Variation in individual growth rates and population densities of Ephemerellid mayflies

Individual growth and densities of six taxa of ephemerellid mayflies were examined inrelation to differences in temperature and food among streams of western North America. For mosttaxa, growth rate was not a simple function of either temperature or food. Growth periods of taxadiffered relative to seasonal changes in temperature. As a result of the interaction between size andtemperature, growth rates and shapes of the growth curve varied among taxa. The significance oftemperature for life history phenomena therefore cannot be easily generalized to explain phenologicalpatterns among stream insects. Also, little evidence was found that implicated food as the cause forobserved differences in growth rates among most sites. Growth rates in streams with high rates ofalgal production (open sites) were similar to growth rates in streams with low algal production (shadedsites). Only sites with long periods of ice cover, and presumably low availability of food, showedmarked reduction in individual growth rates. Densities, however, varied strongly across sites: openstreams had higher densities than shaded streams. These results imply that populations in streamsmay be near carrying capacity and that per capita food availability is similar among streams. Thepresence of such interactions between individual and population processes may help explain patternsat individual, population, and ecosystem levels of organization.
Key words: aquatic insects; Cascade Mountains, Oregon; densities; Ephemerellidae; food; growthrates; life histories; mayflies; streams; temperature.