Impacts of marine invaders on biodiversity depend on trophic position and functional similarity

Impacts of marine invaders on local biodiversity have not been analyzed across invasive species and invaded habitats. We conducted a meta-analysis of 56 field experiments published in 29 papers that examined the effects of marine invaders on local species richness, diversity, and/or evenness. We show that invaders, across studies, typically have negative effects on biodiversity within a trophic level but positive effects on biodiversity of higher trophic levels. For example, both plants and sessile filter-feeders had positive effects on richness and diversity of mobile consumers. The contrasting negative and positive effects on similar versus higher trophic levels are potentially manifested through community-wide antagonism (competition and consumption) versus facilitation (habitat and food provisioning) interactions, respectively. These relation ships extended to functional interactions, as sessile invaders had negative effects on the biodiversity of sessile communities (intra-functional interactions) but positive effects on the biodiversity of mobile communities (inter-functional interactions). Our analyses highlight the importance of pairing attributes of the invader and the impacted organisms to obtain simple predictions of how the diversity of entire communities may respond to species invasions on local scales. We also note that our analysis did not require information on co-evolutionary history but that such data, coupled with long-term large-scale mensurative data, are needed to gain a comprehensive predictive insight into invasion impact.