COMPETITIVE RELATIONSHIPS AND COEXISTENCE IN A GUILD OF CRUSTOSE ALGAE IN THE EULITTORAL ZONE, CAPE-PROVINCE, SOUTH-AFRICA

A number of crustose, fleshy and coralline algae coexist in the eulitt oral zone along the south-western coast of South Africa, in spite of p otentially competing tor the same limiting resource, space. A number o f questions concerning coexistence among crustose algae were addressed in a study conducted at Holbaaipunt, in the south-western Cape Provin ce. To some degree, the crustose algae occur in different zones, and t his may thus reduce competition. It was shown that non-transitive netw orks did not exist among the crustose algae at this site, so do not co ntribute to the coexistence of these crustose algae. While most specie s occur on rock substratum, one species occurs primarily on worm tubes and compacted calcareous material, suggesting that habitat heterogene ity within zones may contribute to the coexistence of these crustose s pecies. Differential susceptibility to, and recovery from, disturbance may also contribute to coexistence. The thinnest species in the mid-l ower eulittoral zone at this site, Spongites yendoi (Foslie) Chamberla in, dominates the space in spite of its position near the bottom of th e overgrowth hierarchy. This may be because its thin thallus slows fas ter lateral growth and less susceptibility to disturbance than its thi cker competitors. Thallus thinness may be maintained by the periodic d eep-layer sloughing of a surface layer. as well as by heavy grazing by Patella cochlear Born.