Dynamics of foundation species across estuarine environmental stress gradients.

摘要

I conducted the first study of the impact of hypoxia (oxygen depletion of waters) on the biological community of Narragansett Bay, and my general findings offer new perspectives on this form of anthropogenic habitat degradation that has reached worldwide epidemic proportions. Rapid mass-mortality and local extinction of mussels (Mytilus edulis) due to hypoxia had catastrophic consequences for the ecosystem, because mussels provide (1) a food resource for many other species, (2) a 3-D living reef structure, and (3) filtration that can control the effects of eutrophication and subsequent hypoxia. Transplant experiments revealed that the tolerance of mussels can vary by 50% depending on prior exposure to low oxygen, but remains quite low. Local extinction of mussels reflected a broader pattern of community collapse and loss of diversity of bivalves (an economically and ecologically important group). However, the quahog (Mercenaria mercenaria ) is the one bivalve that can persist in the most hypoxic sites, and benefits from low-oxygen because predators are either excluded or rendered ineffective by the stress. On intertidal cobble beaches, I have examined how another bivalve, the ribbed mussel (Geukensia demissa), alleviates the substrate instability and solar stress that limits the distribution and abundance of many invertebrates and algae, but is itself dependent on facilitation by cordgrass (Spartina alterniflora). This dependence of one facilitator on another is the first explicit example of a facilitation cascade, and represents a subset of the more general hierarchical organization perspective in which secondary interactions (i.e. facilitation, competition, predation) occur within a community that is dependent on the facilitation of a principle foundations species (cordgrass).