The distribution and abundance of mountain caribou (Rangifer tarandus caribou) in British Columbia has declined. High predation rates as a consequence of forest management and associated changes to the relative abundances of alternate ungulate prey species have been proposed to cause the population declines. A direct link between changes in the forest age structure and declining caribou population trends, however, is lacking. Understanding the underlying mechanism of the population decline is necessary to develop recovery strategies aimed at maintaining a viable mountain caribou population.
The author synthesized demographic and radiotelemetry data from separate studies initiated over the entire distribution of mountain caribou between 1984 and 2002. Her primary goal was to use a comparative approach among identified subpopulations to distinguish between three potential repercussions of forest management (food regulation, predation-sensitive foraging, and predation) that might explain the observed declining population trends. She used information on caribou density per area of forests >140 years within subpopulation ranges and cause of mortality to differentiate between the potential repercussions. Predation was the primary cause of caribou mortality over the entire distribution of mountain caribou. In addition, she found increasingly negative rates of increase as caribou density per area of forests >140 years declined (i.e. inverse density dependence). Both results were consistent with the hypothesis that the decline of mountain caribou is caused by high predation rates.
She then quantified the influence of demographic parameters on subpopulation trends and identified environmental factors correlated with variation in these demographic parameters among subpopulations. Her results indicated that differences in subpopulation trends were best explained by differences in female adult survival rates.
Female adult survival rates were negatively associated with increasing amounts of young forest stands and thus high proportions of suitable habitat for alternate prey species. Thus, her data supports the mechanistic link between the amount of habitat characteristics suitable for alternate ungulates and decreased survival of adult female caribou.
Finally, the author carried out a population viability analysis for mountain caribou. Her results indicate that mountain caribou are likely declining to extinction over the majority of their distribution within <100 years.
