Population dynamics of a damselfish: Effects of a competitor that also is an indirect mutualist

Abstract

We estimated the population consequences to an inferior competitor from interactions with a superior competitor that also is an indirect mutualist. The competitors are two damselfishes, three-spot dascyllus (Dascyllus trimaculatus) and orange-fin anemonefish (Amphiprion chrysopterus), that vie for shelter space on a sea anemone (Heteractis magnified). Anemonefish have an indirect positive effect on dascyllus populations due to the faster growth that anemones experience when they are occupied by Amphiprion. A long-term experiment revealed that anemonefish depressed dascyllus density (number of individuals per area of anemone per area of reef) more than abundance (number of individuals per reef area) because anemonefish caused a greater net increase in anemone surface area. We calculated that the effect of competition alone would depress dascyllus populations to near zero. The indirect positive interaction compensated for ∼67% of the direct negative effect. Failure to consider the indirect positive interaction in this system would lead to a gross underestimate of the strength of competition, and would not reveal the critical importance of the indirect mutualism in maintaining local populations of the inferior competitor. Indirect mutualisms are common in nature, and assessing how they mediate effects of other processes is crucial to a more accurate understanding of natural communities.