Biogeography of Hainan Island, southern China, and the falsification of the ‘shifting-landmass’ hypothesis

Abstract

A radical vicariance-biogeographical theory was proposed in 2016 to account for some of the plant taxa on China's Hainan Island. It involves the landmass undergoing significant changes in its position relative to mainland southern China between the Late Cretaceous and the present. This ‘shifting-landmass’ hypothesis has since been widely adopted and is now invoked to explain the occurrence of 17 faunal and multiple floral components (based on 14 published studies). In this paper, we review geological and biological evidence that falsify this conjecture. First, Hainan shifted no more than c. 47 km to the SE, through extension and strike-slip faulting within the Beibu Gulf Basin, principally in the Late Eocene through Oligocene. Second, our biological appraisal, using data for the claimed supportive clades plus the island's 41 amphibian and 48 land-bound mammal groups (such taxa are poor over-water dispersers and should thus inform tests of the vicariance hypothesis), offers minimal backing. Just three lineages (Goniurosaurus geckos, pheretimoid earthworms and Stedocys spitting spiders) have ‘colonization intervals’ that overlap the 70–23 Mya window that is associated with the hypothesis, as well as being sourced from Vietnam-Guangxi (the proposal's second element). Of the remaining 90 clades that have establishment ages, all but one arrived after 6.5 Mya with most since 2 Mya. The geological evidence indicates that throughout the Cenozoic, Hainan never really functioned as an island due to the frequent eustatic falls, which when combined with the surrounding shallow seabed resulted in the landmass regularly merging with Indochina and mainland southern China. This explains the preponderance of young clades, many of which are non-endemic. Furthermore, there is no obvious preference to the taxa's geographical origins, that is, tropical Indochina or temperate southern China; most are classified as ‘of indeterminate biogeographical origin’.