Impact of the platyhelminthe Obama nungara invasion on earthworm communities in France – PLATWORM

In order to better understand the impact of the Obama nungara flatworm on soils and the fauna that lives there, the PLATWORM project combined several complementary approaches, ranging from field observation to digital modelling. Collection campaigns were carried out in various gardens and green areas in France to study the conditions favouring the presence of this predator, as well as the composition of local earthworm and gastropod communities. The behaviour and diet of O. nungara were then observed in the laboratory, thanks to breeding carried out under controlled conditions, which made it possible to monitor predation, reproduction and survival of individuals.

To identify the prey consumed, a method of genetic analysis of the DNA present in the digestive contents of flatworms was used. This approach, known as “metabarcoding”, makes it possible to accurately determine the species of earthworms and gastropods ingested, based on DNA traces. To do this, new molecular tools were developed to detect gastropods, while existing tools were used for earthworms.

Experiments in mesocosms, i.e. experimental structures that reproduce natural soil conditions, have made it possible to assess the effect of O. nungara on soil structure, porosity, water infiltration and earthworm activity.

Finally, an original ecological model was used to explore the medium- and long-term impact of this species' invasion on soil functioning. Based on simple rules governing relationships between species and between species and the environment, as well as on contrasting climate scenarios, this model reveals the different possible trajectories of the ecosystem and thus anticipates the risks associated with the expansion of the flatworm.

The PLATWORM project has provided a better understanding of the conditions that favour the establishment of the Obama nungara flatworm in France, its impact on soil biodiversity and the risks to ecosystem functioning. Field observations have shown that this species mainly establishes itself in gardens and urban areas where shelter is available, such as under stones, planks or flower pots. Monitoring carried out over several years has revealed populations at different stages of invasion – from recent introduction to the expansion phase – illustrating the rapid spread of the species across the territory.

Experimental and modelling data confirm the high invasive potential of O. nungara, which is able to withstand drought and survive despite eradication attempts. In the laboratory, researchers have observed that this flatworm can live for several months without feeding and produce offspring even after long periods of isolation. These abilities make it a particularly resistant and difficult-to-eradicate alien species.

Predation tests have revealed that O. nungara is an opportunistic predator: it feeds on earthworms as well as snails, slugs and other flatworms, without any marked preference. In an average garden, a single individual could consume several thousand prey items per year. Genetic analyses of digestive contents have identified more than sixty species of prey, including numerous earthworms. These analyses have also shown that the flatworm's diet reflects the diversity of locally occurring species, demonstrating its ability to efficiently exploit available resources.

Experiments conducted in mesocosms have shown that the presence of O. nungara alters soil structure, reducing its porosity, aeration and water infiltration capacity. These changes are likely to affect soil quality and ecological functions in the long term.

Finally, modelling of ecological processes has made it possible to assess the possible consequences of this invasion. The results suggest that soil communities can remain stable as long as climatic conditions remain favourable. However, in the event of prolonged drought, certain earthworm species may disappear locally, leading to lasting imbalances in soil functioning.

The results of the PLATWORM project open up numerous prospects for research and management of invasive soil species.

The amount of data collected on earthworms provides a unique basis for better understanding their distribution in France, depending on soil type. By cross-referencing DNA analysis data with soil characteristics and the presence of other species, it will also be possible to examine in greater detail the predatory pressure exerted by the flatworm Obama nungara on soil organisms, depending on local conditions.

Further work will aim to understand why certain species of earthworms or gastropods are more vulnerable than others. The aim is to identify the physical or behavioural traits that make a species more susceptible to attack by O. nungara. This research combines the study of animal behaviour in the laboratory with the analysis of databases on their ecological characteristics.

Finally, the development of a predictive model of soil functioning is a major advance. By integrating data from the project, this model will make it possible to predict the impact of predators such as O. nungara on soil health and fertility. This type of tool could be used in the future to anticipate the consequences of other biological invasions and help to better protect soil biodiversity, which is essential to the proper functioning of ecosystems.

The increase of international trade in recent decades, as well as insufficient control for the transport of exotic species, have resulted in the accidental introduction of many animal and plant species. One of the most significant ecological impacts of introduced species is predation on native species. Predation can indeed act as a major mechanism of species extinction in invaded communities, affecting as a consequence the whole ecosystem functioning. For instance, the introduction of terrestrial platyhelminthes, recognized as superpredators of soil invertebrate fauna, may represent a threat to earthworms and the numerous ecosystem services that they provide. By significantly modifying the physical, chemical and biological properties of the soil profile; earthworms indeed play a key role in determining the functioning and the biodiversity of the whole ecosystem, as they influence the habitat and activities of many other organisms (plants, animals and micro-organisms). The aims of the PLATWORM project are to delineate, and to predict, the consequences that modifications of earthworm communities, under the effect of a new predation pressure, can have on the functioning of the soil ecosystem, in anthropized environment.
In France, the presence of 10 potentially invasive Platyhelminthe species has recently been reported. The most common, Obama nungara, a generalist predator of soil invertebrates, is now present in 70 departments. In the PLATWORM project, a comprehensive approach will be developed for the study of the impact on soil functioning of the presence of this introduced species. Fundamental knowledge on the disturbance of biodiversity and the modification of the functioning of the soil ecosystem following the introduction of this predator will be acquired. To achieve its objective, the PLATWORM project propose an innovative multidisciplinary approach combining mesocosm experiments, metagenomics, community ecology, study of soil properties and participatory science. All the biological, ecological and human-related data that will be produced by the project will allow modeling the impact of terrestrial platyhelminthes on soil functioning. The establishment of a field-validated ecosystem model will allow both a broad understanding of the targeted ecosystem and predicting potential long-term trajectories of ecosystems invaded by O. nungara. This will therefore provide key management guidance for mitigation actions.