Investigating the influence of forest disturbance on grizzly bear habitat ecology and fitness in the Northern Continental Divide Ecosystem

The Northern Continental Divide Ecosystem (NCDE) grizzly bear (Ursus arctos horribilis) population of northwest Montana was listed as threatened in 1975 under the Endangered Species Act. Recovery objectives for this population have centered predominantly on limiting humancaused mortality and protecting habitat. Consequently, empirically informed habitat management has been an integral component of grizzly bear recovery in the NCDE and will remain a major component of future conservation and management efforts. 

The NCDE has a long history of forest management and wildfire activity. Studies have demonstrated both positive and negative effects of timber harvest on grizzly bears. Grizzly bears are associated with forest edges and early successional forests (Munro et al. 2006, Stewart et al. 2013), which often have higher abundances of preferable forage (Mace and Jonkel 1986, Souliere et al. 2020). Forest management activities that reinstate early successional vegetation can thus have a positive effect on forage quality for grizzly bears (Nielsen et al. 2004). However, roadbuilding and human access associated with timber harvest and silvicultural treatments can have a negative effect on grizzly bear habitat selection and survival (Kasworm and Manley 1990, Mace et al. 1996, 2012, Schwartz et al. 2010, Boulanger and Stenhouse 2014), potentially negating the positive effects of timber harvest and certain silvicultural treatments.  

Wildfire disturbance is likely to have variable effects on grizzly bears. Post-fire, early successional vegetation provides high quality forage, and grizzly bears likely select these postfire stands to take advantage of increases in graminoids, forbs, roots, ungulate prey, and fruiting shrubs that emerge post disturbance (Mace and Jonkel 1986, Hamer and Herrero 1987, Best et al. 2024). On larger temporal scales, historical variation in spatial patterning of infrequent, predominantly stand-replacing fires in the NCDE, has contributed to heterogeneous landscape conditions (Hood et al. 2021). Such conditions likely benefit grizzly bears by providing various landscape features required for denning, access to foods, and security. However, more frequent, and larger high severity fires are projected to increase throughout the Rocky Mountains (Abatzoglou and Williams 2016, Parks and Abatzoglou 2020, Higuera et al. 2021), which could reduce cover and impact regrowth of herbaceous vegetation (Bard and Cain 2020). Future fire activity could change the distribution of food resources across the landscape and potentially reduce expected fire benefits to grizzly bears.

Our objectives are to investigate the effects of forest disturbance from timber harvest and fire on grizzly bears in the NCDE to understand conditions that promote or degrade grizzly bear habitat quality. Balancing grizzly bear conservation while managing forest disturbances is a complex and dynamic challenge for land managers. This proposed research could help address this challenge by providing information to assist managers in developing strategies that balance species conservation and forest management objectives.