Geographical gradients of stability in European land bird communities.

European data from 15 long-term censuses of breeding land bird communities were analysed in order to measure community stability. Stability was here operationally defined as year-to-year persistence of community structure (total density, number of species, diversity, evenness of the species-abundance distribution, species list, and frequencies of species). Central Europe and southern Scandinavia formed a zone of fairly stable communities, but more northern communities were fairly unstable. This result contrasts with several recent arguments, but accords well with the classic dogma that instability of biological communities increases northwards in the northern hemisphere.Three theoretical explanations for the difference in stability between northern and southern communities were explored, and causal mechanisms underlying the hypotheses were specified and tested. Considerable evidence supported the hypothesis that environmental (climatic) unpredictability increases northwards and mainly causes the phenomenon observed. Gradients in productivity may, perhaps in interaction with unpredictability, also contribute to geographical differences in stability, but tests suggested that low diversity is not an important cause of northern instability.