Trees with zygomorphic flowers can persist at lower abundances in diverse communities

Abstract

Zygomorphy (bilateral symmetry) is a form of floral specialization that is thought to restrict pollination to fewer visitors, enhancing the effectiveness and precision of pollen transfer between conspecific flowers. In this way, zygomorphy may help the persistence (i.e., decrease extinction) of small and scattered populations, ensuring cross-pollination even when individuals have few conspecifics nearby. In this study, we use phylogenetic regression models to relate the abundance of 478 Andean species to the zygomorphy of their petals, zygomorphy of their reproductive organs, and shape of their corollas. Our results show that tree abundance correlates with the first trait, but not the last two. Species with zygomorphic petals occur at approximately half the average abundance of non-zygomorphic species. This pattern is particularly pronounced in species-rich mid-altitude forests. Overall, these findings support the idea that the increases in diversification rates found in zygomorphic clades may be due to decreased extinction rates, because these species can persist at lower abundances, rather than (or in addition to) increased speciation rates. More studies that explore zygomorphy in rare species using experimental and phylogenetic methods can help us understand whether and how species loss is avoided.