Most studies were on invasive alien plants (52.8 per cent), which represents almost twice as many as for invasive alien vertebrates (28.9 per cent) or invertebrates (29.2 per cent), with fewer studies having been conducted on invasive alien microbes and fungi (12.3 per cent). More than half of the identified studies were from the terrestrial realm (66.3 per cent), whereas freshwater and marine systems were represented by, respectively, 20.2 per cent and 23.7 per cent of the studies.