Do Alternate Stable Community States Exist in the Gulf of Maine Rocky Intertidal Zone

最近有人提出，潮间带贻贝床和海藻冠层代表了缅因州海湾岩石海岸的交替社区状态。这个理论上有吸引力的想法假设，这些海岸线的栖息地是干扰补丁镶嵌为主的海藻或贻贝社区，并在一个给定的栖息地发生的社会是随机的，并依赖于原来的干扰和招聘的可用性的大小。据推测，大的扰动主要是由贻贝床和藤壶，具有广泛分散的幼虫，而较小的扰动主要是由海藻，具有更有限的扩散。正反馈建议，以保持这两个不同的社区类型的稳定性。伯特尼斯博士在缅因州中部一个潮汐河口的海岸上验证了这一假设。这些栖息地有利于探索这一想法，因为离散贻贝床和海藻冠层发生在附近，很容易操纵。结果不支持交替稳定状态假说。相反，他们认为贻贝床和海藻冠层的出现是高度确定性的。他们发现，贻贝床和海藻冠层迅速恢复到原来的群落类型，但只有在没有食草动物的情况下。海藻冠层占主导地位的栖息地很少水的运动，而贻贝床占主导地位的栖息地高流量。在没有食草动物的情况下，这些群落可以预见地迅速恢复到原来的状态，但有食草动物的存在，即使在两年后，两个群落都没有恢复的迹象。在这个项目中，Bertness博士和合作者将研究这些发现的鲁棒性和后果。特别是，它们将： I.继续监测试验性开垦地，以研究有食草动物存在的自然干扰的时间进程和轨迹，并研究消费者庇护所在恢复自然形成的贻贝床和海藻冠层方面的作用; 二.探讨潮间带贻贝和海藻群落可以代表交替群落状态的假设，但在中间流栖息地的范围较窄; 三.将研究结果的一般性扩展到沿海，波浪暴露的栖息地，最初发展出贻贝床和海藻冠层可以代表交替状态的想法。自然群落可以代表随机决定的交替稳定状态这一概念是一个启发思维的想法，对理解自然系统的结构和动态以及海洋和陆地生态系统的管理具有重要意义。尽管如此，在自然界中交替的稳定的共同体状态的存在本身是很难建立的。在这个项目中概述的工作将批判性地评估交替社区稳定状态假设在缅因州岩石潮间带的海湾，阐明机制的随机性和/或确定性的交替稳定状态在这个系统中，并建立它们最有可能发生的条件。

It has recently been suggested that intertidal mussel beds and seaweed canopies represent alternate community states on rocky shores in the Gulf of Maine. This theoretically appealing idea hypothesizes that these shoreline habitats are disturbance patch mosaics dominated by either seaweed or mussel communities, and that the community occurring in a given habitat is stochastic and dependent on the size of the original disturbance and recruit availability. Large disturbances are postulated to be dominated by mussel beds and barnacles which have widely dispersed larvae,while smaller disturbances are dominated by seaweeds, which have more limited dispersal. Positive feedbacks are proposed to maintain the stability of these two distinct community types. Dr. Bertness tested this hypothesis on the shores of a tidal estuary in central Maine. These habitats are conducive to exploring this idea because discrete mussel beds and seaweed canopies occur in close proximity and are easily manipulated. The results did not support the alternate stable state hypothesis. Instead, they suggest that the occurrence of mussel beds and seaweed canopies is highly deterministic. Largely independent of the size of artificial disturbances, they found that mussel beds and seaweed canopies rapidly returned to their original community type, but only inthe absence of grazers. Seaweed canopies dominate habitats with little water movement, whereas mussel beds dominate habitats with high flows. In the absence of grazers, these communities predictably and rapidly returned to their original state, but with grazers present, neither community showed signs of recovery even after two years. In this project Dr. Bertness and collaborators will examine the robustness and consequences of these findings. In particular, they will: I. Continue monitoring experimental clearings to examine the time course and trajectory of natural disturbances with grazers present and examine the role of consumer refuges in the recovery of naturally occurring mussel beds and seaweed canopies; II. Explore the hypothesis that intertidal mussel and seaweed communities can representalternate community states, but in a narrower range of intermediate flow habitats; and III. Extend the generality of the findings to coastal, wave-exposed habitats where the idea that mussel beds and seaweed canopies can represent alternate states was initially developed. The notion that natural communities can represent stochastically determined alternate stable states is an intellectually stimulating idea that has important implications for understanding the structure and dynamics of natural systems and for the management of marine and terrestrial ecosystems. Nonetheless, the very existence of alternate stable community states in nature has been difficult to establish. The work outlined in this project will critically evaluate the alternate community stable state hypothesis in the Gulf of Maine rocky intertidal zone, elucidate mechanisms underlying the stochastic and/or deterministic nature of alternate stable states in this system and establish the conditions where they are most likely to occur.