Island species and habitats are particularly vulnerable to human disturbances, and anthropogenic changes are increasingly overwriting natural island biogeographic patterns. However, quantitative comparisons of how native and alien assemblages respond to human disturbances are scarce. Using data from 6,242 species of vertebrates, invertebrates and plants, from 7,718 sites on 81 islands, we model how land-use change, human population density and distance to the nearest road affect local assemblages of alien and native species on islands. We found that land-use change reduces both richness and abundance of native species, whereas the number and abundance of alien species are high in plantation forests and agricultural or urban sites. In contrast to the long-established pattern for native species (i.e., decline in species number with island isolation), more isolated islands have more alien species across most land uses than do less isolated islands. We show that alien species play a major role in the turnover of island assemblages: our models show that aliens outnumber natives among the species present at disturbed sites but absent from minimally-disturbed primary vegetation. Finally, we found a homogenization pattern for both native and alien assemblages across sites within most land uses. The declines of native species on islands in the face of human pressures, and the particular proneness to invasions of the more remote islands, highlight the need to reduce the intensity of human pressures on islands and to prevent the introduction and establishment of alien species.

Biodiversity Macroecology and Biogeography University of Goettingen Göttingen Germany

BioInvasions Global Change Macroecology Group Department of Botany and Biodiversity Research University Vienna Vienna Austria

Centre of Biodiversity and Sustainable Land Use University of Goettingen Göttingen Germany

Department of Biosciences Durham University Durham United Kingdom

Department of Invasion Ecology Institute of Botany The Czech Academy of Sciences Průhonice Czech Republic

Department of Life Sciences Imperial College London London United Kingdoms

Department of Life Sciences Natural History Museum London United Kingdom

Ecology Department of Biology University of Konstanz Konstanz Germany

Faculty of Science Charles Department of Ecology University Prague Czech Republic

Food Animal Health Research Program Ohio Agricultural Research and Development Center The Ohio State University Wooster Ohio United States of America

Zhejiang Provincial Key Laboratory of Plant Evolutionary Ecology and Conservation Taizhou University Taizhou China

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