U.S. Geological Survey, Wyoming Cooperative Fish and Wildlife Research Unit, Zoology and Physiology Department, University of Wyoming, Laramie, Wyoming 82071, USA.
Ecology. 2010 Sep;91(9):2742-55. doi: 10.1890/09-1949.1.
Behaviorally mediated trophic cascades (BMTCs) occur when the fear of predation among herbivores enhances plant productivity. Based primarily on systems involving small-bodied predators, BMTCs have been proposed as both strong and ubiquitous in natural ecosystems. Recently, however, synthetic work has suggested that the existence of BMTCs may be mediated by predator hunting mode, whereby passive (sit-and-wait) predators have much stronger effects than active (coursing) predators. One BMTC that has been proposed for a wide-ranging active predator system involves the reintroduction of wolves (Canis lupus) to Yellowstone National Park, USA, which is thought to be leading to a recovery of trembling aspen (Populus tremuloides) by causing elk (Cervus elaphus) to avoid foraging in risky areas. Although this BMTC has been generally accepted and highly popularized, it has never been adequately tested. We assessed whether wolves influence aspen by obtaining detailed demographic data on aspen Stands using tree rings and by monitoring browsing levels in experimental elk exclosures arrayed across a gradient of predation risk for three years. Our study demonstrates that the historical failure of aspen to regenerate varied widely among stands (last recruitment year ranged from 1892 to 1956), and our data do not indicate an abrupt cessation of recruitment. This pattern of recruitment failure appears more consistent with a gradual increase in elk numbers rather than a rapid behavioral shift in elk foraging following wolf extirpation. In addition, our estimates of relative survivorship of young browsable aspen indicate that aspen are not currently recovering in Yellowstone, even in the presence of a large wolf population. Finally, in an experimental test of the BMTC hypothesis we found that the impacts of elk browsing on aspen demography are not diminished in sites where elk are at higher risk of predation by wolves. These findings suggest the need to further evaluate how trophic cascades are mediated by predator-prey life history and ecological context.
当草食动物因捕食者的恐惧而提高植物生产力时，就会发生行为介导的营养级联（BMTC）。基于涉及小体型捕食者的系统，BMTC 被认为在自然生态系统中具有强大而普遍的作用。然而，最近的综合研究表明，BMTC 的存在可能受到捕食者狩猎模式的调节，其中被动（坐等）捕食者的影响比主动（追逐）捕食者大得多。一个被广泛应用于广泛活动的捕食者系统的 BMTC 涉及到将狼（Canis lupus）重新引入美国黄石国家公园，据认为这导致麋鹿（Cervus elaphus）避免在高风险地区觅食，从而使颤杨（Populus tremuloides）得以恢复。尽管这种 BMTC 已被普遍接受并得到高度推广，但它从未得到充分验证。我们通过使用树木年轮获取有关颤杨林分的详细人口统计数据，并通过在三年时间内监测实验性麋鹿围栏内的觅食水平，评估了狼是否通过这种方式影响颤杨。我们的研究表明，历史上颤杨的再生情况在林分之间差异很大（最后一次招募年份范围从 1892 年到 1956 年），而且我们的数据并没有表明招募的突然停止。这种招募失败的模式似乎更符合麋鹿数量的逐渐增加，而不是在狼灭绝后麋鹿觅食行为的迅速转变。此外，我们对可食用年轻颤杨的相对存活率的估计表明，即使在有大量狼群的情况下，黄石公园的颤杨也没有恢复。最后，在对 BMTC 假设的实验测试中，我们发现，在狼对麋鹿捕食风险较高的地方，麋鹿对颤杨种群动态的影响并没有减弱。这些发现表明，需要进一步评估捕食者-猎物的生活史和生态背景如何调节营养级联。
