Sunda–Sahul floristic exchange and pathways into the Southwest Pacific: New insights from wet tropical forest trees

Abstract

Aim Recent investigations on the floristic exchange between Southeast Asia and Australia have shown a clear dispersal directionality bias (West to East) of wet‐adapted plant taxa. However, dispersal routes and directions of wet forest taxa into the South Pacific remain insufficiently known. We here aimed to establish the most likely routes and directions of plant dispersal into the Southwest Pacific islands. Location Southeast Asia, East Asia, Australia, Southwest Pacific. Taxon Dysoxylum s.l. (Meliaceae). This includes Dysoxylum s.s., Didymocheton, Epicharis, Goniocheton, Pseudocarapa, and Prasoxylon. Method We sampled 75% of the species diversity in Dysoxylum s.l., covering the entire distribution range, all genera and major lineages. Phylogenetic relationships of 149 accessions were reconstructed using Bayesian Evolutionary Analysis and two internal constraints. The dispersal–extinction–cladogenesis variant, founder‐event speciation (DEC+J), was used for reconstructing the biogeographic history, and 100 Biogeographical Stochastic Mappings were simulated. Results Dysoxylum s.l. originated and firstly diversified in the western part of its current distribution range (including Indochina) during the Miocene to Pliocene, followed by an overall eastern range expansion towards Malesia, Australia and the Southwest Pacific in the Pliocene. Main Conclusions The south‐eastward expansion of lineages into Wallacea and Australia is in temporal agreement with the convergence of the Asian and Australian tectonic plates since the Miocene. Long‐distance dispersal is the main mechanism that led to the current distribution. Two dispersal pathways into the Southwest Pacific are identified, (1) through New Guinea and the Solomon Islands to Fiji, and (2) from New Zealand to Fiji. For both routes, Fiji was an important secondary source area for dispersal into the Southwest Pacific.

Early diversification dynamics in a highly successful insular plant taxon are consistent with the general dynamic model of oceanic island biogeography

“…3) similar to what Roalson & Roberts (2016) recovered. It should be noted that the arrival of Cyrtandra in the Pacific (7.57 Ma; 6.57-11.10, 95% HPD) dates back about the same time as the emergence of most Pacific islands (see Holzmeyer et al, 2023 for ages of Pacific islands). Our results are consistent with the predictions of the island immaturity speciation pulse (IISP) model of island evolution (Whittaker et al, 2007).…”

“…Studies have supported three main routes for Southeast Asian plant evolution crossing these biogeographic lines (Holzmeyer et al, 2023). First, wet forest-adapted taxa predominantly follow a west-to-east dispersal either from Mainland Asia into Wallacea (Richardson et al, 2012;Chen et al, 2014;Matuszak et al, 2016) or second even farther beyond to New Guinea and Australia (Muellner et al, 2008(Muellner et al, , 2009Nauheimer et al, 2012;Thomas et al, 2012;Grudinski et al, 2014;Favre et al, 2016;Matuszak et al, 2016;Yap et al, 2018;Holzmeyer et al, 2023). A third route, albeit limited, is taken from Australasia westwards predominantly by dry forest-adapted taxa (Cruaud et al, 2011;Richardson et al, 2012;Chen et al, 2014;Johnson et al, 2017) and less so by wet forest-adapted taxa (Hauenschild et al, 2018).…”

“…2), namely, (i) Wallace′s line which passes between the Philippines and Sulawesi and Bali and Lombok, (ii) Huxley′s line which modified Wallace′s line separating Palawan from the rest of the Philippines, and (iii) Heilprin-Lydekker Line further east delimiting a region called Wallacea, comprising Sulawesi, the Moluccas, the Philippines (excluding Palawan), and the Lesser Sunda Islands (Wallace, 1860;Huxley, 1868;Heilprin, 1887;Lydekker, 1896). Studies have supported three main routes for Southeast Asian plant evolution crossing these biogeographic lines (Holzmeyer et al, 2023). First, wet forest-adapted taxa predominantly follow a west-to-east dispersal either from Mainland Asia into Wallacea (Richardson et al, 2012;Chen et al, 2014;Matuszak et al, 2016) or second even farther beyond to New Guinea and Australia (Muellner et al, 2008(Muellner et al, , 2009Nauheimer et al, 2012;Thomas et al, 2012;Grudinski et al, 2014;Favre et al, 2016;Matuszak et al, 2016;Yap et al, 2018;Holzmeyer et al, 2023).…”