Finnish bird fauna-species dynamics and adaptive constraints

Abstract

The adaptive response of Finnish birds to the limits imposed by species dynamics, biological constraints, and environment is described by examining the patterns and underlying mechanisms in the relationships between the number of species, body size, population density, and geographical range size . In the total breeding avifauna the frequency distribution of species with respect to body size was bimodal, while various subsets of species showed unimodal distributions significantly skewed toward the larger body size categories. Abundance distributions of larger sets of species usually followed the canonical lognormal distribution with no significant skewness, while minor groups in general were not normally distributed and exhibited significant leftskewness . The variances of abundance were somewhat higher than the variances of biomass, suggesting that biomass and energy use were more equitably distributed among species than indicated by their abundance. Relatively more large than small species winter in Finland. So, from the energetic point of view, the differences between the summer and winter bird communities were not as great as suggested on the basis of the number of individuals. The average density of birds decreased significantly with increasing body size, but the slope of the relationship was shallower (-0.52) than expected for the energetic equivalence of the species . The Finnish range size of species tended to decrease with increasing body size, partly because the ranges of many larger species have been reduced by human impact. The width of the range increased with density in the total set of species as well as in most of the ecological groups considered, indicating the progress and success of colonization .