Community variability increases with time

We examine the variability of British woodland bird communities in terms of relative abundances, using data obtained in the Common Bird Census 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 and for shorter subsets to examine the effects of temporal scale on community variability (measured as Kendall's coefficient of concordance, W). Community variability increased the longer the communities were observed, indicating that comparisons of variability between different communities or taxa have to take temporal scale into account. We suggest that the increased community variability with time is likely to be related to increased environmental and population variability with time, long-term trends in a number of species, and habitat succession. Community variability was not significantly related to geographical location, plot area, species richness in the community, the proportion of long-distance migrants in the community, two measures of body mass of the 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 that the degree of bird community variability is related to habitat stability.