Temporal Stability as a Density-Dependent Species Characteristic

Abstract

Accumulated annual totals of adults (Nt) of 97 spp. of aphids and 263 spp. of moths were measured over at least 6 yr, at 18 and 53 sites, respectively, in Great Britain [UK] and adjacent mainland Europe. Annual variance .**GRAPHIC**. of Nt is here taken as a measure of the stability of the population density of a species at a site (Nt .varies. mt). Stability was a systematic non-linear function of mean population density (mt) at the different sites over the whole area sampled (more than 2000 km2). Just as spatial stability .**GRAPHIC**. is a power function of mean population density over an area at all times (ms), so temporal stability is a power function of mean population density .**GRAPHIC**. over time at all places (log .**GRAPHIC**. = log a + b log mt). Log mean density accounts for a very large proportion of log temporal stability (more than 90% in most species); the parameter b is highly specific and therefore largely independent of the local environmental variables, including parasites, predators and available resources, which differ between sites. Unlike mean density, stability may not be a function of extrinsic mechanisms, which must differ widely and unsystematically in space and time, but is an intrinsic property of the behavior of the species. The population density is estimated from flying adults and the problems of obtaining adequate density estimates for this kind of investigation are briefly considered.