Harms of introduced large herbivores outweigh their benefits, while both are greater on islands and for higher trophic levels

Abstract

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