Ecological impacts of a global plant invader: synthesizing mean and variance effects using meta-analysis

Invasive alien species (IAS) are one amongst the dominant drivers of global environmental change. The reported effects of IAS on biodiversity and ecosystems are often highly variable across species, habitats, and environmental settings, which constrains our capacity to empirically generalise their overall impact. To date, meta-analyses have investigated the ecological impacts of IAS using only mean effects, while variance effects - an equally important measure in quantifying the efficacy or generalizability of a treatment - has largely remained ignored. Using a global meta-analysis of 586 and 522 effect sizes related to mean and variance measures respectively from 30 studies, here we provide the first quantitative synthesis of the extent to which a global plant invader Lantana camara affects the mean and variance of ecological variables relevant at species, community and ecosystem levels. Our results suggest that for individual response classes, both the mean and variance effects from the species invasion differed considerably in their magnitude and direction, with the effect on most response classes being highly heterogeneous and non-significant. The study design had a significant effect on both the mean and variance effects from the species invasion, but the ecosystem type, continent, and latitude act as weak predictors of both mean and variance effects. By investigating both mean and variance effects, our study provides first quantitative synthesis on ecological impacts of a globally problematic invasive plant. Based on our findings, we highlight the current knowledge gaps and suggest future research directions, particularly the need to focus on causes of variance in invasion impact studies for effective management and restoration of invaded landscapes.