Assessing eradication feasibility of invasive aquatic plants by manual removal in Wallonia

Résumé fourni par la source

Crassula helmsii, Hydrocotyle ranunculoides, Lagarosiphon major, Ludwigia grandiflora, L. peploides, Myriophyllum aquaticum and M. heterophyllum are among the invasive alien aquatic plants (IAAPs) most frequently recorded in ponds and lakes in Wallonia (southern Belgium). All are invasive alien species of Union concern under Regulation (EU) No 1143/2014. Current invasion pressure remains limited: fewer than 100 invaded sites per species have been recorded regionally, and the invaded surface area is <1,000 m² in 80% of field cases. Repeated manual removal (hand pulling) is often prioritized by field managers because it is simple to implement and generally the least disruptive option for ecosystem services (De Baene 2022). However, field trials indicate that some populations cannot be eradicated by manual removal even after several years of effort, and that effectiveness varies with species, invasion extent and site characteristics (Branquart et al. 2024). To support management decision‑making, we developed a feasibility index referring to the expected success rate of manual removal actions targeting a clearly delineated IAP population scored on a 5-point scale (= management effectiveness sensu Booy 2017). It combines IAAP identity, invaded area and the number of local constraints recorded from the field before starting to remove plants. IAAPs were classified into four categories according to their resilience to management, based on field managers’ experience and the scientific literature. Management resilience was found to be lowest for Hydrocotyle ranunculoides and highest for Crassula helmsii , for which only 10% of published studies reported successful eradication (van der Loop et al. 2018). Invaded area was assessed in summer, when the target plants reach their maximum extent. Finally, eight key constraints to manual eradication were identified through a survey of field managers (e.g. rooting depth, bank morphology, sediment thickness, intermixing with native helophytes, and co‑occurrence of Crassula helmsii ). Eradication outcomes from the LIFE RIPARIAS project were positively associated with the feasibility index determined prior to management initiation. Applying the index across invaded waterbodies in Wallonia suggests that approximately 70% of sites can be managed effectively through manual removal, whereas the remaining sites (mostly invaded by Crassula helmsii and Ludwigia spp.) likely require alternative control methods (tarping, mechanical removal or biological control) or containment measures.

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