Species richness of hornworts and liverworts on islands: A preliminary analysis of geographic and anthropogenic drivers

Abstract

Environmental factors related to geography and climate are generally among the most significant drivers of species richness across spatial and temporal scales. Among the most studied attributes explaining patterns of species richness on islands are island area and geographic isolation, the two main factors that founded the Theory of Island Biogeography by MacArthur and Wilson (1967). Even though their theory of the equilibrium on islands represents a generally testable and applicable model, its simplicity disregards taxa specific traits like long distance dispersal capacities and abiotic factors like climate. Furthermore, the impact of human activity which is known to affect biodiversity at different spatial scales, from global to regional, is not included in the model. Bryophytes and especially hornworts and liverworts have comparatively broad distribution ranges due to great long distance dispersal abilities and are known to respond very sensitively to environmental changes which is why pointed as one of the most efficient taxonomic groups used as bioindicators of climate change. Based on a compilation of floristic data from a selection of islands distributed worldwide we investigate the significance of various environmental attributes for patterns in species richness of hornworts and liverworts on islands. We include a set of predictor variables consisting in geographic (distance to the mainland coast, area, proportion of surrounding landmass, latitude, longitude, maximum elevation), climatic (mean annual temperature, mean annual precipitation, climate change velocity and intraannual seasonality quantified by using the minimum annual temperature range and the minimum coefficient of variation in monthly precipitation) and one sociodemographic variable (human population density) to determine what factors explain best variation in species richness across the 95 islands studied. Our results indicate that besides island area and isolation, the traditional geographical variables, elevation, variation in precipitation and climate change velocity are important drivers of species richness of hornworts and liverworts on islands. In contrast, human population density did not seem to explain patterns of species richness. Despite these results, we conclude that different families of predictors, including past ecological conditions and human activities, may explain patterns in species richness, which cannot be explained by traditional geographic conditions alone.