LTREB Renewal: Understanding controls on state-transition on Yellowstone's northern range

灰狼的灭绝和随后重新引入黄石国家公园为了解捕食者如何改变生态系统提供了难得的机会。最初，人们认为狼的减少会导致麋鹿数量的急剧增加，这反过来会导致公园溪流沿线的柳树发生实质性变化。由于麋鹿数量的减少，狼群的重新引入将导致柳树群落的迅速恢复。然而，前15年的研究表明，柳树群落的命运是复杂的，涉及狼、麋鹿、野牛、海狸和可用水之间的相互作用。本研究将重点关注这些捕食者、食草动物和当地水文之间的复杂相互作用如何影响植物对景观的影响。这些知识构成了管理整个美国西部牧场的科学基础，并在此过程中提高了整个地区的人类经济。黄石公园狼群的故事引起了公众的极大兴趣，并将在大众媒体上进行传播。研究人员将与黄石公园的工作人员合作，每年向400万游客提供公民教育。数据和结论将很容易地提供给公众、自然资源管理者和美国各地的研究人员。在世纪之交，黄石国家公园狼群的灭绝使麋鹿的数量增加到使小溪边的植被退化的程度。历史上丰富的柳树消失了，海狸筑坝也停止了。1995年，狼被重新引入黄石国家公园，导致公园里吃草的麋鹿数量急剧下降。这为了解食草动物如何影响植物群落创造了一个不寻常的机会。据推测，狼对麋鹿的影响通过减少对柳树的啃食导致了河岸群落的迅速恢复。研究小组进行了一项为期15年的操纵性实验来验证这一假设，并了解草食和水分供应在控制柳树生长中的作用。实验没有发现狼群通过释放被麋鹿吃掉的植物来改变生态系统的观点。相反，水的可用性控制了柳树的反应。实验表明，在北部地区，麋鹿数量减少对柳树没有影响，这可以解释为海狸没有筑坝，而海狸的筑坝活动可以增加柳树的水供应。而海狸则需要柳树作为食物和筑坝材料。柳树的零星恢复使海狸最近回到了几条小溪。在这些溪流上修建水坝，反过来，可能会加速柳树的恢复。另外，越来越多的野牛消耗柳树可能会与海狸竞争，并阻止它们维护水坝，从而阻止柳树的广泛恢复。研究人员将继续对柳树生长进行实验，并辅以对天然海狸坝对柳树的影响的观察，以了解狼、麋鹿、野牛和海狸的行为如何影响河岸生态系统的状态。将使用最先进的统计模型来分析数据。研究结果将对理解和管理全球半干旱草地景观中的河岸带具有重要意义。

The extirpation and subsequent reintroduction of the gray wolf to Yellowstone National Park has provided a rare opportunity to understand how predators can change ecosystems. Originally it was thought the loss of wolves would lead to a dramatic increase in the number of elk, which in turn would lead to substantial changes in the willows along the park's streams. It was expected that reintroduction of wolves would cause a rapid restoration of willow communities due declines in elk numbers. However, the first fifteen years of research indicates that the fate of the willow communities is complex, involving interactions among wolves, elk, bison, beavers and available water. This research will focus on how these complex interactions among predators, grazing animals, and local hydrology influence plants on landscapes. This knowledge forms the scientific foundation for managing rangelands throughout the western United States, and in so doing, enhances human economies throughout the region. The unfolding story of wolves in Yellowstone is of great public interest and will be communicated in the popular media. The researchers will collaborate with park staff in Yellowstone to offer citizen education to four million visitors annually. Data and conclusions will be made easily available to the public, to natural resource managers, and to researchers throughout the United States. The extirpation of wolves from Yellowstone National Park at the turn of the century allowed elk populations to increase to levels that degraded vegetation along small streams. Historically abundant willows were eliminated and dam building by beaver ceased. The reintroduction of wolves to Yellowstone National Park in 1995 caused a dramatic decline in the number of elk grazing on the park?s northern range, creating an unusual opportunity to understand how herbivores influence plant communities. It has been hypothesized that effects of wolves on elk caused a rapid restoration of riparian communities by reducing browsing on willows. The research team has conducted a 15-year, manipulative experiment to test this hypothesis and to understand the roles of herbivory and water availability in controlling willow growth. The experiment found little support for the idea that wolves changed the ecosystem by releasing plants from consumption by elk. Instead, availability of water controlled willow response. The experiment showed that the absence of effects of effects of reduced elk numbers on willows on the northern range is explained by the absence of dam building by beaver, an activity that enhances availability of water for willows. Beavers, in turn, need willows for food and dam building materials. Patchy recovery of willows has allowed beaver to return recently to a few small streams. Dams built on these streams, in turn, may accelerate the recovery of willows. Alternatively, consumption of willows by increasingly abundant bison populations may compete with beavers and prevent them from maintaining dams, preventing widespread willow recovery. Researchers will continue their experiment on willow growth, supplementing it with observations on effects of natural beaver dams on willows to understand how effects of wolves, elk, bison, and beavers act to determine the state of the riparian ecosystem. State-of-the-art statistical models will be used to analyze data. Results will have important implications for understanding and managing riparian zones embedded within semi-arid grassland landscapes worldwide.