Berry production drives bottom–up effects on body mass and reproductive success in an omnivore

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 longterm 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.

Sea buckthorn berries Hippophae rhamnoides L. predict size and composition of a great tit population Parus major L

“…Although the evidence is not strong, the negative tendency is evident both between and within years. Are the berries important for winter fattening (Hernández 2009, Hertel et al 2018? Perhaps a tit can also eat too many berries.…”

Fluctuating mast production does not drive Scandinavian brown bear behavior

“…Bear hunting is allowed from 21 August and quotas in our study area are usually filled within 2 weeks (Swedish National Veterinary Institute, http://www.sva.se, accessed 10 Mar 2018). The bear study area spanned 20,000 km 2 and was located in Dalarna and Gävleborg counties in central Sweden (61°N, 14°E) approximately 40 km northeast of the Siljansfors Experimental Forest, where berry production was recorded (Hertel et al ; Fig. ).…”

“…Summer precipitation (Jun to Aug) amounts to 200–500 mm. Average temperature during August is 14°C and the growing seasons lasts on average 140 days (Hertel et al ). The rolling landscape of coniferous forest is dominated by commercial forestry plantations of Scots pine ( Pinus sylvestris ) and encompasses an elevational gradient from 200 m to 1,000 m. Bilberry is the most common berry species in Sweden, covering up to 17% of the forest floor in the Swedish boreal forests (Kardell , Nilsson and Wardle ), followed by lingonberry (5% of the forest floor; Kardell ), which produces high amounts of berries on fresh clearcuts.…”

“…The challenge with mast is that the production of fruiting bodies varies among years, often in a synchronized fashion over large spatial areas (Silvertown ) and interannual fluctuations of berry production can reach up to 80% (Kardell , Pelchat and Ruff , McLellan and Hovey ). Bear life‐history traits (e.g., body mass, reproductive success, age at primiparity, litter size, interbirth intervals) have been linked causally to soft and hard mast production for several bear species in North America (Costello et al , López‐Alfaro et al , McLellan ), Japan (Moriwaki et al ), and Europe (Hertel et al ). It is therefore not surprising that bears change their behavior when mast crops fail to find enough food to prepare for hibernation.…”

“…For example, female bears shot during the annual bear hunting season in August were heavier in years of high berry production, and berry production affected reproductive success of a segment of reproductively available females. Females that entered hyperphagia with a lower than average body mass were more likely to produce a litter after autumns of high bilberry production, as compared to autumns of low bilberry production (Hertel et al ). These findings suggest that berries are a limiting resource for brown bears in Scandinavia and that bears should show a behavioral reaction to berry production, such as in North America and Japan.…”