Independently, invasive species and abiotic global change are two dominant driving forces with direct effects on ecological systems. Invasive species can reduce native species abundance and diversity while at the same time altering ecosystem processes. Similarly, some abiotic changes, such as increased carbon dioxide concentrations or increased temperature, modify ecosystem attributes. What remains unknown are the interactive effects of species invasions and abiotic change on native populations, communities, and ecosystems. Will the combined effects of abiotic change and invasion be additive, antagonistic (sub-additive), or synergistic (super-additive)? Here, we quantify interactive and main effects of invasion and abiotic change on native species and ecosystem responses using quantitative and qualitative comparisons of empirical studies. We reviewed empirical papers that manipulated one or more invasive species and one of six global change factors (temperature, carbon dioxide concentration, nitrogen deposition, eutrophication, acidification, and drought) in both terrestrial and aquatic systems.
Our database consists of 48 empirical studies, 36 of which contained data suitable for meta-analysis and 12 of which were suitable for qualitative comparisons of main and interaction effects. Forty-two percent of the studies were from aquatic systems (18% freshwater and 24% marine). Forty-five percent of studies had statistically significant invasion by global change interactions. Of these, 73% were in response to increased temperature, 18% were in response to nitrogen addition, and 9% were in response to increased carbon dioxide concentrations. However, the nature of the interactions varied considerably; in about half of the cases the interaction was synergistic. This suggests that about a quarter of the time, global change and invasion occurring together will generate larger effects than we would predict from each driver (invasion or abiotic change) independently. The potential for offset of impacts was also evident, though the situations in which this occurred were highly variable. The frequency of synergistic interactions, where impacts are enhanced beyond those of each driver alone, highlights the need for continued support of invasion prevention and control strategies in addition to mitigation strategies for abiotic changes.