Managing for biodiversity: impact and action thresholds for invasive plants in natural ecosystems

Abstract

Some studies examining the impacts of invasive plant species in native plant communities have demonstrated non-linear damage functions, whereby community components, such as species richness, are seemingly unaffected by the presence of an invader until it has attained relatively high levels of abundance, whereupon there is a marked decrease with further increases in abundance. Given chronic limitations in the resources available for managing invasive species, it has been argued that the most damaging invaders would be controlled most efficiently by maintaining their abundances below such threshold levels. Because many impact studies do not involve sampling over a wide range of invader abundances it is not possible to estimate the prevalence of threshold relationships. Furthermore, studies that have employed appropriate sampling methods have shown that different life forms exhibit different threshold responses, indicating that maintenance management for biodiversity values should be designed to protect the most sensitive species or groups of species. Since control costs increase with invader abundance, economic and ecological considerations are aligned when invaders are sustainably maintained at relatively low abundances. Adopting such an approach should also minimise negative impacts where damage functions are linear.

Towards a framework for understanding the context dependence of impacts of non‐native tree species

“…Nonlinearities may have critical implications for understanding impacts of non‐native species and for the optimal timing of non‐native species management (i.e. control or eradication; Panetta & Gooden, ). Here we consider evidence for impacts that occur rapidly at low population densities (termed ‘low threshold impacts’), at high invasion densities ('high threshold impacts'; Yokomizo, Possingham, Thomas, & Buckley, ), or impacts that have a unimodal relationship with density (Figure ).…”

Non‐native plants have greater impacts because of differing per‐capita effects and nonlinear abundance–impact curves

“…Understanding the relationship between abundance and impact can optimise strategies for the prevention and management of potential invaders that are likely to become abundant vs. those likely to exert large per-capita effects on recipient systems (Sofaer et al 2018). Likewise, a focus on abundance-impact relationships aligns with long-standing theories on community importance (Power et al 1996;Wootton & Emmerson 2005) and makes available predictive tools for the management of invasive species (Yokomizo et al 2009;Panetta & Gooden 2017). Uncovering whether and how per-capita effects and abundances of nonnative species differ from those of native species can not only place the impacts of non-native species in context but can also clarify the role of eco-evolutionary history on ecological dynamics of invaded systems.…”

Chemical options for the control of Siam Weed (Chromolaena odorata) in natural ecosystems

“…There are now effective treatment options across sites and applicable to different densities, manager preferences and cost structures/resources. Sustained maintenance of infestations of this weed at low cover levels should enable the protection of the biodiversity values at least cost (Panetta & Gooden ).…”