Woodland caribou scientific review to identify critical habitat: chapter 19

Appendix 6.6 (continued)

Discussion

Our models suggested that populations of boreal caribou with poor demographic conditions (e.g., low calf survival and moderate adult female survival) face a high risk of quasi-extinction at any population size. Under moderate demographic conditions (mean calf survival and mean adult female survival), population size plays an important factor in reducing risk of quasi-extinction. Under good demographic conditions (e.g., relatively high calf and adult female survival or high adult female survival and mean calf survival), when other factors that may increase the risk of extinction are absent, small populations of 50 individuals could persist for long periods of time. Of the 57 local populations of boreal caribou in Canada that are considered to be threatened, 46% are small (less than 300 animals), 28% are considered to be declining, and 19% have both conditions. Our models indicated that small, declining boreal caribou populations are in immediate need of enhanced management to improve their chance of persistence.

Our results indicated that adult female survival strongly infl uences boreal caribou population trajectory and that high adult female survival can buffer the effects of poor calf recruitment. This conclusion is supported by fi eld studies that have demonstrated the strong infl uence of adult female survival on ungulate demographics (Nelson and Peek 1984, Eberhardt 1985, Hern et al. 1990. Walsh et al. 1995, Crête et al. 1995, Arthur et al. 2003, Wittmer et al. 2005). Our results also demonstrated the infl uence of calf survival on boreal caribou population trajectory, similar to Bergerud (1971), who showed a strong correlation between calf survival and population growth. Raithel et al. (2007) found that, despite calf survival having relatively low elasticity, the variation in calf survival explained most of the variation in lambda in an elk population.

Our models indicated that, given demographic conditions reported in the literature for four populations of boreal caribou, three have a high risk of extinction. Under relatively poor demographic conditions (e.g., relatively low adult female and calf survival), no population size can eliminate the risk of extinction, although larger populations would take longer to become extinct. The population experiencing good demographic conditions, on the island of Newfoundland (Mahoney and Virgl 2003), exists in the absence of wolves, a predator whose functional and numerical response increases with habitat disturbance (Seip 1991). It is unrealistic to expect that vital rates of boreal caribou remain unchanged over 100 years. For example, adult female survival in the Red Wine Mountains population increased from an average of 0.70 during 1993 - 1997 to 0.90 during 2000 - 2005 (Unpublished data, Wildlife Division, Government of Newfoundland and Labrador). This population has therefore not met the prediction of extinction in the PVA as a result of increasing adult female survival. Nonetheless, our results illustrated that moderate to low adult and calf survival rates increase the risk of extinction and that populations with poor demographic conditions decline rapidly regardless of their population size. Positive change in vital rates, however, particularly of adult female survivorship, can signifi cantly change the outcome of the PVA predictions. Thus, models need to be re-evaluated as new data and new knowledge become available.

Our results demonstrated that the probability of extinction in boreal caribou populations decreases with increasing recruitment rates. Bergerud (1992) reported that 27.7 calves/100 cows yielded a fi nite rate of population increase (λ) value of 1, based on 32 population survey years of both barren-ground and woodland caribou. Our results indicated that this threshold can vary, depending on survival of adult females. In our model, density dependence is incorporated as a logistic Ricker equation (scramble competition), assuming a maximum population growth rate (lambda) of Rmax=1.3. Population growth is affected when abundance reaches the carrying capacity K; below K vital rates of the stage matrix are unchanged (e.g., no density dependence). Although this suggests a ceiling form of density dependence, any form of density dependence below K would otherwise increase extinction risk and suggest an unrealistically high risk of extinction.

Linking carrying capacity with population size (e.g., K= 3Ni) likely introduced some density dependent bias, especially for large populations, that may result in an overestimation of growth rates for large populations. The importance of the CV of survival increases as population abundance approaches K because a high CV causes greater fl uctuations in abundance and thus causes the population to approach or overshoot K more rapidly, when density dependence effects occur.

The primary limitations of our model were that no maximum age or maximum age of breeding were incorporated. These limitations resulted in optimistic projections of extinction risk and likely over-emphasized the importance of adult female survival to risk of extinction and under-estimated the critical population size. The addition of a multi-age matrix model with a maximum age and senescence components would address these issues and produce more realistic estimates of extinction risk relative to population size.

Future modeling efforts should investigate the relationship between the age structure of the initial population on population size and trend over time. Insight into the degree to which a population skewed toward females is able to moderate a decline due to the greater proportion of reproducing individuals and how the proportion of yearlings to adults can infl uence trends would help inform conservation management of boreal caribou. An investigation of the correlation between adult and calf survival would help elucidate the relative importance of these factors and inform the development of management strategies that affect these vital rates.

The recovery strategy for boreal caribou states the “need for large areas of boreal forest with adequate amounts of suitable habitat and low predation rates is a consistent requirement for the conservation of the boreal population of woodland caribou across Canada” (Environment Canada 2007). Given that population vital rates are affected by habitat alteration that favours alternate prey and their predators, the non-spatial PVA provided insight into the effects of a range of demographic conditions on population persistence and the recovery goal of selfsustaining boreal caribou populations.

 

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