Deer, earthworms, and invasive plant removal are associated with degradation of plant communities

Land managers spend substantial resources managing invasive species to mitigate their negative ecological and economic impacts. However, invasive species management is seldom coupled with empirical assessments of management outcomes or of the ecological impacts of the target species. Additionally, the study of invasive species has, until recently, neglected to acknowledge co-stressors on native communities, which often are the drivers of ecological degradation. We assessed the effects of the invasive plant Vincetoxicum rossicum and its removal on understory plant communities, as well as whether these effects were mediated by deer and invasive earthworms. We used paired and open plots with V. rossicum and an open plot without V. rossicum at each of three forested sites in New York State Parks from 2017–2020. Following a nested design, we located sets of open and fenced plots in areas where V. rossicum was experimentally removed and in areas where it was unmanaged. After three years of V. rossicum removal, V. rossicum significantly declined. However, the ultimate goal of management was native plant recovery. Contrary to this aim, native plant diversity and cover increased if V. rossicum was left unmanaged but did not change with V. rossicum removal. Thus, we provide strong evidence that reducing target species abundance does not always translate into native plant recovery. This disparity may be because deer and invasive earthworms are stronger drivers of understory plant communities than V. rossicum, as native plant cover and diversity were lower the more earthworms were in a plot and invasive plant species declined after fencing to exclude deer. Future management should thus prioritize reducing deer and earthworm impacts over invasive plant removal. Our findings underscore the need to collect data to test management assumptions: reducing the abundance of an invader may not always result in native plant recovery, especially if other stressors are the ultimate drivers of change in invaded communities. Land managers spend substantial resources managing invasive species to mitigate their negative ecological and economic impacts. However, invasive species management is seldom coupled with empirical assessments of management outcomes or of the ecological impacts of the target species. Additionally, the study of invasive species has, until recently, neglected to acknowledge co-stressors on native communities, which often are the drivers of ecological degradation. We assessed the effects of the invasive plant Vincetoxicum rossicum and its removal on understory plant communities, as well as whether these effects were mediated by deer and invasive earthworms. We used paired and open plots with V. rossicum and an open plot without V. rossicum at each of three forested sites in New York State Parks from 2017–2020. Following a nested design, we located sets of open and fenced plots in areas where V. rossicum was experimentally removed and in areas where it was unmanaged. After three years of V. rossicum removal, V. rossicum significantly declined. However, the ultimate goal of management was native plant recovery. Contrary to this aim, native plant diversity and cover increased if V. rossicum was left unmanaged but did not change with V. rossicum removal. Thus, we provide strong evidence that reducing target species abundance does not always translate into native plant recovery. This disparity may be because deer and invasive earthworms are stronger drivers of understory plant communities than V. rossicum, as native plant cover and diversity were lower the more earthworms were in a plot and invasive plant species declined after fencing to exclude deer. Future management should thus prioritize reducing deer and earthworm impacts over invasive plant removal. Our findings underscore the need to collect data to test management assumptions: reducing the abundance of an invader may not always result in native plant recovery, especially if other stressors are the ultimate drivers of change in invaded communities.