Disentangling the multiple factors controlling species diversity is a major challenge in ecology. Island biogeography and environmental filtering are two influential theories emphasizing respectively island size and isolation, and the abiotic environment, as key drivers of species richness. However, few attempts have been made to quantify their relative importance and investigate their mechanistic basis. Here, we applied structural equation modelling, a powerful method allowing test of complex hypotheses involving multiple and indirect effects, on an island‐like system of 22 French Guianan neotropical inselbergs covered with rock‐savanna. We separated the effects of size (rock‐savanna area), isolation (density of surrounding inselbergs), environmental filtering (rainfall, altitude) and dispersal filtering (forest‐matrix openness) on the species richness of all plants and of various ecological groups (terrestrial versus epiphytic, small‐scale versus large‐scale dispersal species). We showed that the species richness of all plants and terrestrial species was mainly explained by the size of rock‐savanna vegetation patches, with increasing richness associated with higher rock‐savanna area, while inselberg isolation and forest‐matrix openness had no measurable effect. This size effect was mediated by an increase in terrestrial‐habitat diversity, even after accounting for increased sampling effort. The richness of epiphytic species was mainly explained by environmental filtering, with a positive effect of rainfall and altitude, but also by a positive size effect mediated by enhanced woody‐plant species richness. Inselberg size and environmental filtering both explained the richness of small‐scale and large‐scale dispersal species, but these ecological groups responded in opposite directions to altitude and rainfall, that is positively for large‐scale and negatively for small‐scale dispersal species. Our study revealed both habitat diversity associated with island size and environmental filtering as major drivers of neotropical inselberg plant diversity and showed the importance of plant species growth form and dispersal ability to explain the relative importance of each driver.

This paper presents the field research station of the Muséum national d'Histoire naturelle (MNHN) at Saint-Eugène, through its history, its natural environment, and its equipping. Main habitats encountered on and around the Petit Saut reservoir are described, together with their evolution following dam flooding. A general map of the Saint-Eugène area shows ali the MNHN study sites (mainland and islands). A table summarizes their characteristics of surface and isolation, as weil as main topics under study as yet. R É SUM ÉL'article présente la station de terrain du Muséum national d'Histoire naturelle (MNHN) à Saint-Eugène, à travers son historique, son environnement naturel et ses aménagements. Les principaux habitats rencontrés sur et en bordure de la retenue du barrage de Petit Saut, ainsi que leur évolution consécutive à la mise en eau du barrage, sont décrits. Une carte générale de Saint-Eugène présente l'ensemble des sites d'étude (îles et terre ferme) du MNHN. Un tableau résume leurs caractéristiques de surface et d'isolement, ainsi que les centres d'intérêt dont ils ont fait 1' objet.

Granitic outcrop vegetation was compared in 22 inselbergs of French Guiana, South America, using RLQ and fourth-corner analyses to identify the main relationships between environmental gradients and plant traits. At the scale of the whole territory the distribution of species and species traits was mostly driven by a spatially-structured gradient embracing regional climate (annual rainfall), forest matrix (canopy openness), and inselberg features (altitude, shape, habitats, summit forest, degree of epiphytism, fire events). Biogeographic, environmental and past historical factors contribute to explain the variation observed at coarse scale and two groups of inselbergs are identified. A first group occupies the southern peneplain in a semi-open forest matrix and exhibits a higher representation of suffrutescent species and climbers, a lower representation of upright shrubs, a lower degree of Guiana Shield endemism, and a higher incidence of human use and autochory. All these features suggest an adaptation to more disturbed environments linked to past climate changes and savannization and to human influences. A second group, characterized by opposite plant traits, occupies the northern part of French Guiana and the far south within a closed forest matrix. Within archipelagos (inselbergs at less than 7 km distance), C-score and Mantel tests revealed a random co-occurrence of plant species and an increase of floristic dissimilarity with distance without any concomitant change in plant traits, respectively, suggesting that spatially-structured stochastic factors (limitation by dispersal) were the driving force of vegetation change at fine scale.

Les effets précoces de la fragmentation forestière sur les communautés de petits vertébrés terrestres (lézards et micromammifères) et volants (oiseaux et chauves-souris frugivores), à la suite de la mise en service du barrage de Petit Saut en Guyane française, ont été étudiés. La structure et la composition spécifique des communautés ont été rapidement modifiées dans la zone fragmentée. L'impact de la fragmentation sur l'abondance et la richesse spécifique est contrasté selon les groupes zoologiques. En revanche la chute de diversité spécifique sur les petites îles est manifeste, un an seulement après l'inondation pour les lézards, les micromammifères terrestres et les chauves-souris, et deux ans après pour les oiseaux. Les peuplements insulaires sont dominés le plus souvent par un faible nombre d'espèces généralistes (Kentropyx calcarata pour les lézards, Proechimys cuvieri pour les petits rongeurs, et Artibeus obscurus pour les chauves-souris frugivores). Ces résultats montrent l'impact immédiat de la fragmentation forestière en région tropicale sur les communautés de petits vertébrés et complètent d'autres travaux comparables menés en région néotropicale, dont une courte revue est présentée. Ils soulignent également l'importance de maintenir un pôle de recherche actif sur les problèmes de conservation en Guyane française, eu égard à la valeur patrimoniale exceptionnelle de ces milieux.

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