This is a mid-altitude site that includes the steep-sided ravine of the Rivière des Marsouins, together with a lake (Le Grand Etang) created by the blockage of a valley by a lava-flow from Le Piton de la Fournaise volcano. The ravine cuts into the Bébour plateau, a forested area due west of Le Piton des Neiges, at the Caverne des Hirondelles (at 1,334 m). The site extends from this point down the river to the lowlands at 83 m, and also over the ridge to the south to include Le Grand Etang and its surroundings. Most of the site is remarkably unaltered ecologically, with extensive, rich native plant communities, especially mixed moist evergreen lowland forest; a few stands of exotic vegetation are present. The lake covers about 30 ha. No land-uses are known, although the lake and part of the ravine are accessible by road or track.
See Box and Table 2 for key species. The site is particularly important for Puffinus lherminieri (common breeding visitor), Circus maillardi (8–12 pairs in 1998) and Collocalia francica (several colonies known), but also supports a typical community of native, forest-living birds, most at notably high densities.
Non-bird biodiversity: Endemic plant communities: wide diversity of native plant species.
BirdLife International (2020) Important Bird Areas factsheet: Rivière des Marsouins - Grand Etang. Downloaded from http://www.birdlife.org on 10/07/2020.