Variation in Individual Growth Rates and Population Densities of Ephemerellid Mayflies

Abstract

Individual growth and densities of six taxa of ephemerellid mayflies were examined in relation to differences in temperature and food among streams of western North America. For most taxa, growth rate was not a simple function of either temperature or food. Growth periods of taxa differed relative to seasonal changes in temperature. As a result of the interaction between size and temperature, growth rates and shapes of the growth curve varied among taxa. The significance of temperature for life history phenomena therefore cannot be easily generalized to explain phenological patterns among stream insects. Also, little evidence was found that implicated food as the cause for observed differences in growth rates among most sites. Growth rates in streams with high rates of algal production (open sites) were similar to growth rates in streams with low algal production (shaded sites). Only sites with long periods of ice cover, and presumably low availability of food, showed marked reduction in individual growth rates. Densities, however, varied strongly across sites: open streams had higher densities than shaded streams. These results imply that populations in streams may be near carrying capacity and that per capita food availability is similar among streams. The presence of such interactions between individual and population processes may help explain patterns at individual, population, and ecosystem levels of organization.