The integration of phylogenetics, phylogeography and palaeoenvironmental studies is providing major insights into the historical forces that have shaped the Earth's biomes. Yet our present view is biased towards arctic and temperate/tropical forest regions, with very little focus on the extensive arid regions of the planet. The Australian arid zone is one of the largest desert landform systems in the world, with a unique, diverse and relatively well-studied biota. With foci on palaeoenvironmental and molecular data, we here review what is known about the assembly and maintenance of this biome in the context of its physical history, and in comparison with other mesic biomes. Aridification of Australia began in the Mid-Miocene, around 15 million years, but fully arid landforms in central Australia appeared much later, around 1-4 million years. Dated molecular phylogenies of diverse taxa show the deepest divergences of arid-adapted taxa from the Mid-Miocene, consistent with the onset of desiccation. There is evidence of arid-adapted taxa evolving from mesic-adapted ancestors, and also of speciation within the arid zone. There is no evidence for an increase in speciation rate during the Pleistocene, and most arid-zone species lineages date to the Pliocene or earlier. The last 0.8 million years have seen major fluctuations of the arid zone, with large areas covered by mobile sand dunes during glacial maxima. Some large, vagile taxa show patterns of recent expansion and migration throughout the arid zone, in parallel with the ice sheet-imposed range shifts in Northern Hemisphere taxa. Yet other taxa show high lineage diversity and strong phylogeographical structure, indicating persistence in multiple localised refugia over several glacial maxima. Similar to the Northern Hemisphere, Pleistocene range shifts have produced suture zones, creating the opportunity for diversification and speciation through hybridisation, polyploidy and parthenogenesis. This review highlights the opportunities that development of arid conditions provides for rapid and diverse evolutionary radiations, and re-enforces the emerging view that Pleistocene environmental change can have diverse impacts on genetic structure and diversity in different biomes. There is a clear need for more detailed and targeted phylogeographical studies of Australia's arid biota and we suggest a framework and a set of a priori hypotheses by which to proceed.

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Birth of a biome: insights into the assembly and maintenance of the Australian arid zone biota.

The evolution and subsequent ecological expansion of grasses (Poaceae) since the Late Cretaceous have resulted in the establishment of one of Earth’s dominant biomes, the temperate and tropical grasslands, at the expense of forests. In the past decades, several new approaches have been applied to the fossil record of grasses to elucidate the patterns and processes of this ecosystem transformation. The data indicate that the development of grassland ecosystems on most continents was a multistage process involving the Paleogene appearance of (C3 and C4) open-habitat grasses, the mid-late Cenozoic spread of C3 grass-dominated habitats, and, finally, the Late Neogene expansion of C4 grasses at tropical-subtropical latitudes. The evolution of herbivores adapted to grasslands did not necessarily coincide with the spread of open-habitat grasses. In addition, the timing of these evolutionary and ecological events varied between regions. Consequently, region-by-region investigations using both direct (plant fossils) and indirect (e.g., stable carbon isotopes, faunas) evidence are required for a full understanding of the tempo and mode of grass and grassland evolution.

Evolution of Grasses and Grassland Ecosystems

OCBIL theory aims to develop an integrated series of hypotheses explaining the evolution and ecology of, and best conservation practices for, biota on very old, climatically buffered, infertile landscapes (OCBILs). Conventional theory for ecology and evolutionary and conservation biology has developed primarily from data on species and communities from young, often disturbed, fertile landscapes (YODFELs), mainly in the Northern Hemisphere. OCBILs are rare, but are prominent in the Southwest Australian Floristic Region, South Africa’s Greater Cape, and Venezuela’s Pantepui Highlands. They may have been more common globally before Pleistocene glaciations. Based on the premise that natural selection has favoured limited dispersability of sedentary organisms, OCBILs should have elevated persistence of lineages (Gondwanan Heritage Hypothesis) and long-lived individuals (Ultimate Self Hypothesis), high numbers of localised rare endemics and strongly differentiated population systems. To counter such natural fragmentation and inbreeding due to small population size, ecological, cytogenetic and genetic mechanisms selecting for the retention of heterozygosity should feature (the James Effect). The climatic stability of OCBILs should be paralleled by persistence of adjacent semi-arid areas, conducive to speciation (Semiarid Cradle Hypothesis). Special nutritional and other biological traits associated with coping with infertile lands should be evident, accentuated in plants, for example, through water-foraging strategies, symbioses, carnivory, pollination and parasitism. The uniquely flat landscapes of southwestern Australia have had prolonged presence of saline lakes along palaeoriver systems favouring evolution of accentuated tolerance to salinity. Lastly, unusual resiliences and vulnerabilities might be evident among OCBIL organisms, such as enhanced abilities to persist in small fragmented populations but great susceptibility to major soil disturbances. In those places where it is most pertinent, OCBIL theory hopefully lays a foundation for future research and for better informed conservation management.

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OCBIL theory: towards an integrated understanding of the evolution, ecology and conservation of biodiversity on old, climatically buffered, infertile landscapes

Part I. Introduction: 1. Mediterranean-type climate (MTC) ecosystems and fire 2. Fire and the fire regime framework 3. Fire related plant traits Part II. Regional Patterns: 4. Fire in the Mediterranean basin 5. Fire in California 6. Fire in Chile 7. Fire in the Cape region of South Africa 8. Fire in southern Australia Part III. Comparative Ecology, Evolution and Management: 9. Fire-adaptive trait evolution 10. Fire and the origins of Mediterranean-type vegetation 11. Plant diversity and fire 12. Alien species and fire 13. Fire management of Mediterranean landscapes 14. Climate, fire and geology in the convergence of Mediterranean-type climate ecosystems References Index.

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H.A. Martin

Papers

Cenozoic climatic change and the development of the arid vegetation in Australia