fRI Research: Karen Graham, Gordon Stenhouse, Terry Larsen, Dr. Laura Finnegan, and Doug MacNearney
Bear Research, Conservation, and Education Center – Washington State University: Joy Erlenback and Dr. Charles Robbins
December 2016
Executive Summary
Both woodland caribou and grizzly bears are identified as threatened species in Alberta. Current management efforts to recover caribou populations are focused on a strategy to reduce wolf predation impacts that have resulted in low recruitment within provincial caribou herds. Wolves are seen as the primary predator impacting caribou herds although a number of other carnivores are found within recognized caribou ranges, including cougars, black bears and grizzly bears.
To gain a better understanding of possible impacts of grizzly bears on central mountain caribou populations, we investigated three separate but inter-related topics concerning grizzly bear predation activities. These included 1) gaining a better understanding of grizzly bear and caribou habitat selection patterns to determine if these species co-occur over space and time as a form of predation risk, 2) investigating grizzly bear GPS location clusters to understand predation behaviour within caribou ranges, and 3) conducting captive feeding trials with grizzly bears to determine if stable isotope analysis from bear hair could provide insights into diets pertaining to ungulate consumption.
We quantified predation risk of grizzly bear on caribou using resource selection function models. Our assumption was that predation risk was directly related to the habitat selection patterns of both species, and that caribou were responding to this form of risk. More specifically, we investigated grizzly bear predation risk relative to the habitat associations of adult female caribou when selecting 1) calving sites (May 18 – June 19); 2) habitat during the post parturition period (May 18 – July 14) where calves survived or were lost; and 3) habitat seasonally (spring [May 5-June 19]; summer [June 20 – October 7]). We found that there was a relatively high degree of overlap between grizzly bears (males and females with cubs, not solitary females) and the habitat caribou selected during calving. During the post-parturition period, male bears tended to select habitats where caribou calves survived and avoided those habitats where caribou calves were lost. Conversely, females with cubs selected caribou habitat where calves were lost. Considering seasonal models of caribou habitat selection, female grizzly bears with cubs selected caribou habitats during spring, whereas male and female bears used these habitats at random. During summer, caribou predation risk from grizzly bears appeared to be lowest as bears tended to avoid those habitats preferred by caribou. Overall, spatial and temporal overlap was lowest between solitary (without cubs) female grizzly bears and caribou as female bears consistently avoided caribou habitat. We provide maps that show caribou predation risk as areas where the relative overlap (positive selection coefficient) between species is highest.
Over a two year period (2014 and 2015) we visited 448 GPS collar location clusters identified from collar data gathered from 6 male and 6 female adult (≥5 years of age) grizzly bears. At these clusters, we found 70 carcasses (16%), 33 scats containing hair (7%), 100 foraging sites (22%), 212 beds (47%), and 33 clusters where activity was unknown (7%). We identified the ungulate species from hair found in scats at clusters with no evidence of ungulate scavenging or feeding using morphological assessments. General habitat characteristics for each cluster type are provided. Of the 70 carcasses found at GPS clusters, 60 (86%) were moose; 2 (3%) were caribou and we identified a single mountain goat, mule deer, deer of unknown species, black bear, beaver and one unidentified animal. Of the 60 moose carcasses found, 36 were calves (51% of the total carcasses and 60% of all moose carcasses). Of the 33 scats containing hair, 15 (45%) were moose, 8 (24%) were not an ungulate species and 10 (30%) were unknown. We found scats containing hair at bed sites 91% of the time, and males were responsible for 61 (87%) of the 70 carcass clusters and 23 (70%) of the 33 hair scat clusters. Females accounted for 9 (13%) of the carcass clusters and 10 (30%) of hair scat clusters. Of the six female bears within this study we did not find carcasses at GPS clusters for three, but we did locate scats with hair for two of these females; one of which was confirmed to be a moose. We identified significant variation in estimates of meat consumption (carcasses and scats with hair combined) between individuals. During the ungulate calving season, 2 males consumed 11.4 and 3.1 moose calves/calving season, and 2 females consumed 4.3 and 0.0 moose calves/calving season. The average number of carcasses fed on by males and females were 17.2 (range 12.5-22.3, n=3) and 5.1 carcasses (range 1.9-10.7, n=3) respectively from 15 May to 30 September. Males spent more time at carcasses (34.9 hrs, (SE=6.12, n=61) than females (18.7 hrs, SE=7.28, n=9). The two caribou carcasses found were identified as an adult male and subadult of unknown sex. Evidence at the clusters suggested that the adult caribou may have been scavenged by the bear, and it is unknown whether the collared bear killed or scavenged the subadult caribou. Using field investigations, expert opinion and duration of time that bears spent at the carcass we separated carcass clusters into scavenging versus probable kills; we suspect that 73% of adult/yearling moose carcasses were scavenging events.
Our investigations on the utility of stable isotopes to estimate the amount of caribou grizzly bears are consuming using feeding trials on captive bears revealed important findings. Blood serum isotope analysis was able to differentiate between three ungulate species (reindeer, elk and moose) within the diet of a grizzly bear following a month of feeding. However, using blood serum would require the capture and handling of many wild bears on a regular basis, which limits the utility of this approach. As non-invasive approaches are preferable, particularly when dealing with threatened species, we also investigated whether isotopic signatures within hair (obtainable via hair snagging) could be used to differentiate ungulate species. We found that the carbon and nitrogen used to synthesize bear hair may take 50 days or more to re-equilibrate after a diet shift occurs. This result was based on ~40 day trials with four adult grizzly bears. Because of this time lag effect, and the relatively few caribou carcasses observed during our field campaign, regardless of our ability to determine stable isotope value discrimination values, we believe that this technique is currently not able to help us understand grizzly bear predation on caribou in the ranges we studied.
In summary, although we found that grizzly bears will kill caribou, our study suggests that grizzly bears likely play a minor role in the dynamics of central mountain caribou populations given current densities and distributions of these two species. However, should caribou numbers decrease further, the loss of any caribou may exacerbate the effects of mortality and further hamper population recovery.
# 14-AU-ERPC-01 / 15-ERPC-02