An understanding of community structure should be based on evidence that the growth and regulation of the component populations in the community are affected in a predictable manner by natural physical disturbances and by interactions with other species in the community. This study presents an experimental evaluation of the effects of such disturbances and competitive interactions on populations of sessile organisms in the rocky intertidal community, for which space can be demonstrated to be the most important limiting resource. This research was carried out at eight stations on the Washington coastline which have been ranked according to an exposure/desiccation gradient and subjected to comparable manipulation and observation. Physical variables such as wave exposure, battering by drift logs, and desiccation have important effects on the distribution and abundance of many of the sessile species in the community. In particular, wave exposure and desiccation have a major influence on the distribution patterns of all the algae and of the anemone Anthopleura elegantissima. The probability of damage from drift logs is very high in areas where logs have accumulated along the intertidal. Log damage and wave exposure have complementary effects in the provision of free space in a mussel bed, as wave shock enlarges a patch created by log damage by wrenching the mussels from the substratum at the periphery of the bare patch. Competition for primary space results in clear dominance hierarchies, in which barnacles are dominant over algae. Among the barnacles, Balanus cariosus is dominant over both B. glandula and Chthamalus dalli; B. glandula is dominant over C. dalli. The mussel Mytilus californianus requires secondary space (certain algae, barnacles, or byssal threads) for larval settlement, but is capable of growing over all other sessile species and potentially is the competitive dominant of space in the community.

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Studies of the benthos between 30 and 60 m at Cape Armitage, McMurdo Sound. Antarctica, reveal an epifaunal community in which sponges and their asteroid and nudibranch predators predominate. Field experiments demonstrated that, with the exception of Mycale accrata, the growth rates of the sponges are too slow to measure in one year. Mycale, however, was observed to increase its mass as much as 67%. Because of its more rapid growth rate, Mycale appears to be the potential dominant in competition for substratum space, the resource potentially limiting to the sessile species. This conclusion is supported by observations of Mycale growing over and, in some cases, apparently having smothered many other sessile species representing at least three phyla. The densities and size frequency distributions of all the predators were measured; numerous feeding observations allowed an accurate appraisal of dietary compositions. Because of the predators' very slow consumption rates, however, direct measures of ingestion and its impact on prey populations were not possible. Estimates of the ingestion rates were derived from measurements of predator respiration rates, growth rates, and gonad growth. Data from the field surveys and the energetics studies suggest that Mycale is prevented from dominating the space resource by the predation of two asteroids. Perknaster fuscus antarcticus and Acodontaster conspicuus. Adult Perknaster specialize on Mycale, and the sponge provides a small proportion of the diet of A. conspicuus. Acodontaster conspicuus and the dorid nudibranch Austrodoris memurdensis are the most important predators on three species of rossellid sponges (Rossella racovitzae, R. nuda, and Scolymastra joubini). Despite this relatively heavy consumption and despite the fact that none of these sponges has a refuge in growth from potential mortality from A. conspicuus, very large standing crops of the rossellid sponges have accumulated. This accumulation appears to result from predation on larval and young A. conspicuus and Austrodoris by Odontaster validus, which is primarily a detrital feeder and apparently acts as a filter against the settlement and survival of the A. conspicuus and Austrodoris larvae. In addition, predation upon adult A. conspicuus by O. validus and the actinian urticinopsis antarcticus annually kills approximately 3.5% of the A. conspicuus population. This mortality exceeds the apparent rate at which A. conspicuus escape the larval filter.

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This paper addresses questions of community and patch stability as defined by the population biology of dominant plants in the context of different areas within a large kelp forest. We ask ( 1) "Do large-scale episodic events override biological mechanisms as major community structuring processes?", (2) "Are different local areas characterized by different processes?", and (3) "How persistent are the patches or biological structure over decadal and local spatial scales?" We evaluate these questions with regard to the effects of various types of disturbance for as much as three decades on the populations of several species of kelp in the large kelp forest off Point Lorna, San Diego, California. The most sensitive population factors we studied include recruitment, density, and survivorship. Patch stability was evaluated with regard to the persistence of patches already well established in 1971-1972. The study sites offer a cross-shore transect through the central part of a large kelp forest at depths of8, 12, 15, 18, and 21m; two additional sites at the north and south ends of the forest offer a longshore transect along the 18-m contour.There were marked differences among the decades with regard to the intensity of the disturbances. Compared with the 1980s, the two preceding decades were relatively benign. The 1980s had two extreme disturbance events: the 1982-1984 El Nino-Southern Oscillation (ENSO) was the most severe El Nino event in the last century, which included very warm, nutrient-depleted water, and a short but intense storm in January 1988 appeared to have been the most severe in perhaps 200 yr. The storm changed age-specific kelp mortality patterns and caused the first large-scale understory mortality in several decades. By sweeping away drift algae it caused intense local urchin grazing. The storm was followed by a strong La Niiia event marked by cool, nutrient-rich water in 1988-1989. Differences in kelp recruitment and survivorship in different areas of the kelp forest are influenced by gradients in longshore currents, temperature, light, wave energy, floc, planktonic propagules, and physical disturbance. The areas are characterized by different plant population patterns and the effects of several species of herbivores. The massive disturbances of the 1980s obliterated much of the structure in the kelp forest. Certainly the disturbances caused many lag effects including outbreaks of understory algae such as Desmarestia ligulata, intraspecific competition, changes in grazing patterns, etc., which in tum resulted in between-area variation in recovery rates. However, in all cases this variation was overshadowed by the overwhelming competitive dominance of Macrocystis pyrifera. Most of the understory patches on the transect lines, some of which had persisted for 7 yr, died out by the end of 1990.The population biology of Macrocystis was remarkably similar in most areas, as the cohort longevity and survivorship curves were very similar, and the plant and stipe densities tended to level off in only a few ...