.

Human hunting pressure might change interactions among bears, if it is focused on one gender or if it significantly reduces bear density. In addition, we have so many bears that bear-watching has become a profitable business as well as a popular activity for local folks.

A study in southcentral Alaska found that where brown bears are not hunted, males clearly dominate at most fishing sites except when fishing is poor; at good fishing sites, males were far more common than females. Big bears need to spend more time fishing to get enough fish to eat; fishing is very inefficient for them when capture rates are low. However, where bears are hunted, male brown bears were not more common than females at good fishing places. Hunting pressure is heavier on males than on females, reportedly, which would reduce their numbers. In addition, where they are pursued, males may become more wary than females, such that they sacrifice good fishing for perceived safety. Whatever the reason, fewer males meant that females, especially those with cubs, had more access to fishing sites.

Add bear-viewing humans into the mix of interactions, and researchers found that adult male brown bears tended to avoid the humans, showing more avoidance in areas where hunting pressure was higher. Males also avoided humans more when there were alternative foraging sites in the same region, but if there were no alternative sites, male tolerance of human viewers increased — and presumably females then got fewer fish.

A study in British Columbia found that females with cubs spend less time fishing when big males were active; these females had about a third less consumption of salmon than undisturbed females. Bear-viewing activity tended to displace the big males, providing mother bears with increased access to fish. Mere presence of humans had little effect on foraging of females.

Near the Mendenhall Visitor Center, most of the black bears that use the area are females and young bears; there are few adult males. Observers believe that here, too, bear-viewing humans provide smaller bears with a safe area to forage, where they are not displaced by big males. It would be interesting to learn where the adult male black bears in this region go to forage!

At Pack Creek on Admiralty Island, most of the brown bears foraging on salmon in the estuary near the viewing area are females and subadults, and the males tend to forage farther upstream, away from most humans. On Admiralty, hunting is generally harder on brown bear males than females, especially in spring, so perhaps males are rightfully more wary.

Thus, the general consensus seems to be that females avoid big males when they can and take advantage of their absence when possible, including finding temporary refuge near bear-viewing activities.

• Mary F. Willson is a retired professor of ecology.

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