Data from: Habitat diversity associated with island size and environmental filtering control the species richness of rock-savanna plants in neotropical inselbergs

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.