Abstract
Biological invasions are a defining feature of the Anthropocene, but the factors that determine the spatially uneven distribution of alien plant species are still poorly understood. Here, we present the first global analysis of the effects of biogeographic factors, the physical environment and socio-economy on the richness of naturalized and invasive alien plants. We used generalized linear mixed-effects models and variation partitioning to disentangle the relative importance of individual factors, and, more broadly, of biogeography, physical environment and socio-economy. As measures of the magnitude of permanent anthropogenic additions to the regional species pool and of species with negative environmental impacts, we calculated the relative richness of naturalized (= RRN) and invasive (= RRI) alien plant species numbers adjusted for the number of native species in 838 terrestrial regions. Socio-economic factors (per-capita gross domestic product (GDP), population density, proportion of agricultural land) were more important in explaining RRI (~50 % of the explained variation) than RRN (~40 %). Warm-temperate and (sub)tropical regions have higher RRN than tropical or cooler regions. We found that socio-economic pressures are more relevant for invasive than for naturalized species richness. The expectation that the southern hemisphere is more invaded than the northern hemisphere was confirmed only for RRN on islands, but not for mainland regions nor for RRI. On average, islands have ~6-fold RRN, and >3-fold RRI compared to mainland regions. Eighty-two islands (=26 % of all islands) harbour more naturalized alien than native plants. Our findings challenge the widely held expectation that socio-economic pressures are more relevant for plant naturalization than for invasive plants. To meet international biodiversity targets and halt the detrimental consequences of plant invasions, it is essential to disrupt the connection between socio-economic development and plant invasions by improving pathway management, early detection and rapid response.
Highlights
Human-mediated dispersal of species into new regions has become a key feature of the Anthropocene (Waters et al 2016), redefining biogeographical realms (Capinha et al 2015) and homogenizing the worlds’ biota (Winter et al 2010; La Sorte et al 2014)
Of the five hypotheses that have been proposed to explain the geographic patterns of alien species richness and which we tested with our data set, we could clearly confirm the mainland–island differentiation
We did not find support for the expectation that socio-economic pressures are more important for naturalized than invasive species (Williamson 2006), as we found that socio-economic pressures explain more of the variation in relative richness of invasive species (RRI) than in RRN
Summary
Human-mediated dispersal of species into new regions has become a key feature of the Anthropocene (Waters et al 2016), redefining biogeographical realms (Capinha et al 2015) and homogenizing the worlds’ biota (Winter et al 2010; La Sorte et al 2014). The number of naturalized species that have become invasive, i.e. that spread widely (Blackburn et al 2011) and cause deleterious impacts on the environment and human societies (Lambertini et al 2011; Simberloff et al 2013), is substantial (McGeoch et al 2010; Vilà et al 2011) and rapidly increasing (Catford et al 2009; Hulme et al 2013). Much research has been devoted to testing the validity of these hypotheses and improved insights are highly needed for invasive alien species policy and management, a comprehensive analysis of the drivers mediating richness of naturalized and invasive plants at global extent is still missing
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