Abstract

Competitors, consumers, and environmental stresses are everpresent factors that most organisms face during their lifetime. On the Pacific northwest coast of North America there are three forms of coralline algae (Rhodophyta, fam. Corallinaceae) differentially distributed within and around large tide pools in the mid and low intertidal region. Observations and experiments were conducted to test whether species of three different forms (finely branched, coarsely branched, and encrusting forms) were differentially susceptible to consumers, competitors and environmental stress. The results indicate that the distributions of these three forms are distinctly different. Although no single factor appears to be controlling this zonation during the time scale of this study, the three forms are differentially susceptible to these potential selective agents. The finely branched coralline ( Corallina vancouveriensis Yendo), found immediately outside of the pools on emergent substrata, is the most resistant to desiccation. This is apparently because its bushy structure holds more water than the other forms preventing drying during times of emersion. The coarsely branched coralline, Calliarthron tuberculosum (Post. & Rupr.), is the most resistant to consumers; it was not eaten by any of four moliuscan herbivores tested. Again it appears to be the structure of the plant that provides this protection. When this species is ground and placed in agar, it is readily eaten by all four herbivores. The encrusting corallines (approximately six species) appear to be the best spatial competitors, occupying the majority of the space within the pools, and were the winners of all overgrowth encounters with articulated coralline species.