As we study wildlife, there are some basic concepts we often hear repeated. One of the most common (and the one I want to delve into for this column) is the idea of home range. Understanding how bears use their habitats gives us great insight into these unique animals, but it never fails that as we learn even the most basic information about a species, advances in technology require us to relearn and question conventional wisdom. 

Home range for bears, or really any species, is defined as the area of habitat that an individual occupies most frequently. For bears, the size of their home range can vary tremendously depending on the individual or the availability of resources. While sex, body size, reproductive status, and other attributes can influence habitat use, every home range still needs to include the “bear” necessities: food, water, cover, enough space to make the animal feel secure, and opportunities to reproduce. The more spread out these resources are, the larger the average home range will need to be. 

One study that really highlights this variation comes from research on brown bears (Ursus arctos) in the Brooks Range of Alaska, USA. Thirty-six adult bears were collared and tracked for multiple years to study their habitat use. As expected, the average home range of males was larger than that of females, but females with young cubs also tended to have smaller home ranges than females with yearlings or no cubs at all. The researchers attributed this difference to the limited mobility of young cubs and the drive of sows to protect their offspring when they are most vulnerable. 

The same study also examined the role of diet and its influence on the size of a bear’s home range. The Brooks Range stretches across northern Alaska and into the Yukon Territory of Canada. It runs east to west and lays entirely north of the arctic circle. In this relatively resource-limited environment, bears demonstrate a variety of foraging adaptations. Some individuals focus on chum salmon (Oncorhynchus keta) as a primary food source, while others feed more on vegetation, berries, and mammalian prey. The collared bear population included individuals with both foraging tendencies, but bears that consumed more salmon were found to have considerably larger home ranges. The researchers concluded that productive salmon streams often occurred long distances from preferred denning habitats. To access the nutritional benefit of the salmon and the security of good dens, some bears were covering a lot of ground. 

It's important to remember that wildlife do not use their habitats evenly, and home ranges rarely come in the shape of a simple circle or square. They tend to be irregular, molded around the geography, resources, and threats in the area. This can make measuring them particularly challenging, but new technologies continue to emerge that offer new solutions. 

The first step to studying how wildlife use their habitats is to fit them with tracking devices to monitor their movements. Historically, VHF or Very High Frequency radio collars were the go-to technology for this kind of work in bears. These collars send out a radio signal that can be detected with a special antenna (either handheld or attached to something, like an airplane). Once detected, biologists use the antenna and the radio signal to triangulate the animal’s approximate location. Unfortunately, this is usually a manual process, which limits the total amount of data that can be collected.  

Enter the GPS collar. As the name implies, these newer devices use Global Positioning System technology to detect and report the location of collared animals using satellites. The data is collected automatically at intervals of hours to days depending on the needs of the researcher. This provides infinitely more data and allows for more detailed evaluations of movement and habitat use. But all that extra information comes at a price. GPS collars are far more expensive than their VHF counterparts, and ongoing data collection using satellites carries additional costs, much like a cellular data plan for a phone. These added costs can be a limiting factor for getting new projects started. 

Regardless of how locations are collected, the data must be analyzed if it’s going to be put to good use. A combination of mapping and statistical analysis turns clusters of locations into measures of home range. Just like the collars, programs and methods for doing this work continue to evolve. One of the most interesting things about the Brooks Range study is the way the researchers discussed their data analysis. They started with a common method for calculating home range size, but found that some of the estimates were suspiciously large. Concerned that the analysis wasn’t working well for bears with more irregular movement patterns, the researchers selected a different approach and reevaluated their data. For bears with highly centralized movement patterns, the two analyses produced similar results, but for those with more irregular movements, the adjusted approach provided results that were more in line with previous studies. 

I appreciate that the researchers were transparent about their findings and presented both analyses. Different statistical methods are being developed constantly and each has inherent strengths, weaknesses, and caveats. Picking the right analysis requires critical thinking and a practical understanding of the species we study. Sometimes it’s as much an art as it is a science. 

To prove this point, the researchers for the Brooks Range study included a comparison of their findings with 12 previous studies for brown bears across the arctic. In total, these studies evaluated 11 populations and used six different methods for analysis. The estimated home range sizes varied from 58 square miles (149 square kilometers) for female bears in the far upper Northwest Territories of Canada to nearly 3000 square miles (7760 square kilometers) for male bears in Northern Sweden. While many patterns hold true regardless of how the work is done, this definitely highlights the challenges of doing good science with constantly changing technologies and the pitfalls of making comparisons between studies that use different approaches. With that in mind, the Brooks Range research team concluded their article with a call for researchers around the world to be more collaborative and use consistent methods to aid in making better comparisons. Getting people to agree on much of anything seems difficult these days, but it sounds like a good idea to me. 

References 

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