Interactive effects of invasive species and environmental change: A meta-analysis

Bianca E. Lopez, NECASC, Jenica M. Allen, Miller Worley Center for the Environment, Mount Holyoke Colege, South Hadley, MA, Jeffrey S. Dukes, Purdue Climate Change Research Center, Purdue University, West Lafayette, IN, Jonathan Lenoir, Department of Ecologie et Dynamique des Systèmes Anthropisés, Université de Picardie Jules Verne, Amiens, France, Montserrat Vilà and Alberto Maceda-Veiga, Department of Integrative Ecology, Estación Biológica de Doñana (EBD-CSIC), Sevilla, Spain, Dana M. Blumenthal, Rangeland Resources & Systems Research, USDA, Agricultural Research Service, Fort Collins, CO, Bethany A. Bradley and Emily Fusco, Environmental Conservation, University of Massachusetts, Amherst, Amherst, MA, Evelyn Beaury, Organismic and Evolutionary Biology, University of Massachusetts Amherst, Amherst, MA, Brittany B. Laginhas, Organismic & Evolutionary Biology, University of Massachusetts, Amherst, MA, Toni Lyn Morelli, U.S. Geological Survey, Northeast Climate Adaptation Science Center, Amherst, MA, Cascade J. B. Sorte, Ecology & Evolutionary Biology, University of California, Irvine, CA, Raj Whitlock, Institute of Integrative Biology, University of Liverpool, Liverpool, United Kingdom

Background/Question/Methods

Ecosystems are undergoing rapid changes due to human pressures, including the spread of non-native invasive species and abiotic global environmental changes (GECs). Understanding how ecosystems respond to these coincident stressors is critical for informing ecosystem conservation, adaptation, and management. However, recent studies on the effects of multiple stressors have shown that “ecological surprises” are not uncommon, with one stressor commonly amplifying or dampening the effects of another, leading to unpredictable combined effects. Thus, we designed a meta-analysis to examine how the combined effects of invasive species and GECs on ecosystems compare to the effects of each stressor alone. The meta-analysis used 424 responses from 94 published studies that reported both individual and combined ecological effects of invasive species and one of six GECs (elevated CO2, drought, nitrogen deposition, O2 depletion, altered pH, warming). We calculated standardized effect sizes (Hedges’ d) for the invasive, GEC, and combined treatments and classified interactions as additive (expected based on summed individual treatment effects), antagonistic (mitigating), or synergistic (amplified to a higher magnitude than the individual stressor effects). We used Bayesian hierarchical models to examine differences in effect sizes across studies and Fisher’s tests to examine differences in interaction types.

Results/Conclusions

Most studies manipulated nitrogen deposition (n = 43), warming (n = 30), or drought (n = 21) to study their interactive effects with biological invasions, while too few studies have examined interactions between pH (n = 3), O2 (n = 3), or CO2 (n = 2) and invasion to draw robust conclusions. We found that across studies, combined invasion and GEC effects were more strongly negative than the effects of the individual stressors. Antagonistic interactions were the most common type across studies, but synergistic effects were seen in over 25% of cases. There were also significant differences in interaction types across GECs when considering the direction of effects, with nitrogen addition more often leading to more-positive-than-expected synergistic effects than warming or drought in the presence of invasive species. Interaction types differed depending on the type of measured ecosystem response as well, with negative synergistic interactions and negative effects more common for native species survival, body size/growth, and physiology than ecosystem-, community-, and other species-level responses. These results suggest that invasive species prevention and management may become more important with ongoing GEC, particularly in systems experiencing warming temperatures or drought.