Quicklime-based eradication attempt of Xenopus laevis as a model for controlling pondscape invasions

Abstract

Aquatic non-native invasive species are notoriously difficult to eradicate, particularly in pondscapes where populations can spread rapidly, persist in unmanaged refugia, and recolonise treated sites. In such contexts, high-intensity management interventions may be justified, balancing short-term collateral impacts against the prevention of permanent establishment and long-term damages. Chemical eradication methods, such as rotenone or herbicide application, can be effective but raise ethical and environmental concerns. Here, we evaluate quicklime (calcium oxide, CaO) application as a more sustainable alternative control tool for pondscape invaders compared to other chemical methods, using the African clawed frog (Xenopus laevis) invasion in Belgium as a case study. When applied to water, quicklime hydrates exothermically to calcium hydroxide (Ca(OH)), which releases OH ions upon dissolution, temporarily and rapidly increasing pH to lethal levels. In winter 2023, three ponds with breeding populations of X. laevis of low ecological value were drained, fenced, and treated with quicklime. Treatment effectiveness was assessed through pH measurements, visual surveys, and environmental DNA (eDNA) quantification. Immediately after treatment, large numbers of deceased post-metamorphic individuals were recovered, indicating treatment-induced mortality. Eight weeks post-treatment, eDNA concentrations were markedly lower in two of the three ponds (reductions of 100% and 80%) compared to those during the same period one year later. Although eDNA concentrations increased again during the following summer suggesting partial population recovery through survival and/or recolonisation, they remained lower than pre-treatment conditions. Water pH returned to near baseline levels within one month. We provide the first field-based preliminary evidence that quicklime can induce large-scale mortality in X. laevis populations in small to medium-sized ponds. We discuss practical considerations, limitations, and broader applicability, proposing quicklime as a high-intensity option within integrated management strategies for pondscape invaders.