Woodland Caribou demographic data and range boundaries

As global climate change progresses, wildlife management will benefit from
knowledge of demographic responses to climatic variation, particularly for
species already endangered by other stressors. In Canada, climate change
is expected to increasingly impact populations of threatened woodland
caribou (Rangifer tarandus caribou) and much focus has been placed on how
a warming climate has potentially facilitated the northward expansion of
apparent competitors and novel predators. Climate change, however, may
also exert more direct effects on caribou populations that are not
mediated by predation. These effects include meteorological changes that
influence resource availability and energy expenditure. Research on other
ungulates suggests that climatic variation may have minimal impact on
low-density populations such as woodland caribou because per-capita
resources may remain sufficient even in “bad” years. We evaluated this
prediction using demographic data from 21 populations in western Canada
that were monitored for various intervals between 1994 and 2015. We
specifically assessed whether juvenile recruitment and adult female
survival were correlated with annual variation in meteorological metrics
and plant phenology. Against expectations, we found that both vital rates
appeared to be influenced by annual climatic variation. Juvenile
recruitment was primarily correlated with variation in phenological
conditions in the year prior to birth. Adult female survival was more
strongly correlated with meteorological conditions and declined during
colder, more variable winters. These responses may be influenced by the
life history of woodland caribou, which reside in low-productivity refugia
where small climatic changes may result in changes to resources that are
sufficient to elicit strong demographic effects. Across all models,
explained variation in vital rates was low, suggesting that other factors
had greater influence on caribou demography. Nonetheless, given the
declining trajectories of many woodland caribou populations, our results
highlight the increased relevance of recovery actions when adverse
climatic conditions are likely to negatively affect caribou demography.