Intra‐ and Inter‐Specific Ecological Impacts Vary Across a Gradient of Abundance of an Invasive Species, <i>Bothriochloa ischaemum</i> , in a Mixed‐Grass Prairie

Managing biological invasions is one of the top priorities of biodiversity conservation. Invasive plants are a well-known threat to native plant and animal communities, and understanding their ecological impacts is critical to developing individualized management strategies. While much is known about the impacts of invasive plants, there are still questions about the per capita effects along invasion abundance gradients across levels of biological organization. In this study we investigate how the ecological impacts of the invasive grass <i>Bothriochloa ischaemum</i> vary across a gradient of invasion and whether effects are consistent across population (abundance and functional traits of a dominant native grass, <i>Schizachyrium scoparium</i>) and community (species richness and composition) levels. We found that most of the ecological impacts of <i>B. ischaemum</i> scale linearly with its abundance across population and community levels. Increasing invasion reduces the height and abundance of the dominant native <i>S. scoparium</i> individuals and shifts their functional trait composition. Increasing invasion also reduces cover of native C₃ and C₄ grasses, total foliar cover, subdominant foliar cover, species richness, and leads to shifts in species and functional group composition. However, the impact on legume abundance saturated at low invader abundance (1%-15% cover) and remained constant as invader abundance increased. We show that the direct ecological impacts of invasive species may be compounded by shifts in the functional traits of dominant native species toward more conservative traits and shifts in species and functional group composition, leading toward a shift in population and community structure and function.