Direct effects of a non-native invader erode native plant fitness in the forest understory

The direct role of non-native plant invaders in driving negative population- and community-level processes of native species has been recently questioned. Addressing this controversy requires determining quantitatively if invaders negatively affect native population fitness. Because the invasion of non-natives often coincides with other anthropogenic stressors, experiments that partition the putative impact of non-natives from other known stressors and assess their potential synergies are required. While many studies have examined the effects of non-natives on components of native plant performance, studies that decompose the net fitness effects of non-natives from other anthropogenic stressors on population growth rate are lacking. We used 6 years of detailed demographic data to parameterize a size-dependent integral projection model to examine the individual and combined effects of an allelochemical-producing invader (Alliaria petiolata) and an overabundant ungulate herbivore (Odocoileus virginianus) on the population dynamics of an understory perennial (Trillium erectum). We show that Alliaria consistently and negatively affects the population dynamics of Trillium. Specifically, this invader reduces native population growth rate and alters the size distribution of the population at equilibrium. Alliaria also works in concert with the known negative impacts of overabundant white-tailed deer, illustrating the additive effects of anthropogenic stressors on native plant dynamics. Synthesis. Alliaria's effects on vital rates differed in magnitude and sign across the native's life cycle, highlighting the importance of detailed demographic analyses. Our study provides novel empirical support for the claim that non-native invasive species can significantly and directly reduce the fitness of native plants.