Land Birds on Northern Islands: A Sampling Metaphor for Insular Colonization

Abstract

Land birds were censused on 44 islands (size range 0.5-582 ha) in the archipelago of Aland, southwest Finland, in 2 (or 3) breeding seasons. Species-abundance relations in the data of the 7 island type groups, defined on the basis of habitat composition, show statistically significant deviations from each other as assessed by rarefaction and by fitting Fisher''s logseries model to the data. Within each group the distributions were similar irrespective of sample size. A sampling metaphor for insular colonization is elaborated assuming that colonization is a Poisson process. The expected mean population size of each species is determined by average regional abundance of the species and sampling efficiency of any given island. Habitat composition is an important determinant of sampling efficiency. Within each island type group it is immaterial for the increase of S [species numbers] whether sample size is increased in time or in space. This emphasizes the stochastic nature of actual colonization events and warns against making conclusions concerning turnover proneness of different species without data on population numbers; less abundant species are expected to turn over frequently. Expectations of the random placement model were compared with the observations. The model was in agreement with data within, but not between island type groups. These patterns indicate that each island type is sampling breeding pairs from an avifaunal universe of its own and habitat composition is an important determinant of the pool of actual colonists. The relation between S and A [ecological space] is indirect and is mediated by a chain of interrelated factors that comprise habitat composition, differences in species-specific colonization probabilities and species-abundance relations. S and A alone are imperfect characteristics of the ecologically interesting features of the colonization process and not too much attention should be paid to their mutual correlations.