Calf Survival of Woodland Caribou in a Multi-Predator Ecosystem

Abstract

The proximate role of predation in limiting caribou (Rangifer tarandus) populations is well documented, but the long-term effects of predation pressure on selection of calving areas and the subsequent impacts to calving success remain unclear. We examined the relationships among calf survival, predation risk, and vegetation characteristics among 3 calving areas and across spatial scales in the Besa-Prophet River drainage of northern British Columbia. Fifty woodland caribou (R. t. caribou) neonates were collared and monitored twice daily for the first month and once weekly during the next month of life in 2 summer field seasons (2002 and 2003). Predation risk was estimated using resource selection functions (RSFs) from Global Positioning System (GPS) locations of 15 grizzly bears (Ursus arctos) and 5 gray wolf (Canis lupus) packs. The Normalized Difference Vegetation Index (NDVI) derived from Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM) data were used to quantify large-scale characteristics of vegetation (indices of biomass and quality). We incorporated small-and large-scale characteristics (i.e., predation risk, vegetation, and movement of woodland caribou calves) of neonatal calving sites into logistic regression models to predict survival for the calving (25 May-14 Jun) and summer (15 Jun-31 Jul) seasons. Predation risk and vegetation characteristics were highly variable among calving areas and calving sites, and parturient woodland caribou responded to these characteristics at different scales. Minimizing gray wolf risk and selecting against areas of high vegetation biomass were important at large scales; areas with high biomass were likely associated with increased predation risk. Calving in areas high in vegetation quality was important across scales, as parturient woodland caribou took higher levels of predation risk to access areas of high vegetative change. Models using small-scale characteristics of calving sites to predict survival performed better in the calving season than in summer. Large-scale characteristics predicted survival of woodland caribou neonates better in summer than in the calving season, probably in part because of the unexpected role of wolverines (Gulo gulo) as the main predator of woodland caribou calves during calving. Gray wolves were the main cause of mortality during the summer. Movement away from calving sites corresponded to higher calf survival and appeared to be in response to increased access to forage during the peak demands of lactation and/or minimizing gray wolf risk in the summer. High variation in predation risk and vegetation attributes among calving areas and at calving sites within calving areas, with no differences in calf mortality related to that variation, illustrates the importance of behavioral plasticity as a life-history strategy for woodland caribou.Wildlife Monographs 165: 1-32

Using ultrasound measurements of rump fat to assess nutritional condition of woodland caribou in northern British Columbia, Canada

“…Some of the pregnant woodland caribou in the Greater Besa-Prophet Area may be at or near this limit (Table 1). Neonatal calf weights for woodland caribou in our study area [males, 8.09 ± 0.52 kg, x -± SE, n = 19; females, 7.78 ± 0.28 kg, n = 31 (Gustine et al, 2006)], however, were similar to the 10-year average of calf weights (males, 8.04 ± 0.07 kg; n = 244; females, 7.50 ± 0.07 kg, n = 267) from barren-ground caribou in generally excellent condition (Denali herd; Adams, 2005). Low body fat, therefore, may not necessarily result in lower calf weights for those caribou that do reproduce, although it is not currently known if winter body-fat levels of 6-7% are low enough to evoke a 'threshold' effect in fetal development.…”

“…1). Descriptions for this study area are available in Gustine et al (2006). Caribou were not immobilized for capture, rather we captured individuals with a net-gun (Rongstad & McCabe, 1984) from a helicopter (Bell Jet Ranger II-206B) and 'hobbled' them with leather restraints.…”

Comparison of seasonal habitat selection between threatened woodland caribou ecotypes in central British Columbia

“…Spring corresponded with the melting of snow and emerging green vegetation on south-facing slopes. Calving encompassed the typical calving period for woodland caribou (Bergerud et al, 1984;Bergerud & Page, 1987;Gustine et al, 2006a). Summer/fall began when the majority of snow had melted from the mountainous areas.…”

“…Habitat selection by caribou likely involves a trade-off between forage quality and abundance, and risk of predation (Bergerud et al, 1984;Rettie & Messier, 2000;Barten et al, 2001;Gustine et al, 2006a). The influence of these factors on selection by caribou may vary at different scales.…”

“…The influence of these factors on selection by caribou may vary at different scales. At course scales (e.g., seasonal range), caribou likely select or are able to exist in areas with a low risk of predation and then select for forage at finer scales within those areas (Bergerud et al, 1990;Rettie & Messier, 2000;Gustine et al, 2006a).…”