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.

In the last decades, thousands of investigations confirmed the detrimental effects of species translocated by man outside of their native ranges (nonindigenous species, or NIS). However, results concluding that many NIS have null, neutral, or positive impacts on the biota and on human interests are as common in the scientific literature as those that point at baneful impacts. Recently, several scholars confronted the stand that origin per se is not a reliable indicator of negative effects, suggesting that such conclusions are the expression of scientific denialism, often led by spurious purposes, and that their numbers are increasing. When assessed in the context of the growing interest in introduced species, the proportion of academic publications claiming that NIS pose no threats to the environment and to social and economic interests is extremely low, and has not increased since 1990. The widely prevailing notion that many NIS are effectively or potentially harmful does not conflict with the fact that most have mixed (negative, neutral, and positive) impacts. When based on solid grounds, reports of positive or neutral impacts should not be labeled as manipulative or misleading unless proven otherwise, even if they may hamper interest in‐ and funding of research and control bioinvasion programs.