Format: ORAL

Giovanni Vimercati, Leon Strotz, Sven Bacher

University of Fribourg, Fribourg, Switzerland

Alien species cause various environmental impacts worldwide. Negative impacts have received considerable attention, as impactful alien species, named invasive, are a major driver of biodiversity decline. The IUCN Environmental Impact Classification for Alien Taxa (EICAT) has been developed to assess negative impacts on native biodiversity. Alien species can also have positive impacts, e.g., through habitat/food provisioning or dispersal facilitation. However, these impacts have never been systematically examined, mostly due to the absence of a standardized protocol to assess their magnitude and diversity. We have filled this gap by developing EICAT+, a framework which structurally resembles IUCN EICAT, can be applied to all alien taxa at different spatial and organizational scales, and uses five semi-quantitative scenarios to describe the magnitude of observed positive impacts of alien taxa on native biodiversity. In EICAT+, the highest levels of impact magnitude are assigned to alien taxa increasing the area of occupancy of native species through local population re-establishment or preventing local extinction. EICAT+ also describes the underlying impact mechanisms by which alien taxa facilitate native biodiversity. To illustrate the functionality and utility of the framework, we have applied EICAT+ to studies reporting positive impacts of the alien tree black locust (Robinia pseudoacacia) and used IUCN EICAT to assess its negative impacts. About one third of all impacts were positive. Most impacts, both negative and positive, involved changes in population size of native species and were caused by alterations to abiotic conditions. Species that benefit from the introduction of the black locust show a preference for nutrient-rich soils and higher light intensity, in contrast to species negatively affected. Our results align with the existing literature on the black locust and demonstrate how the combined use of IUCN EICAT and EICAT+ enables to obtain a comprehensive understanding of alienspecies impacts on native biodiversity.