The impacts of invasive species generally increase with their abundance, and the abundance–impact relationship may be a practical way to assess and make important predictions about the impact of invasive species. Here, I summarize how abundance-impact relationships can help to understand the impact of invasive plants and explore the types of evidence needed to estimate those relationships. I give examples from three disparate systems in which abundance-impact relationships have helped to either pinpoint or scale the impact of invasive plants in USA natural areas: (1) estimating the impact of invasive wetland plants in Illinois on native plant communities, (2) scaling experimental data to assess which native plants are most impacted by invasive garlic mustard, and (3) parsing the impacts of cheatgrass and fire on individual native plant species throughout the Great Basin.
Results/Conclusions
Abundance-impact relationships can be estimated from experimental studies, longitudinal studies (i.e. long-term monitoring), and horizontal studies (i.e. landscape surveys), and unique caveats must be given to each line of evidence. Using longitudinal surveys, invasive plants had greater impacts on Illinois wetland communities than did native plants because invasive plant species were more abundant and had greater per-capita effects. This indicates a unique role of invasive plants in the community assembly of Illinois wetlands. An experimental removal study showed that garlic mustard disproportionately impacts native plants that depend on mycorrhizal fungi, but not all of these effects translated to patterns of impact in broad-scale longitudinal studies. Cheatgrass is known to both outcompete native plants and displace them by altering fire-cycles. A horizontal study using thousands of plots across the Great Basin confirmed that a set of native plant species were impacted directly by cheatgrass while others were more impacted by fire. This study allows us to identify plant species that may be useful for restoration in a landscape where cheatgrass-fire cycles are an enduring reality. It is important to understand the impacts of invasive species, and abundance-impact relationships are a robust way to do this. Many government agencies, research groups, municipalities, parks, and nature reserves conduct long-term and large-scale monitoring of plant communities. These data can be used to estimate the impacts of invasive plants and guide management to address those impacts.