Lines in the land: a review of evidence for eastern Australia's major biogeographical barriers to closed forest taxa

Abstract

The influence of climatic changes occurring since the late Miocene on Australia's eastern mesic ecosystems has received significant attention over the past 20 years. In particular, the impact of the dramatic shift from widespread rainforest habitat to a much drier landscape in which closed forest refugia were dissected by open woodland/savannah ecosystems has long been a focal point in Australian ecology and biogeography. Several specific regions along the eastern coast have been identified previously as potentially representing major biogeographical disjunctions for closed forest taxa. Initially, evidence stemmed from recognition of common zones where avian species/subspecies distributions and/or floral communities were consistently separated, but the body of work has since grown significantly with the rise of molecular phylogeographic tools and there is now a significant literature base that discusses the drivers, processes and effects of these hypothesised major biogeographical junctions (termed barriers). Here, we review the literature concerning eight major barriers argued to have influenced closed forest taxa; namely, the Laura Basin, Black Mountain Corridor, Burdekin Gap, Saint Lawrence Gap, Brisbane Valley Barrier, Hunter Valley Barrier, Southern Transition Zone and East Gippsland Barrier. We synthesise reported phylogeographical patterns and the inferred timing of influence with current climatic, vegetation and geological characteristics for each barrier to provide insights into regional evolution and seek to elicit common trends. All eight putative biogeographical barriers are characterised currently by lowland zones of drier, warmer, more open woodland and savannah habitat, with adjacent closed forest habitats isolated to upland cool, wet refugia. Molecular divergence estimates suggest two pulses of divergence, one in the early Miocene (~20–15 Mya) and a later one from the Pliocene–Pleistocene (~6–0.04 Mya). We conclude with a prospectus for future research on the eastern Australian closed forests and highlight critical issues for ongoing studies of biogeographical barriers worldwide.

Predominant colonization of Malesian mountains by Australian tree lineages

“…At the same time, temperatures rose again and moist tropical habitats expanded in Australia until the mid‐Miocene climatic optimum. Global cooling since the mid‐Miocene (c. 14 Ma) was offset in Australia by the continued rafting towards the equator, at least in the north but intensive aridification occurred, leading to an overall contraction of mesic biomes, which remained mostly in montane refugial areas along the eastern coast of the continent and in eastern New Guinea, where mountains had been present since the early Oligocene (Bryant & Krosch, ; Byrne et al, ; Macphail, ; Martin, ; Quarles van Ufford & Cloos, ).…”

“…The nature of habitats containing the lineages that came to colonize Malesian mountains and nutrient‐poor soils, is more obscure because of the complex plate‐tectonic and climatic history of Australia since the Oligocene that caused recurrent expansions and contractions of different forest types. Nevertheless, there are indications that moist habitats were mostly present in colder upland areas, a least since the middle Miocene (Bryant & Krosch, ; Byrne et al, ; Martin, ), providing the nearest source population of plants adapted to Malesian mountain habitats.…”

Perpendicular axes of differentiation generated by mitochondrial introgression

“…). These two barriers have been implicated in subspeciation in several bird species and other closed‐forest taxa including invertebrates, lizards, frogs, mammals and plants (Ford ; Schodde & Mason ; Schodde ; Bryant & Krosch ).…”

A revision of Gyrocochlea-grade Charopidae from mid-eastern Queensland and redescription and generic reassignment of three Gyrocochlea-grade species (Eupulmonata: Charopidae)

“…Mid-eastern Queensland is separated from the humid, wet tropics and the dry to moist subtropical forests of SEQ by two dry corridors of open eucalypt forest: the Burdekin-Lynd Gap in the north and the St Lawrence Gap in the south. This has isolated the region into refugial pockets of rainforest (Webb & Tracey 1981;Bryant et al 2016) and probably affected subsequent charopid distribution including that of the Gyrocochleagrade species. The nine species dealt with in this study represent one known genus and seven new genera including the re-assignment of the three previously known 'Gyrocochlea' species from this area.…”