Assessing the geographic dichotomy hypothesis with cacti in South America.

The Cactaceae is one of the most conspicuous and ecologically important plant families in the world. Its species may have specialist or generalist pollination systems that show geographic patterns, which are synthesised in the Geographic Dichotomy Hypothesis. Here, we assess this hypothesis in five countries in both tropical and extratropical regions, evaluating the pollinator visitation rate and pollinator identity and abundance. We calculate the Shannon diversity index (H') and evenness (J) and evaluate differences between latitude parameters with a Student t-test. Overall, we found more specialised pollination systems in all tropical sites; the richness, diversity and evenness of pollinators was reduced in comparison to extratropical regions, where the pollination system was generalised. Our results support the geographic dichotomy hypothesis in the cacti of South America, suggesting that environmental factors underlying the latitudinal patterns can help to explain differences in the pollination syndrome between tropical and extratropical regions.