An individual’s annual home range is the area it uses to acquire what it needs for the year, such as finding food, shelter, and mates. For brown (grizzly) bears, their movements are limited to when they leave their dens in spring and when they enter them in the fall. Annual home range size can be influenced by many things, including the size of the individual bear, its sex, whether or not a female has cubs, the productivity of its habitat, and climate. A team of biologists from the National Park Service and U.S. Fish and Wildlife Service put GPS collars on male and female brown bears in the Brooks Range and used the collar data to estimate each bear’s annual home range.
The annual home ranges of male bears were 4-9 times larger than those of female bears. While this result is similar to studies from other areas, the difference between males and females was among the largest ever reported. The team expected that larger bears of the same sex would require larger ranges to acquire all the food they needed, but surprisingly, they did not find evidence to support this. In coastal environments, brown bears with access to super abundant salmon can have incredibly small annual home ranges (5 mi2). In contrast, Brooks Range bears that fed on salmon had larger annual home ranges than those that did not. The reason, the biologists suspect, is that bears in the Brooks Range region generally have to travel relatively far (50 miles) between prime denning habitats in the mountains and salmon streams at lower elevations.
So how much space does a brown bear use in a year? The team used two different methods to estimate this. The first one has been widely used in other studies (it is called the Kernel Density Estimator or KDE). The other is a newer method that incorporates the bear’s movement to estimate space use (called the dynamic Brownian Bridge Movement Model or dBBMM). Using the regularly used method, some bears of both sexes had the largest annual home ranges reported anywhere in the world. However, these very large annual home ranges were from bears whose movements were not confined by a defined area and instead traveled to somewhere distant in the summer. This type of behavior does not fit well with traditional concepts or estimates of annual home range, and so the biologists followed up by estimating range size using the other method. Using this method, they had an answer to their question: males used an average of 124,500 acres (195 mi2) and females 33,400 acres (52 mi2) in a year.
With large-scale industrial development proposed in this roadless region, the size and drivers of bear annual home ranges has numerous management implications. Brown bears with large annual home ranges in northcentral Alaska, where primary productivity is relatively low and denning habitat is often far from salmon-bearing streams, are likely to move outside conservation units and encounter more risks as they interact with humans and human infrastructure.
Source: U.S. Department of the Interior, National Park Service
