J Syst Evol

• Research Article •    

Slowing taxon cycle can explain biodiversity patterns in the Pacific: insights into the biogeography of the tropical South Pacific from molecular data

Gunnar Keppel1,2,*, Francis J. Nge3,4, Thomas Ibanez2   

  1. 1 UniSA STEM and Future Industries Institute, University of South Australia, GPO Box 2471, SA 5001, Adelaide, Australia;
    2 AMAP, Université de Montpellier, CIRAD, CNRS, INRAE, IRD, Montpellier, France;
    3 IRD-Institut de Recherche pour le Développement, Avenue Agropolis BP 64501, Montpellier, 34394, France;
    4 School of Biological Sciences, Faculty of Science, The University of Adelaide, Adelaide, SA 5000, Australia
    *Corresponding author. Postal:Room F2-50, Mawson Lakes Campus, GPO Box 2471, Adelaide, 5001 SA, Australia. E-mail:gunnar.keppel@unisa.edu.au;ORCID:https://orcid.org/0000-0001-7092-6149
  • Received:2023-07-01 Accepted:2023-09-21

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 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 for 16 archipelagos in the tropical and subtropical South Pacific. We then evaluate whether results support the taxon cycle as a plausible mechanism for these observations. We find that most species in the Pacific arrived within the last 5 million years 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 to understand the dynamic nature and biodiversity patterns of Pacific Islands.

Key words: colonization, diversification, island biogeography, Pacific, phylogeography, taxon cycle