The unseen invaders: introduced earthworms as drivers of change in plant communities in North American forests (a meta-analysis).

Globally, biological invasions can have strong impacts on biodiversity as well as ecosystem functioning. While less conspicuous than introduced aboveground organisms, introduced belowground organisms may have similarly strong effects. Here, we synthesize for the first time the impacts of introduced earthwormson plantdiversity and community composition in North American forests. We conducted a meta-analysis using a total of 645 observations to quantify mean effect sizes of associations between introduced earthwormcommunities and plantdiversity, cover of plantfunctional groups, and cover of native and non- native plants. We found that plantdiversity significantly declined with increasing richness of introduced earthwormecological groups. While plantspecies richness or evenness did not change with earthworminvasion, our results indicate clear changes in plantcommunity composition: cover of graminoidsand non- native plantspecies significantly increased, and cover of native plantspecies (of all functional groups) tended to decrease, with increasing earthwormbiomass. Overall, these findings support the hypothesis that introduced earthwormsfacilitate particular plantspecies adapted to the abiotic conditions of earthworm-invaded forests. Further, our study provides evidence that introduced earthwormsare associated with declines in plantdiversity in North American forests. Changing plant functional compositionin these forests may have long-lasting effects on ecosystem functioning.