Reversed latitudinal gradients in total density and species richness of birds breeding on Finnish mires

Abstract

Latitudinal gradients in the total density and species richness of breeding mire birds were studied on the basis of 72 censuses made on flark fens in middle and northern parts of Finland. Flark fens represent an ornithologically important but common mire type, which support fairly high densities of many species. It has been shown previously that the average density and species richness of birds breeding on Finnish mires increase towards the north. It is shown here that these "reversed" gradients cannot be attributed to size variation in mire areas studied in different regions. Even after the effect of area on density is removed, species richness increases northwards in samples including the same number of pairs (90 pairs; about 0.9 species more per 100 km north) or areas of the same size (1 km2; about 1 .3 spp. more per 100 Ian north) . The northward increase in total density is statistically significant, but largely obscured by the considerable heterogeneity of densities among mires in the same geographical area . Differences in the proportion of major habitats probably contribute markedly to the northward increasing densities. In particular, flark fens are common in the north (about one-half of the area of treeless mires) but less so in the south (one-fifth or even onetenth of the area).