The population dynamics of aphids on fire weed: a comparison of local populations and metapopulations

Abstract

Local populations of four species of aphids (Macrosiphum Valerianae, Aphis varions, A. helianthi, and A. salicariae) occur on shoots of fireweed (Epilobium angustifolium) in the Rocky Mountains of Colorado, U.S.A. Through exhaustive and repeated nondestructive sampling of the local populations on over 3800 shoots of fireweed, information was obtained on the dynamics of both the local populations and the metapopulation of each species. The dynamics of the metapopulation were analyzed in terms of the frequency of shoots occupied by aphids and the median number of aphids per occupied shoot. There were significant differences between species in both parameters, but high densities were not necessarily accompanied by high frequencies. Local populations were initiated throughout the summer, and there was marked variation in the duration and size of these populations. Most populations lasted only a few weeks, but others lasted up to 14 weeks. This turnover of local populations within the metapopulation implies that local populations must be studied in order to understand the dynamics of the metapopulation. There is a discussion of the relationships between population processes occurring at different levels of spatial heterogeneity.