A collaboration between the Laboratoire des Sciences du Climat et de l’Environnement (CEA-CNRS-UVSQ) and the Bureau de Recherches Géologiques et Minières has just published a study in the journal Science Advances which highlights the environmental consequences of changes in land use on the island of Mayotte as a result of increasing demographic and migratory pressures.
Mayotte has experienced strong demographic growth since it became a French department in 2011. This is partly due to a large influx of migrants from neighboring islands. The resulting overpopulation has led to profound changes in land use, with uncontrolled urbanization of the island, land use changes including accelerated deforestation, and the abandonment of traditional agricultural practices (the ‘mahorais garden’, a type of agroforestry) in favor of intensive banana and cassava monocultures. These changes are leading to significant environmental pressures. The erosion is accelerating, increasing the supply of sediment to the reservoirs used to provide the island’s drinking water and, ultimately, the lagoon. The filling of these reservoirs by excessive sedimentation is reducing their storage capacity, in the context of successive water crises since 2016. Deforestation and changes in land use are also threatening the unique biodiversity of this island and increasing the risk of natural disasters (e.g. floods, landslides).
In this study, sediment cores were collected from the Dzoumogné reservoir, one of the main reservoirs on the island. These sedimentary archives were used to reconstruct the impact of recent human pressures on the degradation of water and soil resources by combining an original approach using environmental DNA analysis, sediment tracing, and retro-observation of erosion.
The main results of this study highlight the occurrence of two periods of accelerated soil erosion: the first just after the island’s departmentalization from 2011 to 2015, followed by a second increase between 2019 and 2021, due to sustained migratory and demographic pressures. The decrease in erosion rate between these two periods is associated with a reduction in rainfall, which plunged the island into the water crises of 2016 and 2017. Environmental DNA analyses have revealed changes in biological communities before/during and after these changes in land use. These changes have increased connectivity between parts of the landscape that were previously little affected by erosion (e.g. forest, traditional crops) and the reservoir. These measurements also highlight the impact of the intensification of agricultural practices and the decrease of the reservoir water level after 2016 on water quality degradation. Urgent conservation measures are needed to avoid major socio-environmental crises and protect the resources of this area for future generations.
Deforesting, burning, and clearing agricultural land for banana monoculture – © Olivier Evrard
Study carried out in the framework of the LESELAM project (Lutte contre l’Erosion des Sols et l’Envasement du LAgon de Mayotte), financed by Europe (European Agricultural Fund for Rural Development), the French State (Contrat de Convergence), the Agence Régionale de la Santé de Mayotte and the BRGM.
Link to the publication: Uncontrolled deforestation and population growth threaten a tropical island’s water and land resources in only 10 years