Density-dependent impacts of invasive Vincetoxicum rossicum (pale swallowwort) and native Asclepias syriaca (common milkweed) on plant traits and competitive interactions

Abstract

Invasive plants have become an increasingly pressing ecological problem, and the ability of invasive plants to out-compete native plants seriously threatens both biodiversity and ecosystem functionality. One invasive plant species increasing in abundance in temperate regions of the United States is Vincetoxicum rossicum (pale swallowwort), which has been observed to displace native confamilial Asclepias syriaca (common milkweed) in habitats where both species overlap. We sought to examine the effects of trait characteristics, trait variation, and density-dependent interactions on the competition between these species. We examined field-grown trait variability across an environmental gradient in western New York and applied a density-based competitive factorial design in a greenhouse setting. Both species grew in a wide range of environmental conditions in the field, but pale swallowwort exhibited more trait variation and greater specific leaf area than common milkweed, potentially reflective of increased efficiency in resource capture. Swallowwort density was a significant predictor of plant traits in the field. In the greenhouse, interspecific competition was more intense than intraspecific competition, with negative impacts of milkweed density on swallowwort growth. Although its trait variability allows it to survive in many environments, on an individual basis at low densities, swallowwort is not a strong competitor. However, this invader gains competitive advantages over native species due to Allee effects, which increase its ecological impact and success as density increases. Swallowwort is likely to remain a successful invader in disturbed temperate ecosystems and continue to have negative impacts on communities where it achieves high densities.