Complex Interactions Spell Trouble for Mountain Caribou

Big game species in North America have fascinating survival strategies and few are more interesting than the mountain caribou of the Pacific Northwest. Mountain caribou live in a region that receives snowfall in amounts better measured in feet than inches; 30 or more feet each winter. Unlike most deer species, mountain caribou in these high snowpack ecosystems do not migrate to lower elevations to escape the snow, but instead ascend from early winter habitat in lower elevation cedar-hemlock forests to high elevation spruce-fir forests. The caribou then use the snow to lift them up into the branches of trees where they subsist on a diet comprised entirely of arboreal lichen.

Because of abundant rain and snowfall, forest fires are rare in this region and prior to extensive timber harvest, old-growth forest dominated. Old growth spruce-fir forest produces the slow growing arboreal lichens eaten by woodland caribou during winter, but does not produce a lot of other forages that large herbivores can eat after the snow melts. An environment with a limited variety of food would appear to be a bad place to live, but if you are a species adapted to a diet composed only of lichens for over 6 months/year, it can be a good location. An area that cannot support many large herbivores can support even fewer large predators, which is why the old-growth forest and alpine tundra in the mountains of British Columbia, Idaho, and Washington support caribou populations.

This fine balance between prodigious snowfall, old-growth forest, low predator density, and mountain caribou works as long as the system remains intact. However, populations of mountain caribou across much of the subspecies’ range are in trouble. The precarious state of mountain caribou in southwestern British Columbia has stimulated research which suggests that human-caused changes to the system may have triggered a cascade of events detrimental to caribou.

A large body of research in British Columbia suggests the following scenario. Old growth timber has been logged during the past 50 or more years. Harvest of old-growth timber generally favors browsing herbivores, like caribou, by promoting growth of nutritious forbs, shrubs, and seedlings on the forest floor. At the same time, timber harvest reduces the amount of old growth timber and therefore the arboreal lichen caribou need during winter. Recent studies suggest there is sufficient old growth timber remaining to meet the winter nutritional needs of the caribou.

So, why are caribou having problems when the only change in their environment was logging of old growth forest, which generally favors browsing herbivores? A clue comes from caribou mortality patterns. In declining caribou populations, most mortality of adult caribou is during summer and predation is the primary cause. Not only is adult mortality unsustainably high, but survival of calves is exceedingly low. The most likely reason mortality has increased is that logging did exactly what would be predicted; it favored browsing herbivores. In this instance, the herbivores favored were moose, elk, and deer. With increasing numbers of these other deer species, large predators, particularly cougars and wolves, were able to increase. Larger predator populations increased predation on caribou, and because caribou were never in high densities, even incidental killing of caribou by predators has caused caribou populations to decline.

Interestingly, without knowledge of predation’s role in this story, the decline of caribou with increasing moose, elk, and deer populations could be incorrectly interpreted as competition between caribou and these other deer species. However, several studies clearly show food resources are sufficient for stable or growing mountain caribou populations and that predation is the root cause of the caribou’s decline.

Thus, a change in habitat may have short-circuited the mountain caribou’s strategy of surviving in an area with little food by having high adult survival, sufficient reproduction to offset low mortality rates, and too few caribou to support more than low densities of predators. Now that we understand the problem, the challenge will be to determine how to manage the system so that needs of people, predators, and caribou can be balanced.

A research paper by Kinley and Apps suggest the following in areas managed for mountain caribou:

• Change hunting regulations to liberalize harvest of predators, elk, moose, and deer
• Alter forest management practices to favor older forests 
• Transplant caribou into areas capable of supporting them

For a more information on the plight of mountain caribou, see:

D. R. Seip. 1992. Factors limiting woodland caribou populations and their interrelationships with wolves and moose in southeastern British Columbia. Canadian Journal of Zoology 70:1494-1503.

A. T. Bergerud and J. P. Elliot. 1986. Dynamics of caribou and wolves in northern British Columbia. Canadian Journal of Zoology 64:1515-1529.

T. A. Kinley and C. D. Apps. 2001. Mortality patterns in a subpopulation of endangered mountain caribou. Wildlife Society Bulletin 29:158-164.
H. U. Wittmer, A. R. E. Sinclair, and B. N. McLellan. 2005. The role of predation in the decline and extirpation of woodland caribou. Oecologia 144: 257–267. 

Trophy Points: Big Game Research On Line is complied and edited by David G. Hewitt, a Professional Member of the Boone and Crockett Club and the Stuart W. Stedman Chair for White-tailed Deer Research at the Caesar Kleberg Wildlife Research Institute.