Creel Scott, Christianson David
Department of Ecology, 310 Lewis Hall, Montana State University, Bozeman, Montana 59717, USA.
Ecology. 2009 Sep;90(9):2454-66. doi: 10.1890/08-2017.1.
Recent increases in the height and growth ring width of willow (Salix spp.) and other woody plants in the Greater Yellowstone Ecosystem (GYE) have been attributed to a behaviorally mediated trophic cascade from wolves (Canis lupus) to elk (Cervus elaphus) to willows. This hypothesis predicts that individual elk consume less willow in response to the presence of wolves, but this prediction has not been directly tested with data from elk. We collected 727 fecal samples from elk in the Gallatin Canyon portion of the GYE over three winters and used microhistological methods to quantify the proportion of willow in each sample. We then tested the effect of wolf presence on willow consumption by elk, controlling for the effects of snow conditions, sex, and habitat type. During the period of study, 8-17 wolves occupied the study area, and wolves were locally present on 49% of 260 sampling days, stratified at two-week intervals across three drainages. Over the three years combined, willow consumption was related to snow conditions, wolf presence, and a wolf X sex interaction. As expected, willow consumption increased with deeper and less penetrable snow, and this effect was strong. Contrary to expectation, willow consumption increased in the presence of wolves. As with other aspects of antipredator behavior, wolves had different effects on willow consumption by males and females. Finally, we aggregated the data to estimate winter-long mean willow consumption within each drainage; at this broader scale, willow consumption again increased as predation risk increased. In summary, willow consumption was more strongly affected by snow conditions than by the presence of wolves. Interactions between elk and willow were affected by wolves, but not as predicted by the hypothesis that wolf presence favors willow release through a reduction in the selection of willow by individual elk. If a trophic cascade is operating, our results suggest that a decline in the size of the elk population (to roughly one-half its size immediately prior to wolf recovery) may be more important than changes in the willow consumption of individual elk. Finally, reduced grazing of herbaceous vegetation may be equally important for vegetation dynamics.
黄石国家公园生态系统（GYE）中柳树（柳属）及其他木本植物近期树高和年轮宽度的增加，被归因于一种行为介导的营养级联效应，即从狼（犬属）到麋鹿（鹿属）再到柳树。该假说预测，麋鹿个体在有狼存在时会减少对柳树的啃食，但这一预测尚未通过麋鹿的数据得到直接验证。我们在三个冬季从GYE的加勒廷峡谷地区的麋鹿身上收集了727份粪便样本，并使用微观组织学方法量化每个样本中柳树的比例。然后，我们测试了狼的存在对麋鹿柳树消耗量的影响，并控制了雪情、性别和栖息地类型的影响。在研究期间，8 - 17只狼占据了研究区域，在260个采样日中的49%，狼在当地出现，在三个排水区域以两周为间隔进行分层。综合三年的数据，柳树消耗量与雪情、狼的存在以及狼×性别的相互作用有关。正如预期的那样，柳树消耗量随着积雪更深且更难穿透而增加，且这种影响很强。与预期相反，在有狼存在的情况下柳树消耗量增加。与反捕食行为的其他方面一样，狼对雄性和雌性麋鹿的柳树消耗量有不同影响。最后，我们汇总数据以估计每个排水区域内整个冬季的平均柳树消耗量；在这个更广泛的数据范围内，随着捕食风险增加，柳树消耗量再次增加。总之，柳树消耗量受雪情的影响比受狼的存在的影响更大。麋鹿与柳树之间的相互作用受到狼的影响，但并非如假说所预测的那样，即狼的存在通过减少个体麋鹿对柳树的选择而有利于柳树的释放。如果营养级联效应在起作用，我们的结果表明，麋鹿种群数量的下降（降至狼恢复前数量的大约一半）可能比个体麋鹿柳树消耗量的变化更为重要。最后，草本植物放牧量的减少对植被动态可能同样重要。
