From widespread invasion to isolated refugia: progress towards eradication of Xenopus laevis in Portugal

Résumé fourni par la source

The African clawed frog Xenopus laevis is an invasive species first detected in Portugal in 2006. It was recorded in both artificial lentic habitats and natural lotic systems in the municipalities of Oeiras and Sintra, occupying an estimated maximum area of 50 km². Given its well-documented invasive potential worldwide, an eradication program was initiated in 2010 and is still ongoing. Consistently low capture rates in recent years in the areas that have been monitored since the beginning of the eradication efforts, along with the apparent restriction of the remaining populations to a small number of largely disconnected refugia, indicate that the program has reached an advanced stage. Here we provide an assessment of the local conditions that have likely contributed to the success of eradication efforts in Portugal. We compare 24 sites where the species was successfully removed and has remained absent for five or more years with 14 sites where incomplete removal and/or recolonization have hindered success. Small shallow pools in flowing systems and artificial ponds that can be drained have proven most amenable to complete eradication. In contrast, deep natural pools and deep artificial water bodies continue to act as refugia and sources of dispersing individuals; these are the typologies where electrofishing, which is the main method used in this project to capture metamorphs, is less effective. In man-made ponds with water volumes of up to 1000 cubic meters and depths of up 1 meter, eradication is feasible using a combination of physical removal of specimens and partial or complete draining. In natural pools, volume appears to be less important than depth, with pools deeper than 1 meter showing a lower likelihood of successful eradication due to the difficulty of detecting the last remaining individuals. Another factor limiting eradication success is proximity to breeding sites: locations within 500 meters of active reproduction areas have proven particularly difficult to control, likely due to frequent recolonization. Additionally, in order 1 and 2 tributaries, hydrological distance to the headwater was an important factor; stream pools located closer to the source (100 meters or less) are more likely to be successfully cleared, likely because upstream sections can be fully screened, and also because there are few lentic structures near those sections, reducing the risk of undetected breeding populations. To eliminate the remaining populations, we propose an integrated approach combining continued removal efforts with the installation of physical barriers to prevent dispersal, targeted drainage where feasible, and reinforcement of native predator populations known to consume X. laevis eggs in natural systems.

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