How much space does a brown bear in the Brooks Range use?

Bears in the Brooks Range have to travel a long way between where they den and an active salmon stream.

NPS/Matt Cameron

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.

Factors influencing Arctic brown bear annual home range sizes and limitations of home range analyses

Abstract

Home range size is a basic ecological index related to individual's realized niche. Its size can be influenced by body size, sex, maternal status, population density, habitat productivity, spatiotemporal variation of resources, climate, predation risk, and disturbance. Home range estimation can also be greatly affected by methodology and sampling regime. We used Global Positioning System collar data to assess what factors influenced the size of annual home ranges (space use during a single active season) of 28 female and 8 male brown bears (Ursus arctos) that denned in the Brooks Range of northcentral Alaska, USA, from 2014 to 2017. We used 2 methods to estimate annual home ranges, the Kernel Density Estimate (KDE) and the dynamic Brownian Bridge Movement Model (dBBMM). Contrary to expectations, we did not find that larger bodied bears of the same sex had larger annual home ranges. Annual home ranges of male bears (mean [standard deviation]; 504 [312] km² and 3,886 [4,279] km², using dBBMM and KDE, respectively) were 3.7–9.4 times larger than that of females (135 [86] km² and 411 [738] km², respectively). We found that greater chum salmon (Oncorhynchus keta) consumption was associated with larger annual home ranges for both sexes. In contrast, coastal brown bear populations that consume high levels of salmon often have small annual home ranges. We suggest that the relatively long distance (up to 100 km) between salmon streams and another key resource, denning habitat, is a reason for the positive association between salmon consumption and annual home range size. Although age was not in our top model for annual home range size, younger bears tended to have larger annual home ranges. We documented the fact that individuals of both sexes had the largest annual home ranges of any we could find for brown bears worldwide, using a traditional measure of space use (KDE). However, very large annual home ranges were associated with nonlocalized movements and the alternative method (dBBMM) to delineate these ranges provided more realistic range estimates. We discuss options and limitations of estimating space use and recommend caution when comparing space use between studies. With large-scale industrial infrastructure approved for development in this previously undeveloped region, the size and drivers of bear annual home ranges have numerous management implications. Brown bears with large annual home ranges in northcentral Alaska, where primary productivity is relatively low and denning habitat often far from salmon-bearing streams, are likely to move outside conservation units and encounter more risks as they interact with human infrastructure.

Joly, K., M. D. Cameron, M. S. Sorum, D. D. Gustine, W. Deacy, and G. V. Hilderbrand. 2022. Factors influencing Arctic brown bear annual home range sizes and limitations of home range analyses. Ursus (33e11): 1-12.