Slowing taxon cycle can explain biodiversity patterns on islands: Insights into the biogeography of the tropical South Pacific from molecular data

Abstract

Islands in the tropical Pacific Ocean are renowned for high biodiversity and endemism despite having relatively small landmasses. However, our knowledge of how this biodiversity is formed remains limited. The taxon cycle, where well‐dispersed, earlier colonizers become displaced from coastal to inland habitats by new waves of colonizers, producing isolated, range‐restricted species, has been proposed to explain current biodiversity patterns. Here, we integrate the outcomes of phylogenetic studies in the region to investigate the sources, age, number of colonizations, and diversification of 16 archipelagos in the tropical and subtropical South Pacific. We then evaluate whether the results support the taxon cycle as a plausible mechanism for these observations. We find that most species in the Pacific arrived less than 5 Mya from geographically close sources, suggesting that colonization by new taxa is a frequent and ongoing process. Therefore, our findings are broadly consistent with the theory of the Taxon Cycle, which posits that ongoing colonization results in the gradual displacement of established lineages. Only the oldest archipelagos, New Caledonia and Fiji, do not conform to this trend, having proportionally less recent colonization events, suggesting that the taxon cycle may slow on older islands. This conclusion is further validated by New Caledonia having lower diversification rate estimates than younger islands. We found that diversification rates across archipelagos are negatively correlated with area and age. Therefore, a taxon cycle that slows with island age appears to be a suitable concept for understanding the dynamic nature and biodiversity patterns of the Pacific Islands.

Flying snails: immigrant selection and the taxon cycle in Pacific Island land snails

“…Unfortunately, requisite fossil and phylogenetic data are lacking for many PILS. For example, a recent multi-archipelago, multi-taxon phylogenetic examination of the taxon cycle identified only four land snail genera from two families with sufficient phylogenetic data for inclusion in analyses (the bulimulid genus Naesiotus and partulid genera Eua, Partula, and Samoana; Keppel et al 2023). Given the state of knowledge, this key assumption that widespread, SAE, and SIE species represent a chrono-sequence is largely inescapable.…”

“…The taxon cycle explains this seeming contradiction: because they lack local adaptations, widespread species incur a competitive disadvantage in more favorable highelevation habitats. This competitive disadvantage may increase with island age, as greater niche filling interferes with colonization and diversification by new taxa (Keppel et al 2023). Thus, the taxon cycle -but not immigrant selection alone -predicts that widespread species would be more common at lower elevations (Ricklefs andBermingham 2002, Matos-Maravi 2020).…”