Unravelling the drivers of island species richness in tropical savannas

1. Despite their ecological and conservation significance and potential to enrich our understanding of species and habitat dynamics, natural island habitats in seasonal tropical terrestrial environments remain poorly studied. In particular, the mechanisms regulating species diversity in these systems are largely unresolved. 2. We examined how island area, geographic isolation and habitat heterogeneity and availability influence species richness in campos de murundus—‘fields of earth mounds’—a distinctive ecosystem of South American tropical savannas. We analysed three key biological groups that structure these systems (trees, herbs and termites) using a comprehensive inventory of 373 murundu islands sampled within 11 1-ha plots across the extensive seasonal floodplains of east-central Brazil. 3. Bayesian mixed-effects models showed that tree and herb richness increased with murundu island area, consistent with predictions from island biogeography. Geographic isolation and environmental heterogeneity had no detectable effects at the island scale. Termite richness showed weak relationships with the predictors and no clear association with area or isolation. Island area explained most variation in plant richness, whereas termite assemblages were mainly associated with spatial eigenvectors at intermediate and fine spatial scales. At the landscape scale, tree alpha diversity increased with total abundance and gamma diversity and decreased with beta diversity, suggesting nested assemblages towards larger islands. 4. These results indicate that plant richness in hyperseasonal savannas is driven primarily by murundu island area and local habitat amount, with little evidence for dispersal limitation or strong environmental filtering. Landscape patterns suggest metacommunity dynamics linked to habitat configuration and long-term ecosystem engineering processes involved in murundu formation. In contrast, termite communities appear only weakly structured by the predictors considered. 5. Synthesis. Murundu island area explains a substantial proportion of the variation in tree and herb richness, consistent with Island Biogeography Theory. On average, a 10% increase in island size corresponds to increases of seven tree species and 7.8 herb species. Termite diversity responds weakly to area and isolation, suggesting stronger roles for dispersal constraints, nest-site availability and species interactions. Landscape structure influences plant but not termite diversity.