Invasive Plant (Microstegium vimineum) Reduces Abundance and Richness of Insect Communities

Abstract

Plant invasions are known to have negative impacts on native plant communities, yet their effects on higher trophic levels have not been well documented. Past studies investigating the effects of invasive plants on herbivores and carnivores have been largely observational in nature and thus lack the ability to tease apart whether differences are a cause or consequence of the invasion. In addition, understanding how plant traits and plant species compositions change in invaded habitats may increase our ability to predict when and where invasive plants will have effects that cascade to animals. We experimentally introduced a non-native plant (Microstegium vimineum, Japanese stiltgrass) to evaluate impacts on arthropods. We also investigated possible mechanisms through which the invader could affect associated arthropods, including changes in native plant species richness, aboveground plant biomass, light availability, and vegetation height. In experimentally invaded plots, arthropod abundance was reduced by 39%, and species richness declined by 19%. Carnivores experienced greater reductions in abundance than herbivores (61% versus 31% reduction). Arthropod composition significantly diverged between experimentally invaded and control plots, and particular species belonging to the families Aphididae (aphids), Formicidae (ants) and Phalacridae (shining flower beetles) contributed the most to compositional differences. Among the mechanisms we investigated, only the reduction in native plant species richness caused by invasion was strongly correlated with total arthropod abundance and richness. In sum, our results demonstrate negative impacts of M. vimineum invasion on higher trophic levels and suggest that these effects occur, in part, indirectly through invader-mediated reductions in the richness of the native plant community. The particularly strong response of carnivores suggests that plant invasion could reduce top-down control of herbivorous species for native plants. The cascading effects of plant invasion on arthropods reported here reinforce motivation to control M. vimineum spread.