COMMUNITY VARIABILITY INCREASES WITH TIME

We examine the variability of British woodland bird communities in ter ms of relative abundances, using data obtained in the Common Bird Cens us from 1971 to 1992. Eighteen woodland plots with long-term data were analysed. The analyses were made for the whole time period of 22 yr a nd for shorter subsets to examine the effects of temporal scale on com munity variability (measured as Kendall's coefficient of concordance, W). Community variability increased the longer the communities were ob served, indicating that comparisons of variability between different c ommunities or taxa have to take temporal scale into account. We sugges t that the increased community variability with time is likely to be r elated to increased environmental and population variability with time , long-term trends in a number of species, and habitat succession. Com munity variability was not significantly related to geographical locat ion, plot area, species richness in the community, the proportion of l ong-distance migrants in the community, two measures of body mass of t he community, generation time, or the proportion of species with long- term national trends. The local bird communities in scrub forest plots changed more over time than those in deciduous forest, indicating tha t the degree of bird community variability is related to habitat stabi lity.