Macroscale patterns in the per capita effects of plant invasions

Evelyn Beaury, Organismic and Evolutionary Biology, University of Massachusetts Amherst, Amherst, MA, Helen R Sofaer, U. S. Geological Survey, Hilo, HI, Regan Early, Centre for Ecology and Conservation, University of Exeter, Cornwall, Cornwall, United Kingdom, David Barnett, National Ecological Observatory Network (NEON), Boulder, CO, Dana M. Blumenthal, Rangeland Resources & Systems Research, USDA, Agricultural Research Service, Fort Collins, CO, Jeffrey Corbin, Department of Biological Sciences, Union College, Schenectady, NY, J. Dietz, University of Oregon, Jeffrey S. Dukes, Purdue Climate Change Research Center, Purdue University, West Lafayette, IN, Ines Ibanez, University of Michigan, Ian Pearse, United States Geological Survey, Fort Collins, CO, Laís Petri, School for Environment and Sustainability, University of Michigan, Ann Arbor, MI and Bethany A. Bradley, Environmental Conservation, University of Massachusetts, Amherst, Amherst, MA

Background/Question/Methods
Invasive plant species negatively impact native ecosystems, economies, and human health, yet a surprising paucity of studies quantify invasive species impacts or assess how impacts vary across geographies. Previous studies suggest invasion impact varies as a function of invader abundance, range, and effects per individual or per unit of cover, otherwise known as per capita effects (PCEs). While factors determining the range and abundance of invaders have been studied, little is known about macroscale patterns in PCEs and how they contribute to overall impact. In this study, we used 25,197 plots of plant abundance data from Federal agencies across the U.S. to test whether and how PCEs might vary with environmental characteristics known to modify species interactions. We quantified PCEs using the slope of the relationship between non-native species’ cover and native plant diversity; steeper slopes indicate stronger PCEs resulting in greater declines in native diversity. We hypothesized that PCEs vary across gradients of resource availability (measured as soil water availability), the number of non-native species present, and human activity.
Results/Conclusions
At the continental scale, native plant richness and diversity declined as invader cover increased, but native plant evenness significantly increased. The slope of this relationship varied significantly with water availability and non-native species richness; PCEs were greatest in areas of high water availability and high numbers of non-native species indicative of invasional meltdown. Human activity did not influence PCEs, suggesting that human activity likely has a direct effect on invader cover but does not influence the impact of invaders once established. Therefore, our results suggest that the environment modifies the effects of invasion, but the conditions that facilitate invasive plants reaching high abundance (e.g., disturbance via human activity) are not necessarily the same conditions in which invaders have the greatest per capita effects (e.g., high resource areas). These results also support evidence that invasion impacts are largely context-dependent. By providing a first assessment of how PCEs of invasion vary at the macroscale, our study provides valuable information for understanding, predicting, and mitigating invasive plant impacts across ecosystems of the United States.