The peninsular effect and habitat structure: bird communities in coniferous forests of the Hanko Peninsula, southern Finland

Abstract

The hypothesis of the peninsular effect predicts that the number of species declines towards the tip of a peninsula, this being attributed to an equilibrium between colonization and extinction. I tested the hypothesis on the Hanko Peninsula, southern Finland, by collecting quantitative data on birds breeding in pine and spruce forests along the peninsula. I also made habitat descriptions to check whetherdifferences in the number of species and bird densities could be explained by variation in habitat structure. Rarefaction analysis did not reveal any significant differences in the numbers of bird species between different parts of the peninsula in pine or spruce forests . Nor did the total density of birds in the pooled data, or in the pine forests reveal the expected pattern . The total bird density in spruce forests was highest in the middle of the Hanko Peninsula. This pattern, and also density differences in single species and species groups along the peninsula, couldbe plausibly explainedby habitat structure. Both the analyses of separate habitat variables and DCAordinations support this conclusion . The most important factor determining bird densities was theluxuriance of the habitat, but the bird communitieswere also affected by tree species composition andthe size class distribution of the trees. It thus appears that habitat differences on a larger scale, e.g. in the proportions of habitat types, also contribute to the diversity gradients observed on many peninsulas. I discuss scale problems inherent in traditional peninsular studies and the implications of thepeninsular effect for conservation policy .