Abstract
Obligate herbivores dominate studies of the effects of climate change on mammals, however there is limited empirical evidence for how changes in the abundance or quality of plant food affect mammalian omnivores. Omnivores can exploit a range of different food resources over the course of a year, but they often rely on seasonally restricted highly nutritious fruiting bodies during critical life stages. Brown bears Ursus arctos in Sweden are dependent on berries for fattening before entering hibernation. We used a ten‐year time series to evaluate the effect of temperature and snow on annual variation in berry abundance and how this variation affected bears. We found marked interannual variation in berry production of bilberry Vaccinium myrtillus and lingonberry V. vitis‐idaea, that we could attribute in part to temperature during plant dormancy and flowering and precipitation during fruit ripening. Both, autumn weights of female bears and spring weights of yearling bears increased linearly with bilberry abundance. When bilberry abundance was low, lightweight female bears had a lower reproductive success than females in better condition. This effect vanished when food abundance was above average, indicating that lightweight females could compensate for their initial weight during good bilberry years. Our study highlights the importance of considering individuals’ dynamic responses to variation in food availability, which leave some more vulnerable to food shortage than others. Individual life‐history heterogeneity in response to resource variation likely affects long‐term population recruitment. Our findings emphasize that Scandinavian bears can be dependent on a single food resource during a critical period of the year and are therefore less resilient to environmental change than expected for an omnivore. Future climate scenarios predict ambiguous trends for weather covariates that affected crucial stages of berry phenology, preventing a clear prognosis of how climate change may affect long‐term bilberry production.
Highlights
Omnivores have a simple digestive system and often rely on digestible and highly nutritious fruiting bodies, like berries or nuts, during limited time periods (Smith and Follmer 1972, Bojarska and Selva 2012)
We address how omnivorous mammals are affected by annual variation in fruit production
Our objective was to quantify interannual variation in bear food abundance during hyperphagia to 1) evaluate to what extent climatic events during key stages of berry plant phenology synchronize fruit production, 2) determine whether fruit production varies among sampling years in a synchronized fashion, and if so, 3) quantify the effects of varying fruit production on life history traits of brown bears
Summary
Omnivores have a simple digestive system and often rely on digestible and highly nutritious fruiting bodies, like berries or nuts, during limited time periods (Smith and Follmer 1972, Bojarska and Selva 2012). The few studies evaluating the indirect effects of climate change on small omnivorous hibernators found a crucial dependence between weight acquisition during hyperphagia and the timing of den entrance and emergence for reproductive success and winter survival (Inouye et al 2000, Ozgul et al 2010, Tafani et al 2013). We analyzed whether interannual variation in bilberry and lingonberry abundance could explain variation in brown bear life history traits in our study population, i.e. autumn mass of hunter-killed females, spring mass of yearlings, and female reproductive success (implantation and early cub survival). We predicted that female reproductive success would be lower after years of simultaneous failure of lingonberry and bilberry and hypothesized that body mass at the beginning of hyperphagia may buffer this effect Unifying these three components, our overarching hypothesis was, that weather events affect bilberry and lingonberry abundance, and, if synchronized, they indirectly affect bear life history
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