Increasing diversity of invasives increases impacts on native communities
Increasing native plant diversity can have a positive effect on ecosystem functions, such as productivity, stability, and resistance to exotic invasions. However, little is known about how increased diversity of invasive species influences invasion success and overall impacts on native plant communities. Instead, investigations of invasive species effects are generally limited to single species studies, and thus ignore the possibility of synergistic interactions among simultaneously invading species. We explored both the individual and cumulative impact of multiple plant invaders on native species in experimentally assembled North American intermountain prairie plant communities. We created common garden communities comprised of four native grass and forb species and invaded them with either twelve individuals of a single invasive species, a pair of invasives species represented by six individuals each, or four invasive species represented by three individuals each. In all treatments, we controlled for density by keeping the spacing and number of individual plants consistent while varying the identity of invaders in each plot. Plants were grown for 150 days and all above ground biomass was harvested, dried, and weighed.
Diverse groups of invaders had disproportionately stronger negative impacts on native plant communities when compared to single invader effects. The proportion of total biomass of native plants in four invader communities was more than two times lower than single invader communities. Concomitantly, invader total biomass significantly increased as diversity of invaders increased. Two invasive species, Centaurea stoebe and Linaria dalmatica, were significantly larger in four invader assemblages compared to when they invaded native communities alone. Our results suggest that invasive species may interact in ways that create communities in which the ecosystem wide effect of invasive species is greater than the sum of its parts. As a consequence, estimates of invader impacts in isolation of other invaders are likely to underestimate the overall impact of invasion on ecosystem function.