Imagine you have to survive on an island with one hundred other people. Depending on the size of the island and how much everyone eats, you might struggle to find enough water, food and shelter. If nobody new ever joins your island, over time the choices for romantic partners would feel, well… slim. Everyone, eventually, becomes related to everyone else. Not exactly a great situation.
The challenge for bears is similar. Grizzlies require large, intact habitats to thrive–for an adult male, a home range (where a bear dens, breeds and so forth) can be up to five hundred square miles, or the size of Nashville, Tennessee! Development such as roads, houses, farms, and energy infrastructure shrink and separate bear habitat, so that habitat “islands” may be too small to sustain healthy populations of grizzlies, or bears struggle to travel between them to feed in new areas or breed with non-relative bears.
Today, conservation efforts have grown the grizzly bear population to about 1,800 individuals in the Lower 48. Numbers, however, don’t tell the full story when it comes to the health of species. We care not only about how many grizzlies are out there, but also whether they have quality habitat and can travel freely enough between habitats to survive and breed sustainably. This is the idea of “connectivity”–how connected different habitats are for a species–and it is critical to the genetic health and survival of animal populations.
Thousands upon thousands of grizzly bears once thrived throughout western North America. From the mid-1800s to 1970s, however, their numbers and habitat dwindled dramatically due to hunting and human expansion. When the grizzly was listed under the Endangered Species Act in 1975, grizzly bear habitat in the contiguous United States was a paltry 2 percent of its historic size.
Currently, grizzly bears in the United States are managed in six different “recovery zones” across Washington, Idaho, Montana and Wyoming. The largest of these is the Northern Continental Divide Ecosystem (NCDE), which includes Glacier National Park, with over 1,000 bears, and the Greater Yellowstone Ecosystem (GYE), including Yellowstone and Grand Teton National Parks, with over 700 bears.
The problem is that these different ecosystems and respective populations are disconnected. There are not easy pathways for bears to travel between them, and so they have become like islands in a sea of human development. Among many challenges, isolation limits the genetic pool of populations, and can result in inbreeding. A lack of genetic diversity hampers a population’s ability to adapt to environmental changes, increases susceptibility to disease, and can even lead to local extinctions. Nobody wants to be stranded on an island.
Connectivity is therefore essential to the health of grizzly bears across our landscapes, and connecting bear “recovery zones” is one of the long-term objectives of the U.S. Fish and Wildlife Service (USFWS). The hope is that it happens naturally. The GYE and the NCDE grizzlies are now only 45 miles apart, a distance which has shrunk over time. The connection between these two populations is critical particularly to the long term genetic health of GYE bears, which have been genetically isolated for over one-hundred years. According to the USFWS, “Due to its connectivity to large populations in Canada, the NCDE has the potential to serve as an important genetic corridor between Canadian grizzly bear populations and the GYE.”
Yet there are also two highways between the NCDE and GYE. While it’s not impossible for a bear to cross a road, it is stressful and can lead to bear mortality, either due to vehicle collisions or other human-bear interactions near roadsides. This may hinder natural migration.
In that case and in the case of other geographically separated populations, different solutions are needed, such as wildlife corridors. These corridors aim to string protected areas together with smaller linkage habitats. Establishing them begins with thoughtful mapping of where animals are likely to travel, as has been done already between the GYE and NCDE recovery zones. Land managers, legislators, scientists, the public and other stakeholders must then collaborate to reduce obstacles for bears through these linkages by preventing additional development within them; reducing human-bear conflict on public and private land; and facilitating movement across roads, as has been creatively facilitated with wildlife bridges over large highways.
Successful migration and interbreeding between grizzly populations could take generations of bears to complete, yet if we truly wish to maintain grizzlies as strong, healthy creatures on our landscapes, we can’t just protect isolated islands of bears–we need to connect them.