Origin matters: alien consumers inflict greater damage on prey populations than do native consumers

Abstract Aim Introduced alien species are frequently implicated in ecosystem disruption and biodiversity loss, but some ecologists have recently argued that efforts to manage ecosystems should be refocused on known problematic species without regard to whether such species are native or alien. This argument rests on the premise that native and alien species in general do not differ in their impacts. Although there are numerous cases that suggest alien predators and herbivores can sometimes cause severe declines or even local extinctions of native species, very few studies have compared the impacts of native and alien consumers on native populations. Location World‐wide. Methods We have conducted a meta‐analysis on a global dataset to compare the effects of native and alien predators and herbivores on native populations occupying a broad range of terrestrial and aquatic environments. Results The distribution of positive, negative and neutral effects on native prey abundance differed significantly by consumer origin, with alien consumers associated with more negative and fewer positive effects than expected, opposite the finding for native consumers. The effect size of alien consumers was 2.4 times greater than that of native consumers and did not differ between predators and herbivores. The impact of alien consumers did not differ significantly in aquatic (lakes, rivers, oceans) versus terrestrial (continental, island) habitats. Similarly, there was no significant interaction between consumer origin and location, as consumers had similar effects in insular (freshwater, island) and open (continental, marine) systems – contrary to the notion that alien species impacts are mainly problematic for island biota. Main conclusions We hypothesize that the ecological naïveté of native biota facilitates their enhanced suppression by alien predators and herbivores relative to native enemies. Our results counter the assertion that the biogeographical origin of species has no bearing on their ecological impact.