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

• Published in: Oikos 1983-01, Vol.41 (2), p.255-273
• Main Authors: Haila, Yrjö, Järvinen, Olli
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Animals; Autoecology; Aves; Aviculture; Biological and medical sciences; Birds; Breeding; Coniferous forests; Deciduous forests; Ecological competition; Forest habitats; Fundamental and applied biological sciences. Psychology; Habitats; Trees; Wildlife habitats
• ISSN: 0030-1299; 1600-0706
• DOI: 10.2307/3544273

The breeding land birds (about 65 spp. annually) of the island Ulversö (5.8 km2) were compared with the avifauna of Main Åland (970 km2, distance 10 km). About 54% of Åland's 121 species breed annually on Ulversö. Census data from different habitats were used in order to assess the importance of various factors leading to faunal impoverishment; misleading conclusions follow from presence-absence data. Many rare species are absent from Ulversö, but they would probably be absent from comparable areas on Åland, 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. In the long run, many rare species would be expected to breed on Ulversö by chance, but suitable habitats are lacking; thus we estimate that the long-term effect of habitat impoverishment explains 48% of the absences. Interspecific interactions (competition, predation) seem important in three cases, and the remaining four absences were traced to effects of dispersal history, or they remained obscure. Six corresponding habitats were censused on Ulversö and Main Åland. 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 between Ulversö and Main Åland. We attribute apparent cases of density compensation and habitat shifts 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 Ulversö were compared with expectations derived from the habitat censuses made on Main Åland. Two fifths of the bird community of Ulversö consist of species that are equally abundant on Ulversö and Main Åland. Another two fifths, mainly species favoured by the mosaic-like or bushy habitats of Ulversö, are more abundant on Ulversö than on Main Åland. Our analysis leads to the conclusion that many typical features of insular communities can be traced to autecological responses (e.g., those due to differences in habitat structure). /// Гнездукщиеся сухопутные птицы (около 65 видов ежегодно) на острове Ульверзë (5,8 к m2) сравнивались с авиафауной Больших Аландов (970 к m2, удаленность 10 км). Около 54% из 121 вида аландских птиц гнездуются ежегодно в Ульверзë. Данные переписи из разных местообитаний использованы для оценки знеачения разных факт