Meta-analysis on the effects of exotic plants on the fitness of native plants.
Exotic plant species can reduce the abundance and biodiversity of native plant species. However, the underlying demographic mechanisms for the population declines of native plants through individual fitness components are unclear. To assess the impact of exotic plant species on the fitness components of native plants (establishment, growth, biomass, reproduction, and survival), we used a global meta-analysis of 75 articles representing 232 data points. We examined how the characteristics of native and exotic species, environmental conditions, and methodological factors modify the magnitude and direction of impacts on native fitness. We found that the impact of exotic plant species on native plants varied across fitness components. Exotic plant species generally reduced the biomass, reproduction, and survival of native plants, while they had no impact on the establishment and growth of natives. Most studies included in the data set were conducted in temperate grasslands and forests focusing on a few life-forms of exotic species, making it difficult to draw more specific conclusions of the impacts of exotic plant species on native plants. Overall, our results indicate that exotic plants may limit the population growth of co-occurring native plant species primarily by reducing the biomass, reproduction, and survival of individual plants, most likely through resource competition. To enable generalisations to be made, we suggest that the impacts of exotic plant species on native plants should be assessed based on multiple fitness components, preferably across different habitat types. Moreover, we encourage impact studies to be carried out beyond temperate regions using a wider range of exotic plant species.