![]() stylosa, including its expansion pace and possible disruptions on habitats, species, processes and usages that make up the natural coastal ecosystems of Society Islands. However, additional research is needed to assess the detailed ecological and social impacts of the introduction of R. This mapping exercise has brought forth for the first time the exact location of all introduced mangroves within the Society Archipelago. #French coloree professionalThis estimate was ground-truthed during a field campaign with the help local volunteers, professional tourist guides, decision-makers, and local scientists, which allowed to gauge local perception on this relatively recently introduced species, and to set up a network of volunteers to participate in the national ROM. The islands with the largest areas of mangroves are Huahine, Tahaa and Raiatea, with about 12, 10 and 8 ha respectively. This mapping exercise, carried out by photo-interpretation of Google Earth® images, has led to the first estimate of mangrove area in French Polynesia: in 2019, mangroves encompassed a little over 40 hectares scattered on 6 islands within the Society Archipelago. In this context, the IUCN French Committee, the Federation of Environmental Protection Associations (FAPE or Te Ora Naho) and the University of Nantes have joined forces to comprehensively map introduced mangroves in French Polynesia, under the aegis of the French Mangroves Monitoring Network (ROM). ![]() Should it be considered as potentially beneficial for the ecosystem services it provides, as is the case in areas where it is naturally occurring, or should it be seen as potentially harmful to native biodiversity as an invasive alien species? Up to now, the lack of comprehensive scientific data on the exact extent of mangroves in French Polynesia prevented an accurate overview of the situation. stylosa in French Polynesia is unusual, since its voluntary introduction raise legitimate questions on the way it should be managed. Whilst international trends are towards reinforcing the protection of mangroves, the situation of R. This expansion has possibly occurred at the expense of other natural ecosystems, particularly the so-called ‘sub-mangrove’ which includes several native coastal species that fill a similar ecological role, but also Paspalum vaginatum salt marshes. Since its introduction in the early 1930s in Moorea, French Polynesia for oyster farming, the Rhizophora stylosa mangrove species has naturalized, and then spread to almost the entire Society Archipelago (islands of Tahiti, Huahine, Raiatea, Tahaa, and Bora-Bora). From this perspective, it is necessary to gain a more in-depth understanding of the specific factors that promote the success of Avicennia germinans seeding. Therefore, it would be appropriate to redirect conservation policies towards reforestation efforts in the Saloum rather than in Casamance and to focus these actions on the perpetuation of Avicennia germinans rather than Rhizophora mangle, which has no difficulty in reproducing. ![]() In the long term, these regeneration gaps may prevent the mangrove from colonising the upper tidal zones in the Saloum. Furthermore, this study confirms, on a regional scale, observations in the literature noting the lack of Avicennia germinans reproduction on a local scale. In the Saloum, Rhizophora mangle is undergoing a process of densification in mangroves and appears to reproduce well in both regions. ![]() In Casamance, a colonisation process is underway. However, the dynamics of zonation differ between the two main mangrove hydrosystems of Senegal. ![]() Over the last 20 years, the mangroves of Senegal have increased in surface area. The accuracy of the discriminations was evaluated with kappa indices, which were 0.8 for the Saloum delta and 0.83 for the Casamance estuary. Using Landsat ETM+ and Landsat 8 OLI images merged to a 15-m resolution with a pansharpening method, a processing chain that combines an OBIA approach and linear spectral unmixing was developed to detect changes in mangrove zonation through a diachronic analysis. This study presents a method for mapping plant formations to monitor and study changes in zonation within the mangroves of Senegal. The mangrove areas in Senegal have fluctuated considerably over the last few decades, and it is therefore important to monitor the evolution of forest cover in order to orient and optimise forestry policies. ![]()
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