Questions: Do islet area, number of biotopes and distance from the mainland shape lichen species richness on islets? Are there any species- or substrate group-specific trends associated with these factors?

Location: Islets of the west Estonian Archipelago, Estonia.

Methods: A species list was compiled for each of the 32 islets and the relative abundance of each species was estimated. The lichens were divided into seven groups according to their substrate preferences. Generalized linear model (GLIM) analysis was applied to test the effect of the islet traits on the number of lichen species on the islets and in the substrate groups. The probability of presence/absence and abundance of the most frequent species according to the islet traits were tested with GLIM and general linear mixed model.

Results: The lichen flora of the islets consisted of 326 taxa, the number of lichen species per islet varied from 2 to 197. Total number of species per islet and within the substrate groups was positively correlated with islet area and with number of biotopes, and negatively correlated with distance from the mainland; however, these relationships varied among the substrate groups. Although individual lichen species showed variation in responses, general trends in island biogeography were evident.

Conclusions: The distribution pattern of lichens on the studied islets follows the theory of island biogeography: the number of species per islet depends on isolation, area and biotope diversity. Species specific traits, such as dispersal strategy and growth form, as well as availability of a particular substrate are important for formation of the lichen flora on islets.

Nomenclature: Randlane & Saag (1999, 2004) and Santesson et al. (2004) for lichens; Leht, M. (ed.) 1999 for vascular plants.

Abbreviations: AIC = Akaike's information criterion; GLIM = Generalized linear model; TLC = Thin layer chromatography.