Effects of roads and motorized human access on grizzly bear populations

Grizzly bear

A grizzly bear (Ursus arctos).

Since they were listed as a threatened species (1975 in the US, 2010 in Canada), grizzly bear numbers have started to climb. But recently, across their range, a record number are being killed by collisions with vehicles.

These higher numbers could be due to larger population sizes increasing the chances of vehicle strikes, or they could be a result of humans encroaching more into grizzly territories and increased motorized traffic. As people continue to move into more rural areas, this causes more incidents with bears as they are attracted to human activities such as roadkill along roadsides, trash, and home gardens.

A new paper reviews the literature on the effects of roads on grizzly bears, and whether management practices can be effective at mitigating their harm. 

Roads affect bears directly and indirectly via four different mechanisms: increased mortality from human encounters, habitat displacement, habitat fragmentation, and habitat loss. Human encounters are the largest direct cause of death for grizzly bears where they co-occur; in North America, humans cause 77-90% of deaths. Roads increase human access into bear habitat, and most deaths occur near roads. Indirectly, roads also cause significant changes to bear habitat. 

How roads affect bears

Schematic of mechanisms of grizzly bear response to roads. The main effect is mortality, which ultimately reduces density. Secondarily, displacement and direct habitat loss potentially affect reproductive output and density (from Proctor et al. 2019).

Fortunately, the authors found management of motorized movement through grizzly bear habitat is effective at reducing mortality. They made several suggestions for managers.

First, habitat quality should be considered. The higher the food availability in a habitat, the lower the density of roads and other motorized traffic. 

Second, movement of populations should also be considered in management plans. Habitat connectivity measures, such as wildlife bridges, should be utilized when necessary for bears to move across roads. In the U.S., connectivity between the Greater Yellowstone Ecosystem and Glacier region is considered critical to maintain the long-term genetic viability of grizzly populations. Conservation corridors between these two regions, and improved management strategies such as installation of wildlife bridges, will be highly important. 

Third, reduction of roads should be focused on areas containing threatened populations of grizzly bears. In British Columbia, Canada, where there are ten times the number of grizzly bears as in all the lower 48 states of the U.S., there are still several populations that are threatened or at much lower numbers than would be possible without human disturbance.


Proctor, M.F., McLellan, B.N., Stenhouse, G.B., Mowat, G., Lamb, C.T. and Boyce, M.S. 2020. Effects of roads and motorized human access on grizzly bear populations in British Columbia and Alberta, Canada. Ursus 2019(30e2): 16-39.

Robbins, J. Grizzly Bear Death Rates Are Climbing. The New York Times. February 10, 2020.

2020-03-10T10:54:06-04:00 March 9th, 2020|

About the Author:

Elizabeth Schultheis
I am a postdoctoral researcher at Michigan State University. My work includes research in plant biology and science education, professional development for scientists on science communication, and teacher professional development on data in the classroom. I am a co-founder of Data Nuggets, an innovative approach to bring cutting edge research and authentic data into K-16 classrooms.