Harms of introduced large herbivores outweigh benefits to native biodiversity.

Introduced species significantly impact native biodiversity worldwide, with extensive research on harms but relatively less focus on benefits. Here, using the IUCN Environmental Impact Classification for Alien Taxa (EICAT) and EICAT+ frameworks, we assess 2021 negative and positive impacts of introduced large mammalian herbivores globally. We show that negative impacts are more common, and of higher magnitude than positive impacts, i.e. affect populations, not only the performance of individuals. Native species on islands and at higher trophic level experience greater impacts. Reported impact magnitudes decline over time only for positive impacts. Most positive impacts are caused indirectly through changes in species interactions and ecosystem properties, often following negative impacts on native plants through herbivory and disturbance. We therefore advise caution regarding the intentional introduction of large mammalian herbivores for conservation purposes (rewilding, assisted colonization) without rigorous assessment of their impacts on native communities.