SPATIAL COMPETITION IN MARINE HARD SUBSTRATE COMMUNITIES: AN OVERVIEW

Competition for living space is typically more intense than competition for other resources, such as food or nutrients, for sessile epibenthos inhabiting hard substrates (reefs, hardgrounds, rocky intertidal, shells); accordingly, a number of strategies have evolved for successfully competing for space in these settings. This presentation will emphasize spatial competition occurring on modern coral reefs, but will include examples from other modern marine environments and the fossil record as well. Competitive strategies commonly employed by a number of taxa include: 1) pre-emptive settlement (arriving first in open space), 2) rapid growth, resulting in direct overgrowths, shading or bulldozing and 3) the use of allelochemicals (toxins) to prevent larval settlement and/or maintain or capture living space. In addition, cnidarians have developed a variety of exotic competitive mechanisms, such as extracoelenteric digestion and the use of acrorhagi, sweeper tentacles and oversized (“sweeper”) polyps. Solitary organisms (bivalves, sedentary polychaetes) are often inferior competitors against clonal organisms (sponges, corals, bryozoans). Thus, while solitary organisms may be important colonizers on newly available substrates, they tend to be quickly replaced by clonal organisms. Organisms that secrete calcareous skeletons (corals, coralline and calcareous algae) are typically at a disadvantage when competing against more rapidly growing, nonskeletonized taxa, such as many algae. Although competitive interactions for space occurring between skeletonized epibenthos may be one of the few commonly preserved examples of direct (interference) competition, interactions between skeletonized (coral) and nonskeletonized organisms (fleshy algae) may leave little fossil evidence. Therefore, the phase shift from coral and calcareous algae dominated to soft, frondose and filamentous algae dominated systems currently taking place on many modern coral reefs may not appear in the fossil record to be the result of direct competition between the two groups.