How can we protect biodiversity from invasive species? The example of Réunion

Réunion has a remarkable wealth of biodiversity, which is on the UNESCO World Heritage List. A third of the island is covered by natural zones free from any human activity, with a wide range of endemic plants (28% of species). Unfortunately, that biodiversity is under threat of invasion by alien species, which IPBES recognizes as the 5th largest cause of increased biodiversity loss worldwide. Some 129 invasive alien species – such as Rubus alceifolius, common gorse and black wattle – are spreading rapidly, at the expense of endemic species.

How can such invasive species be fought on the island? An action-research project led by researchers from CIRAD has been under way since 2018 to come up with an answer. Using the range of available data, the team identified 60 303 ha – almost half the vegetated area on the island – as priority conservation areas. "These areas, which are very largely inland, contain a wealth of biodiversity and/or better-quality habitats, which it is thus very important to preserve”, says Mathieu Rouget, an ecologist with CIRAD. Some 30% of those zones are currently free from invasive alien plants.

However, merely identifying priority conservation areas is not enough, and this is one of the advantages of the study: "From the outset, we designed the project with an operational aim in mind", Mathieu Rouget points out. The team went on to identify zones of even higher priority within the initial 60 303 ha. The area was split according to operational criteria, with an intervention strategy focusing on invasion fronts, accessibility, and existing operations. In the end, the study recommended restoring 4392 ha already invaded by alien plants as the top priority, and monitoring 16 019 ha of non-invaded zones.

The resulting map can be used to set realistic operational targets for players on the ground. However, it will have to be scaled up: in 2020, the local council and the Office National des Forêts (French national forestry office) intervened on 612 ha, by means of 136 operations. ”More than 93% of operations in 2020 were in priority intervention zones", says Mathieu Rouget. "However, our work has shown that further investment is required to protect the island's biodiversity more effectively."

This action-research project has demonstrated the importance of ensuring that the entire range of players – managers, decision makers and researchers – works together. ”Without the operational criteria identified thanks to players on the ground, there is a risk of producing scientific results that do not fit the situation in the field", Mathieu Rouget adds. "Managers reassess the intervention programme every year, and it now takes account of our recommendations.” 

The study falls under the umbrella of a framework agreement between CIRAD and the Réunion National Park, which are leading a working group* associating the University of Réunion, the local environment, planning and housing agency, SPL-EDDEN, the Office national des Forêts, the Conservatoire botanique national de Mascarin, the regional council and the Réunion departmental council. The working group is now seeking to improve the priority intervention maps, to optimize the work being done on the ground: future recommendations will be fuelled by more detailed vegetation maps and greater consideration of invasion dynamics.