Nonnative invasive species are overlooked in biological integrity assessments

Multimetric indices (MMIs) are common tools used to assess the biological status of ecosystems. However, not all the components of biological integrity have been equally addressed, since most of MMIs do not consider nonnative species, which are a great threat to ecosystems integrity. We performed a systematic review of how nonnative species have been considered in MMIs to assess biological conditions worldwide. Using a systematic review of the literature we identified to what extent nonnative species have been included as a disturbance factor for determining reference conditions and if specific metrics related to both nonnative and native species have been taken into account in the MMIs around the world. We also identified the most frequent species origin metrics that have been included in the MMIs. We searched for articles published in English up to 2016 that applied or developed an MMI in the Web of Science database (Clarivate Analytics). Following the PRISMA guidelines, we analyzed 409 articles. We found that only 1.2% of the studies used nonnative species as a disturbance factor in determining reference conditions. About 66% of articles did not test or did not provide the information if they tested metrics on native or nonnative species. Most of the articles (53%) that tested native or nonnative metrics were fish-based MMIs. We found 97 different metrics, of which 69% were related to native species, whereas 31% were related to nonnative ones. Native richness was the most used metric in studies that considered species origin (23.5%). Our main results evidence that nonnative species are overlooked in biomonitoring programs. Owing to the well-known impacts of them on ecosystem integrity, their inclusion in definition of reference conditions and in MMIs metrics should be considered as bioassessment criteria.