Origins of mangrove ecosystems and the mangrove biodiversity anomaly

1. Mangrove species richness declines dramatically from a maximum in the In do-West Pacific (IWP) to a minimum in the Caribbean and Western Atlantic. E xplaining this 'anomalous' biogeographic pattern has been a focus of discus sion for most of this century. 2. Two hypotheses have been put forward to explain the mangrove biodiversit y anomaly. The 'centre-of-origin hypothesis' asserts that all mangrove taxa originated in the IWP and subsequently dispersed to other parts of the wor ld. The 'vicariance hypothesis' asserts that mangrove taxa evolved around t he Tethys Sea during the Late Cretaceous, and regional species diversity re sulted from in situ diversification after continental drift. 3. Five lines of evidence are used to test between these two hypotheses. Fi rst, we review the mangrove fossil record. Second, we compare modern and fo ssil distributions of mangroves and eight genera of gastropods that show hi gh fidelity to the mangrove environment. Third, we describe species-area re lationships of mangroves and associated gastropods with respect to area of available habitat. Fourth, we analyse patterns of nestedness of individual plant and gastropod communities in mangrove forests. Fifth, we analyse patt erns of nestedness of individual plant and gastropod species. 4. All five lines of evidence support the vicariance hypothesis. The first occurrences in the fossil record of most mangrove genera and many genera of gastropods associated with mangrove forests appear around the Tethys Sea f rom the Late Cretaceous through the Early Tertiary. Globally, species richn ess in any given mangrove forest is lightly correlated with available area. Patterns of nestedness at the community and species-level both point towar ds three independent regions of diversifrcation of mangrove ecosystems: Sou th-east Asia, the Caribbean and Eastern Pacific. and the Indian Ocean regio n.