Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Introduced mangroves in the Society Islands, French Polynesia (South Pacific): invasive species or novel ecosystem?

Abstract

Biotic mixing, where introduced and native species mix into a newly formed ecosystem, is an increasing phenomenon worldwide. It particularly affects islands, where the number of introduced plants often exceeds native ones, and where invasive alien species are one of the main threats to terrestrial biodiversity. The mangrove tree species Rhizophora stylosa, first introduced in French Polynesia (South Pacific) in the early 1930s, is now well established and found in nearly all the inhabited islands of the Society archipelago (Bora Bora, Huahine, Moorea, Raiatea, Tahaa, Tahiti). It forms dense thickets on coastal habitats and estuaries, and gradually colonizes native tidal grassland marshes and nearshore rocky habitats. In order to assess its current expansion, we mapped its distribution from satellite images interpretation and ground-truthing fieldwork. Results indicate that mangroves currently cover a total of 41 ha in the Society Islands. Expansion rate seems relatively slow, particularly in Moorea where only 2.8% of the coastline is colonized over 90 years after its introduction. A preliminary study of the socio-cultural perception of mangroves was undertaken through an online questionnaire sent to local stakeholders and NGOs. It revealed that although more than 70% of the respondents recognized R. stylosa as an introduced species, 40% considered that it had positive impacts and only 20% that it was a harmful species. A more detailed ecological and socio-economical cost-benefit analysis is still needed to provide clearer conclusions on the invasive status of mangroves in the Society Islands, and the most appropriate management strategies to be implemented by local authorities in collaboration with island communities.