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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 earthworms on plant diversity 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 earthworm communities and plant diversity, cover of plant functional groups, and cover of native and non-native plants. We found that plant diversity significantly declined with increasing richness of introduced earthworm ecological groups. While plant species richness or evenness did not change with earthworm invasion, our results indicate clear changes in plant community composition: cover of graminoids and non-native plant species significantly increased, and cover of native plant species (of all functional groups) tended to decrease, with increasing earthworm biomass. Overall, these findings support the hypothesis that introduced earthworms facilitate particular plant species adapted to the abiotic conditions of earthworm-invaded forests. Further, our study provides evidence that introduced earthworms are associated with declines in plant diversity in North American forests. Changing plant functional composition in these forests may have long-lasting effects on ecosystem functioning. [ABSTRACT FROM AUTHOR]

1. Roads are an important conduit for the spread of invasive species. Road age is a key factor that could influence the susceptibility of roads to invasion as older roads are typically subject to higher cumulative levels of human disturbance and propagule pressure than younger roads. We investigated the effects of road age on the spread of non-native earthworms, which act as ecosystem engineers. 2. We sampled earthworms and habitat variables at 98 roads in the boreal forest of Alberta, Canada, to determine the influence of road age on non-native earthworm occurrence at the landscape level. The extent and rate of local spread were also assessed at seven sites adjacent to old and young roads. Generalized estimating equations and zero-inflated negative binomial regression were used to analyze landscape- and local-level results, respectively. We used our models to create maps that predict the current and potential future extent of earthworms in north-eastern Alberta. 3. Probability of earthworm occurrence and extent of spread increased as road age increased. Areas closer to agriculture and towards the south and west of our study area were also significantly more likely to be invaded by earthworms. 4. Our spread model indicated that approximately 9% of the boreal forest of north-eastern Alberta is likely invaded by earthworms currently. This is projected to increase to 49% of suitable forest habitat over the next 50 years as human development intensifies in this region. 5. Synthesis and applications. Although the effects of roads and linear features are commonly investigated in relation to native species, our results emphasize the importance of considering the impacts of linear feature creation on the spread of invasive species. We demonstrate that road age in particular can be an important factor affecting the spread of invasive species. In the boreal forest, reducing the number of roads being constructed, restricting traffic, and reclaiming temporary roads will be critical to reduce the future extent of earthworm invasions. [ABSTRACT FROM AUTHOR]