Community Responses to Experimental Nutrient Perturbations in Oligotrophic Lakes: The Importance of Bottlenecks in Size-Structured Populations

Abstract

Most aquatic metazoans exhibit major changes in size as they mature. Accordingly, they often use different foods and habitats, and are differentially vulnerable to various predators, competitors, and abiotic conditions as they grow. In short, small/young individuals may have very different ecologies than larger/older individuals in the same population. In Zooplankton, for example, larger individuals may be strongly limited by size-selective predation by fish planktivores (Brooks and Dodson 1965). Small organisms, on the other hand, are more susceptible to invertebrate predators (Zaret 1980) and possibly to resource limitation (Romanovsky and Feniova 1985; McCauley and Murdoch 1987). In consequence, the dynamics and evolution of various sizes/ages may be set by a complex array of ontogenetically changing environmental factors. Similarly, among fish, large individuals are often nearly invulnerable to piscivore attack, but may interfere with each other’s access to resources. On the other hand, small individuals may suffer heavy piscivore predation (Mittelbach 1981), resource shortages during “critical stages” of growth (Hjort 1926), and both exploitative and interference competition (Werner et al. 1983). At population and community levels, then, fitness limitation through various size-dependent processes may strongly influence patterns of relative success, the form and strength of ecological interactions, and potentially the flow of mass through the ecosystem.