Data and code from: Unravelling the drivers of island species richness in tropical savannas

Despite their ecological and conservation relevance and their potential to
advance our understanding of species–habitat relationships, natural island
habitats in seasonal tropical terrestrial systems remain inadequately
explored. In particular, the processes governing species diversity in
these environments are still poorly understood. Here, we examine how
island size, geographical isolation, and habitat heterogeneity and
availability affect species richness in campos de murundus—literally
“fields of earth mounds”—a distinctive ecosystem within South American
tropical savannas. Our study targets three major biological groups that
dominate and structure murundu communities—trees, herbs, and termites—and
is based on an extensive inventory of these taxa across 373 murundu
islands sampled within eleven 1-ha plots distributed throughout the vast
seasonal floodplains of east-central Brazil. Bayesian mixed-effects models
indicated that tree and herb species richness increased with murundu
island size, consistent with predictions from island biogeography theory.
By contrast, neither isolation nor environmental heterogeneity or habitat
availability exerted detectable effects on the species richness of trees,
herbs, or termites at the murundu island scale. At the landscape scale,
tree alpha diversity was largely driven by metacommunity attributes
directly associated with landscape habitat amount, increasing with total
abundance and declining with beta diversity. In contrast, termite species
richness was weakly explained by the environmental variables considered,
showing no clear association with island size, isolation, or environmental
heterogeneity. Overall, island size accounted for most of the explained
variation in plant species richness, whereas termite assemblages were more
strongly associated with spatial eigenvectors at intermediate and fine
spatial scales. We conclude that, for woody and non-woody plant
communities in hyperseasonal savannas, island species richness is
primarily determined by murundu island size and habitat amount, with
little evidence of dispersal limitation or strong influences of
environmental heterogeneity. At the landscape scale, tree species richness
did not respond directly to habitat amount; however, total abundance and
gamma diversity increased with habitat amount, which in turn resulted in
higher alpha diversity. Beta diversity appears to be more closely linked
to the nested composition of species on small islands within larger ones
than to spatial turnover. In contrast, termite communities are only weakly
structured by the predictors tested, suggesting that stochastic processes,
local habitat constraints, and species-specific nesting behaviours play a
more prominent role.