The environment of an organism includes both biotic and abiotic (physiochemical) factors. Communities of marine organisms encompass not only the seaweed communities but also the animal communities, of which the benthic grazers and their predators are most important to seaweed ecology. Thus, the biotic interactions of seaweeds include not only competition with other seaweeds (both within and between species) and with sessile animals but also predator–prey relations at several trophic levels, and facilitation; the mix of such interactions will change as the individual changes with age and environmental history.

Biotic interactions are complex, and their study often requires large-scale and long-term observations and manipulations in the laboratory, as well as in the field. Interactions can be positive (e.g. facilitation, mutualism, and commensalism), negative (e.g. competitive exclusion, consumption) or neutral, where there is no effect of one species on another. Studies on biotic interactions in the marine environment have traditionally focused on competition but more recently facilitation has been recognized as an important way in which biota interact. The minireviews of Olson and Lubchenco (1990), Carpenter (1990), Paine (1990), and Maggs and Cheney (1990) remain useful frameworks, as are the more recent syntheses found within Marine Community Ecology (Bertness et al. 2001) and Marine Ecology (Connell and Gillanders 2007).