Community Interactions on Marine Rocky Intertidal Shores

"The ultimate goal of an ecological survey should be to discover and measure the main dynamic relations between all organisms living on an area over some period of time" (Elton 31 ). This is a tall order but it describes the aim of many "community" ecologists pretty well. Most studies have not got past the first stage of identifying and describing the community, because of the very great difficulties of sampling and taxonomy (Fager 32). Even then, direct measurement of dynamic relations is difficult. Recourse is sometimes made to fitting available data, such as the abundances of the different species, to various mathematical distributions. If the data fit a particular model, the underlying assumptions of the model are then presumed to represent the mechanisms which produced the abundances. However, where it is possible to measure directly the dynamic relations between organisms and their physical and biological environment, this is to be preferred to such indirect methods. Such direct measurement is difficult in most habitats, but it is possible in a few. The marine rocky intertidal is one of these. Because it is so accessible, it has been studied for a long time and the species are well known. Because many of the common species are sessile or slow moving and not hidden in the substrate, their abundance and other population characteristics can be estimated readily. Lastly, the populations are amenable to experimental manipulation in the field; their abundance and distribution can be altered in one place and left unchanged in another nearby. This last characteristic is particularly important. To measure dynamic relations between organisms one must either look for correlations between changes in the abundances and distributions of the coexisting organisms, or else make the changes experimentally. The last is usually better because replicate treatments and undisturbed controls can be set up at the same time (Connell 22). Most ideas and models about how organisms live together, i.e. about "ccommunity structure," are based upon little, if any, direct evidence. For example, a common theme is that the "niche spaces" of species have been determined by the evolution of adaptations to reduce competition. If this were so, one would expect to see competition occurring where the ranges of two