Disentangling natural and anthropogenic drivers of native and non-native plant diversity on North Sea islands

Aim: Biodiversity on islands is commonly explained by a set of natural drivers such as area, isolation and habitat heterogeneity. However, constant human impact has led to considerable changes in island floras worldwide. This is reflected, among others, in increased numbers of non-native species. Barrier islands are discrete land units, strongly influenced by humans and not displaying significant evolutionary dynamics. This makes them highly suitable for studying contemporary patterns of species richness and underlying processes. We aim to disentangle the effects of established natural and anthropogenic drivers on native and non-native plant species richness at the example of 31 European barrier islands.Location: 31 North Sea barrier islands located off the Dutch, German and Danish coast.Taxon: Native and non-native plant species (spermatophytes and ferns).Methods: Individual relationships of natural and anthropogenic drivers with native and non-native plant species richness are analysed with generalised linear models (GLMs). We use structural equation models (SEMs) to additionally account for interrelations between drivers.Results: Island area was the strongest predictor of native and non-native plant species richness but affected richness mostly indirectly through habitat heterogeneity (non-native species) and island inhabitants (native species). Isolation had a slight negative effect on native and non-native plant species numbers on islands.Main Conclusions: The richness of native and non-native plant species on islands is associated with different drivers, that is, habitat heterogeneity and island inhabitants respectively. This might be caused by distinct underlying processes forming native and non-native richness patterns. Area was confirmed to be the most important driver of species richness but acting primarily through other natural and anthropogenic drivers of plant species richness. We encourage considering both natural and anthropogenic drivers and their interrelatedness to explain contemporary biogeographic patterns of species richness.