Consequences of density-dependent heterotrophic feeding for a partial autotroph

Population regulation in sessile clonal animals has been poorly studied. The prevailing paradigm is that competition is primarily for space, although this is changing. I use a laboratory flume experiment to show that particle capture by the marine intertidal zoanthid Palythoa caesia is negatively related to density (% cover) and positively related to colony size. Using models of colony growth and subsequent population dynamics, which incorporate this density- and size-dependent feeding response, I then show that it is potentially an important component of population regulation. The other important vital rate for regulating population size is fission - as this species is clonal it can reproduce asexually by splitting into 2 or more daughter colonies. Fission rates decrease with density and increase with colony size. When both feeding and fission are density-dependent, model cover is regulated to just under 80%, but when either process is made density-independent, population growth is exponential. Importantly, when no heterotrophic feeding is allowed, and all respiratory requirements must come from the translocation of photosynthetic products from symbiotic zooxanthellae, the model population rapidly becomes extinct, whereas eliminating photosynthesis decreases cover but allows the population to persist. The colony level model shows that individual growth rates of large colonies are increased at low densities, while those of small colonies are decreased at high densities.