Habitat Diversity or Area Per se? Species Richness of Woody Plants, Carabid Beetles and Land Snails on Islands

(1) We examine patterns of species richness of woody plants, carabid beetles and land snails, respectively, on seventeen undisturbed forested islands (area range: 0.6-75 ha) in Lake Malaren, Sweden, in relation to area, isolation, vegetation structure, habitat diversity, habitat heterogeneity, and other factors. (2) The slopes of the species-area relations (z-values in the power model) are 0.10 for woody plants, 0.16 for land snails, 0.36 for carabid beetles, and 0.62 for forest birds. The z-values differ significantly from zero and also differ between all organism groups except woody plants and land snails on one hand, and land snails and carabid beetles on the other. (3) Island area is the best single predictor of species richness in the organism groups examined. Since habitat variables and island area are uncorrelated on the studied islands, we can reject the habitat diversity hypothesis as an explanation for the species-area relations found. (4) Total densities of woody plants and land snails were not correlated with island area, while the total density of carabid beetles was positively correlated with island area. (5) Islands with a high proportion of wet forests have relatively higher species richness of carabid beetles, and the number of land snail species relates positively to the proportion of deciduous forest. The patterns found are discussed in relation to general species richness theories.