Modification of food webs is a frequent cause of shifts in ecosystem states that resist reversal when the food web is restored to its original condition. We used the restoration of the large carnivore guild including gray wolves (Canis lupis), cougars (Felis concolor), and grizzly bears (Ursus arctos horribilis) to the northern range of Yellowstone National Park as a model system to understand how ecosystems might resist reconfiguration after the restoration of apex predators to the food web. The absence of wolves, cougars, and grizzly bears for nearly a century from the northern range was the primary cause of dramatic changes in riparian plant communities. Willows (Salix spp.) were suppressed in height by intense browsing by the dominant herbivore, elk (Cervus canadensis). The loss of activity by beavers (Castor canadensis) coincided with the loss of tall willows. We hypothesized that intense elk browsing interrupted the mutualism between willow and beavers: ecosystem engineering by beavers was a critical component of willow habitat and tall willows were a critical component of habitat for beavers. This interruption made riparian communities resilient to the disturbance caused by the restoration of apex predators. We hypothesized further that reductions in elk browsing attributable to reductions in elk population size were not sufficient to prevent the suppression of willow growth. To test these hypotheses, we conducted a 20-year, factorial experiment that crossed simulated beaver dams with the exclusion of browsing. We found that willows grew to heights expected for restored communities only in the presence of dams and reduced browsing. Willows experiencing ambient conditions remained well below this expectation. We found no difference in heights or growth rates of willows in experimental controls and willows in 21 randomly chosen sites, confirming that the results of the experiment were representative of range-wide conditions. A reorganized community of large herbivores was implicated in the suppression of willow growth. We conclude that the restoration of large carnivores to the food web failed to restore riparian plant communities on Yellowstone's northern range, supporting the hypothesis that this ecosystem is in an alternative stable state caused primarily by the extirpation of apex predators during the early 20th century.

中文翻译：

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将顶级捕食者恢复到食物网中会恢复生态系统吗？黄石公园的大型食肉动物作为模型系统

食物网的改变是生态系统状态变化的常见原因，当食物网恢复到原始状态时，生态系统状态将无法逆转。我们利用黄石国家公园北部地区灰狼（ Canis lupis）、美洲狮（Felis concolor）和灰熊（Ursus arctos horribilis ）等大型食肉动物群体的恢复作为模型系统，以了解生态系统如何抵抗重新配置。在顶级掠食者恢复食物网之后。北方山脉近一个世纪以来没有狼、美洲狮和灰熊，这是河岸植物群落发生巨大变化的主要原因。由于主要食草动物麋鹿（Cervus canadensis）的强烈啃食，柳树（Salix spp.）的高度受到抑制。海狸（ Castor canadensis ）活动的丧失与高柳树的消失同时发生。我们假设，麋鹿的强烈觅食破坏了柳树和海狸之间的共生关系：海狸的生态系统工程是柳树栖息地的重要组成部分，而高大的柳树是海狸栖息地的重要组成部分。这种中断使河岸群落能够抵御顶级掠食者的恢复造成的干扰。我们进一步假设，由于麋鹿种群规模的减少而导致的麋鹿觅食的减少不足以阻止对柳树生长的抑制。为了检验这些假设，我们进行了一项为期 20 年的析因实验，跨越了模拟海狸水坝，排除了浏览行为。我们发现，只有在存在水坝和减少浏览的情况下，柳树才能生长到恢复社区预期的高度。柳树经历的环境条件仍然远低于这一预期。我们发现实验对照中的柳树和随机选择的 21 个地点的柳树的高度或生长速度没有差异，这证实了实验结果代表了整个范围的条件。大型食草动物群落的重组与柳树生长的抑制有关。我们得出的结论是，大型食肉动物恢复食物网未能恢复黄石公园北部山脉的河岸植物群落，这支持了这样的假设：该生态系统处于另一种稳定状态，主要是由于 20 世纪初顶级掠食者的灭绝造成的。