Land Bird Communities on a Finnish Island: Species Impoverishment and Abundance Patterns

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Abstract
The breeding land birds (about 65 spp. annually) of the island Ulverso (5.8 km2) were compared with the avifauna of Main Aland (970 km2, distance 10 km). About 54% of .ALPHA.land''s 121 spp. breed annually on Ulverso. Census data from different habitats were used to assess the importance of various factors leading to faunal impoverishment; misleading conclusions follow from presence-absence data. Many rare species are absent from Ulverso, but they would probably be absent from comparable areas on Aland, too, as shown by rarefaction and other probabilistic techniques. The statistical effect of rarity accounts for 75% of the absences. Lack of suitable habitats explains 12% of the absences. Many rare species would be expected to breed on Ulverso by chance, but suitable habitats are lacking; it is estimated that the long-term effect of habitat impoverishment explains 48% of the absences. Interspecific interactions (competition, predation) seem important in 3 cases and the remaining 4 absences were traced to effects of dispersal history or they remained obscure. Six corresponding habitats were censused on Ulverso and Main Aland. The observed differences in species richness, total density, biomass and the representation of different feeding guilds in different communities tended to agree with differences in the details of habitat structure betwen Ulverso and Main Aland. Apparent cases of density compensation and habitat shifts are attributed to details of habitat structure, such as differences in average tree height and average density of the bush layer. Population numbers of single species on Ulverso were compared with expectations derived from the habitat censuses made on Main Aland. Of the bird community of Ulverso, 2/5 consists of species that are equally abundant on Ulverso and Main Aland. Another 2/5, mainly species favored by the mosaic-like or bushy habitats of Ulverso, are more abundant on Ulverso than on Main Aland. Many typical features of insular communities can be traced to autecological responses (e.g., those due to differences in habitat structure).