Abstract
Aim Terrestrial invertebrates comprise a large proportion of alien species world‐wide, yet a quantitative global synthesis of their effects on native species and ecosystems has not been explored. We conducted a meta‐analysis to examine the ecological impacts of terrestrial invertebrate invaders and to test how impacts are modulated by the invader's trophic position, habitat attributes (i.e. insularity and disturbance) and the study methodology (observational versus experimental). Location Global. Methods We investigated the effects of terrestrial invertebrate invaders on populations, communities and ecosystems by conducting a random effects meta‐analysis using 112 articles reporting data from 710 field and laboratory studies. The analysis included 16 insect, 11 earthworm, 7 slug and 1 nematode invaders. Results On average, across invaders, the presence of invaders reduced plant fitness (52%), animal diversity (33%) and animal abundance (29%). Leaf litter decomposition was 41% higher in the presence of invaders, while other ecosystem‐level variables such as nutrient cycling were not affected in a consistent direction. Invasive predators and detritivores decreased animal abundance, whereas herbivores and omnivores had limited impacts. Single invaders increased soil nitrogen pools while multiple species did not. Insularity and disturbance did not affect the magnitude of the impacts significantly, mainly because there was a large variation among studies. Main conclusions Overall, our study indicates that terrestrial invertebrate invaders have significant consistent effects on populations, communities and ecosystems, with islands and disturbed sites not being more prone to impacts. However, effects vary considerably depending on the type of impact being examined and the trophic position of the invader. There is no evidence that invaders cause larger impacts when multiple species of invaders, rather than single invaders, are involved.