Department of Forest Ecosystems and Society, Oregon State University, Corvallis, OR, USA.
J Anim Ecol. 2014 Jan;83(1):223-33. doi: 10.1111/1365-2656.12123. Epub 2013 Sep 4.
We explored multiple linkages among grey wolves (Canis lupus), elk (Cervus elaphus), berry-producing shrubs and grizzly bears (Ursus arctos) in Yellowstone National Park. We hypothesized competition between elk and grizzly bears whereby, in the absence of wolves, increases in elk numbers would increase browsing on berry-producing shrubs and decrease fruit availability to grizzly bears. After wolves were reintroduced and with a reduced elk population, we hypothesized there would be an increase in the establishment of berry-producing shrubs, such as serviceberry (Amelanchier alnifolia), which is a major berry-producing plant. We also hypothesized that the percentage fruit in the grizzly bear diet would be greater after than before wolf reintroduction. We compared the frequency of fruit in grizzly bear scats to elk densities prior to wolf reintroduction during a time of increasing elk densities (1968-1987). For a period after wolf reintroduction, we calculated the percentage fruit in grizzly bear scat by month based on scats collected in 2007-2009 (n = 778 scats) and compared these results to scat data collected before wolf reintroduction. Additionally, we developed an age structure for serviceberry showing the origination year of stems in a northern range study area. We found that over a 19-year period, the percentage frequency of fruit in the grizzly diet (6231 scats) was inversely correlated (P < 0·001) with elk population size. The average percentage fruit in grizzly bear scats was higher after wolf reintroduction in July (0·3% vs. 5·9%) and August (7·8% vs. 14·6%) than before. All measured serviceberry stems accessible to ungulates originated since wolf reintroduction, while protected serviceberry growing in a nearby ungulate exclosure originated both before and after wolf reintroduction. Moreover, in recent years, browsing of serviceberry outside of the exclosure decreased while their heights increased. Overall, these results are consistent with a trophic cascade involving increased predation by wolves and other large carnivores on elk, a reduced and redistributed elk population, decreased herbivory and increased production of plant-based foods that may aid threatened grizzly bears.
我们探讨了黄石国家公园灰狼（Canis lupus）、麋鹿（Cervus elaphus）、产浆果灌木和灰熊（Ursus arctos）之间的多种联系。我们假设麋鹿和灰熊之间存在竞争，在没有狼的情况下，麋鹿数量的增加会增加对产浆果灌木的啃食，减少灰熊可获得的果实数量。在狼群被重新引入且麋鹿数量减少后，我们假设产浆果灌木（如野樱莓，一种主要的浆果植物）的建立会增加。我们还假设，在重新引入狼之后，灰熊饮食中的果实比例会比引入之前更大。我们在麋鹿密度增加期间（1968-1987 年），比较了灰熊粪便中果实的频率与狼重新引入前的麋鹿密度。在重新引入狼后的一段时间内，我们根据 2007-2009 年收集的粪便（n=778 个粪便样本），按月计算了灰熊粪便中果实的百分比，并将这些结果与狼重新引入前收集的粪便数据进行了比较。此外，我们为野樱莓建立了一个年龄结构，展示了在北部研究区域中茎的起源年份。我们发现，在 19 年的时间里，灰熊饮食中果实的百分比频率（6231 个粪便样本）与麋鹿种群大小呈负相关（P<0·001）。与重新引入狼之前相比，在 7 月（0·3%对 5·9%）和 8 月（7·8%对 14·6%），灰熊粪便中果实的平均百分比更高。所有可被有蹄类动物接触到的测量野樱莓茎都起源于狼重新引入之后，而在附近有蹄类动物围栏内生长的受保护野樱莓既起源于狼重新引入之前，也起源于狼重新引入之后。此外，近年来，围栏外野樱莓的啃食减少了，而它们的高度增加了。总的来说，这些结果与涉及狼和其他大型食肉动物对麋鹿的捕食增加、麋鹿数量减少和重新分布、食草减少以及植物性食物产量增加的食物链级联相一致，这可能有助于受到威胁的灰熊。
