On oceanic islands, ecosystem processes hold on a limited number of species. However, their patterns of coexistence and the resulting variations of assemblage composition along environmental gradients often remain insufficiently known to set up efficient conservation measures in the face of increasing anthropogenic pressure. In particular, stronger human impacts on insular as opposed to continental habitats, as well as the massive introduction of exotic species on islands, modify the functional composition of species assemblages and may alter key ecological functions. Here, we explored the distributions of terrestrial bird species, and the resulting taxonomic and functional compositions of their assemblages, along the main habitat gradients structuring landscapes in La Réunion, an oceanic island located in the Madagascan region. We used multivariate analyses and generalized additive models to analyze a set of 372 point counts, surveyed 10 years before the establishment of La Réunion National Park. So doing, we established a reference point for the impacts of exotic species and human alterations of habitats on the island’s bird assemblages prior to the setting of an ecosystem-level conservation strategy. We found that native and exotic species overlapped at mid-elevations, but the former were tied to native habitats while the latter were associated with urban areas and man-modified landscape mosaics. Species with contrasting trophic preferences were segregated along habitat gradients. However, overall, ecological traits had a weak role in explaining variations in the composition of bird assemblages. We conclude that native and exotic species in La Réunion formed two distinct species assemblages both in terms of traits and habitat preferences, therefore unlikely to interact over available resources. However, several ongoing processes including recent introductions, continued anthropogenic pressures on habitats, and the conservation measures related to La Réunion National Park may all have recently altered these patterns. Performing further analyses of historical datasets complemented with diachronic surveys therefore appears essential to monitor the efficiency of recently-established conservation measures in favour of insular ecosystems.

This archive contains all the data and R scripts necessary to replicate the analyses performed on this article.

R, 4.1.0

QGIS, 3.12