Abstract：
Ecological theory predicts that kelp forests structured by trophic cascades should experience recovery and persistence of their foundation species when herbivores become rare. Yet, climate change may be altering the outcomes of top-down forcing in kelp forests, especially those located in regions that have rapidly warmed in recent decades, such as the Gulf of Maine. Here, using data collected annually from 30+ sites spanning >350 km of coastline, we explored the dynamics of Maine\'s kelp forests in the ~20 years after a fishery-induced elimination of sea urchin herbivores. Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20th century, we found that forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest. Forests collapsed in the southwest apparently because ocean warming has-directly and indirectly-made this area inhospitable to kelp. Indeed, when modeling drivers of change using causal techniques from econometrics, we discovered that unusually high summer seawater temperatures the year prior, unusually high spring seawater temperatures, and high sea urchin densities each negatively impacted kelp abundance. Furthermore, the relative power and absolute impact of these drivers varied geographically. Our findings reveal that ocean warming is redefining the outcomes of top-down forcing in this system, whereby herbivore removal no longer predictably leads to a sustained dominance of foundational kelps but instead has led to a waning dominance (northeast) or the rise of a novel phase state defined by \"turf\" algae (southwest). Such findings indicate that limiting climate change and managing for low herbivore abundances will be essential for preventing further loss of the vast forests that still exist in northeast Maine. They also more broadly highlight that climate change is \"rewriting the rules\" of nature, and thus that ecological theory and practice must be revised to account for shifting species and processes.
摘要：
生态理论预测，当草食动物变得稀有时，由营养级联构成的海带森林应该经历其基础物种的恢复和持久性。然而，气候变化可能正在改变海带森林自上而下强迫的结果，尤其是那些位于近几十年来迅速变暖的地区，比如缅因湾.这里,使用每年从超过350公里海岸线的30多个地点收集的数据，在渔业导致的海胆草食动物消灭后的20年中，我们探索了缅因州海带森林的动态。尽管森林（Saccharinalatissima和Laminariadigital）在20世纪后期已广泛返回缅因州，我们发现缅因州东北部的森林已经经历了缓慢但显著的海带下降，东北部的森林持久性被一个快速的、在西南部广泛的崩溃。西南部的森林倒塌显然是因为海洋变暖直接和间接地使该地区不适合海带。的确,当使用计量经济学的因果技术对变化的驱动因素进行建模时，我们发现前一年夏季海水温度异常高，异常高的春季海水温度，和高海胆密度分别对海带的丰度产生负面影响。此外，这些驱动因素的相对力量和绝对影响在地理上各不相同。我们的发现表明，海洋变暖正在重新定义该系统中自上而下强迫的结果，因此，草食动物的清除不再可预见地导致基础海带的持续优势，而是导致优势减弱（东北）或由“草皮”藻类（西南）定义的新阶段状态的上升。这些发现表明，限制气候变化和管理低食草动物丰度对于防止缅因州东北部仍然存在的广阔森林的进一步丧失至关重要。他们还更广泛地强调，气候变化正在“改写自然规则”，因此，必须对生态理论和实践进行修订，以解决物种和过程的变化。
