Wolves trigger a trophic cascade to berries as alternative food for grizzly bears

Ripple, William; Beschta, Robert; Fortin, Jennifer; Robbins, Charles

Trophic cascades involving large carnivores have generated much recent interest and scientific research which has illustrated new and sometimes surprising linkages involving predators and food webs (Estes et al. 2011; Ripple et al. 2014). We recently wrote an article (Ripple et al. 2013) on potential trophic cascades involving two large carnivores, ungulates and plants. In our article, we explored potential connections among grey wolves (Canis lupus), grizzly bears (Ursus arctos), Rocky Mountain elk (Cervus elaphus) and berry-producing shrubs in Yellowstone National Park. We hypothesized competition between elk and grizzly bears whereby changes in the abundance of elk would change the amount of browsing on shrubs, thus affecting the available fruit to grizzly bears. The commentary by Barber-Meyer (in press) addresses three main concerns with our article including the following: (i) the use of serviceberry (Amelanchier alnifolia) as a focal species in our field study, (ii) selection of study areas in Yellowstone National Park and (iii) an alternative foods hypothesis for grizzly bears. Below we reply to each of these three points. We show a rationale for both the selection of serviceberry and our study areas. We also provide evidence that the alternative foods hypothesis as described by Barber-Meyer (in press) would likely be part of a trophic cascade from reintroduced wolves, to decreases in elk, to increases in berry production and finally to the ability of bears to switch from other foods to consuming more berries.