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Scaling of ecological dominance and resilience in kelp communities PAUL K. DAYTON, ED PARNELL, AND JAMES LEICHTER Scripps Institution of Oceanography, La Jolla, CA 92093-0227 What are the important environmental factors determining resilience in kelp ecosystems and how do they exert their influence? Patterns and processes vary in space and time; often resilience reflects ecological mediation of the physical and biological processes. The Macrocystis kelp forest at Point Loma, San Diego, California exhibited distinct kelp patch structure that persisted from the 1960s until the late 1990s, resisting invasion and severe perturbations including massive storms and El Niños. Under natural conditions important animals including several species of fish, sea urchins, abalones as well as sessile invertebrates also persisted. This resistance stability largely resulted from interspecific competition, dispersal, recruitment, and intensity of predation. Eventually a strong La Niña brought sufficient nutrients to the Macrocystis surface canopy that the canopy became dense enough to eliminate all the understory patch structure through strong shading. As elsewhere, kelps do not resist grazing by sea urchins in large fronts, the formation of which reflect available kelp detrital food and reduced predation by several urchin carnivores. When released from predation, amphipods can denude Macrocystis, and heavy settlement of epiphytic organisms can sink or stress Macrocystis, suggesting stabilizing roles of small canopy fish predators. What are the environmental processes that cause these patterns? Physical processes are important at all scales as they transport both nutrients and propagules. At smaller scales the kelps themselves affect the flow rates around the plants. This is true at the scale of individual Macrocystis plants such that flows may move sharply upwards, resulting in an important dispersal process for the kelp spores. As with Macrocystis, kelps with floating or stipe-supported canopies such as Pterygophora, Eisenia, Laminaria spp., Ecklonia, Lessonia, may interfere with flow, which can result in strong edge effects. The reduction of flow at the scales of the forest edge inhibits nutrient transport and dispersal of propagules into the larger kelp forests. At these scales the most important forcing functions may be breaking internal waves and wind. The former may force deep nutrient rich water and propagules much farther into the forest whereas the latter breaks down the edge effect at the canopy and results in surface water flowing across the top of the forest, thereby potentially advecting surface larvae into the forest. The availability of light limits kelp growth and distribution. At small scales it limits germination and growth, and it is one of the most important factors limiting small-scale distribution and abundance. How general are these patterns and processes over larger spatial scales? In areas with warmer nutrient poor water the surface canopy can be thin resulting in welldeveloped understory canopies deriving nutrients from breaking internal waves. In areas with extreme wave exposure, nutrients and light are less likely to be limiting. The understory canopies are more tolerant to wave exposure and become dominant. Thus resilience of kelp community structure depends upon shifting patterns of ecological dominance, which can reflect several environmental forcing functions. In addition, it also depends upon cascading patterns of predation on the kelps, their epiphytes, and sea urchins. Our understanding of this resilience depends upon synthesis of both ecological succession and resistance to invasion. What of the future? The natural distribution and abundance of most of the associated animals reflects topographical bottom features and biota other than kelps themselves. Anticipated global warming effects may lower the thermocline: but all other things being equal this will reduce the nutrients in the surface waters and thin the Macrocystis canopy. Physical processes such as wind forcing, upwelling and internal waves will continue although perhaps at somewhat different levels; this probably implies that the understory kelps and the rest of the communities may not be much affected by the projected warming. Unfortunately, the mortality related to fishing is so severe, that natural patterns are a myth: most kelp forests have lost most of the functionally important species including most large predators and species such as abalones and many other gastropods, holothurians, and now even sea urchins as a result of fishing. In many cases other predators, especially crustacean and small fish acquire community roles that they did not have before the kelp habitat was so badly overfished.