Competition and Predation: Herring Gulls versus Laughing Gulls

Abstract

Here I overview the relative importance of competition and predation in structuring zooplankton communities and populations, and examine how this changes from habitats with high densities of planktivorous fish to those where fish are absent. I first examine the attributes of exploitation and interference competition and food limitation. Then, I compare size-selective and gape-limited predation by visually feeding planktivorous fish and invertebrate predators, respectively. Next, I seek the reasons why both competition and predation are density-dependent and highly size-selective, causing each to have a considerable impact on (1) community patterns (from a diverse multispecific community to a single species monopolizing resources), (2) population density and its age-structure/size-distribution patterns (from a low-density population of age-diverse and short-lifespan individuals to a cohort of long-lived individuals with synchronized growth and reproduction), and (3) individual morphology, life history, and behavior (from small-bodied individuals with early maturation and apparent antipredation behavior, to large-bodied individuals with large per-offspring investment and reproduction postponed until available food levels allow juvenile survival). Finally, I discuss the disparity between competition and predation, and the reasons why the population density of a dominant zooplankton species may be the same in habitats with different productivity potential, food levels, and reproductive rates.