In previous sections in this book, the emphasis has been on the impact of physical factors on the distribution of plants and animals on the shore. While these factors set the framework and define the limits over which the various life cycle stages of a particular species can exist, the patterns of distribution are subject to modification by a complex of interacting biological factors. Early studies (for example, reviews by Lewis, 1964; Stephenson and Stephenson, 1972) were oversimplifications relating distributions to tidal rise and fall and wave exposure, and much subsequent research has shown that interactions among species can profoundly modify distribution patterns, and often determine these patterns (see reviews by Dayton, 1971; 1984; Connell, 1972; 1975; 1983; 1985; 1986; Underwood, 1979; 1985; 1991; 1992; 1994; Underwood and Denley, 1990).