Life in the “dead heart” of Australia

It was the desolation of Australia’s deserts and dried-up rivers, contrasted with the fossil legacy of giant extinct marsupials and birds, that led the British explorer JW Gregory to label this region ‘the dead heart of Australia’. In fact, despite its harsh and unforgiving climate, the Australian deserts are teeming with life.

Above: The red desert sands of a vegetated dunefield in the Great Victoria Desert near Yulara in the Northern Territory, Australia (photo credit: Stephen Zozaya)

The Australian arid zone comprises more than 70% of the continent and is one of the largest arid systems in the world. In much of the literature, the arid zone is drawn as a homogeneous blob in the center of Australia – a rough oval shape comprising the “deserts”. Indeed, the topography of the region is generally subdued and superficially featureless. But anyone who has visited or flown over the center of Australia will know there are isolated rugged mountains in the northwest, center and south – flat topped mesas and razor-back ridges that rise up from the surrounding lowlands, they conjure up quintessential images of the Aussie outback, glowing red in the late afternoon sun. Even the vast intervening desert lowlands, far from being homogeneous, comprise a discontinuous mosaic of sandy dunefields with unwaveringly consistent longitudinal dunes, stony gibber deserts, clay plains, ephemeral rivers and salt lakes.

Cover image article: (Free to read online for a year.)
Pepper, M, Keogh, JS. Life in the “dead heart” of Australia: The geohistory of the Australian deserts and its impact on genetic diversity of arid zone lizards. J Biogeogr. 2021; 48: 716– 746. https://doi.org/10.1111/jbi.14063

It was the desolation of the deserts and dried-up rivers, contrasted with the fossil legacy of giant extinct marsupials and birds, that led the British geologist and explorer JW Gregory to label this region ‘the dead heart of Australia’. In fact, despite its harsh and unforgiving climate, and contrary to Gregory’s unflattering description, the Australian deserts are teeming with life. I have been working with Scott Keogh on the evolutionary history of lizards in the Australian arid zone for 16 years – the diversity of lizards here is higher than anywhere else on earth! My original training as a geoscientist has been instrumental in driving my research towards understanding how changes in the landscapes and climate across Australia over the past 15 million years have structured genetic diversity between and within species that live here. Over this relatively short period of geological time, previously wet and humid landscapes across Australia were radically transformed. The late Cainozoic saw wide-scale contraction of tropical and temperate forests, unprecedented levels of erosion, cessation of major drainages and the disappearance of extensive inland lakes, with intensifying aridity culminating in the development of dunefields across most of the central continent. How has this geohistory of the Australian desert landscapes harbored and structured the diversity in plants and animals that we see today? Which areas of Australia’s deserts have the most biodiversity? Which areas have the least? And how old (or young) are the species that live there?

Stony desert country with flat-topped mesas from the Kanku-Breakaways near Coober Pedy in South Australia
(photo credit: Damien Esquerré)

The very cool thing about working on the biogeography of the Australian arid zone is that the mountain range systems here are ancient – they have been geologically stable for a hundred million years, which means the contribution of geological uplift to speciation is negligible. Trying to understand speciation processes and drivers is hard enough, so removing mountain building from the speciation equation simplifies things immensely (as a comparison, our close neighbor New Zealand has built its enormous mountain ranges in the last 5 million years, so you can imagine the impact this would have had/has on organisms evolving there!). Equally cool about the Australian landscapes is that the vast deserts that dominate the interior of the continent are thought to have formed as recently as one million years ago during the height of the Pleistocene glacial cycles. This complete transformation of landscapes across so much of the continent would have had unprecedented consequences on the evolutionary history of plants and animals living there – some would have gone extinct, unable to tolerate the drier and more inhospitable conditions. To survive, others would have been forced to contract their distributions to wetter, more climatically stable areas where they could persist (typically mountainous regions) until arid conditions eased. Other more generalist species may have been preadapted to aridity and sand and instead were able to expand their distributions and diversify in these new environments.

Currently we don’t know very much about the evolutionary forces at play in Australia’s deserts (compared to those in Australia’s forested fringe). It is amazing what the genes of living species can reveal about the evolution of ancestors that haven’t existed for millions of years. However, without knowledge of past landscape and climate change in Australia’s deserts, evolutionary biologists cannot make sense of the genetic patterns they see in the plants and animals today. As the geohistory of Australia’s arid zone is not widely understood by biologists, we wanted to review its geological development and contemporary landscapes in an accessible way (not many biologist want to spend their time deciphering the lingo of the geology literature!), and in doing so we describe a series of biogeographical hypotheses centered on how geomorphology, evolutionary history and contemporary ecological factors interact to shape diversification patterns in Australia’s desert lowlands.

An aerial drone photo of vegetated dunes in the northern Simpson Desert in the Northern Territory
(photo credit: Paul Hesse)

The main message from our review is that for much of Australia’s biota living in the broad centre of the continent, their deeper history was shaped by an environment characterized by vast fluvial systems, feeding enormous volumes of water into permanent mega-lakes and transporting large quantities of sand across the landscape. This extensive surface water dried out first in the west, and elsewhere fluvial systems switched to an ephemeral state and progressively became saline. But wet pulses in history would have reactivated these inland rivers from time to time as precipitation waxed and waned with glacial cycles (even during the driest period in the Last Glacial Maximum there were large rivers in the southeast and enhanced run-off from the highlands). When they were dry, their sand filled valleys could be blown out by wind, forming extensive sandplains across the continent that would have been stabilized by a vegetation progressively shifting to dry woodlands, and open shrublands and grasslands. Geochronological evidence suggests that the formation of dunefields began in the mid-Pleistocene, accelerating as the climate became increasingly arid in later glacial cycles. These dunes reflect episodic accumulation, and patchy, rather than widespread, activity, so the deserts in Australia never looked like the vast mobile dune landscapes of the Sahara. With this in mind, it becomes more apparent how species could have persisted and diversified in the arid zone despite the enormous and turbulent climatic changes of the Pleistocene. Indeed, human populations were faced with the same severe climatic conditions, and likely responded in a similar way. From renowned Australian archaeologist Mike Smith: “… if we take the desert as a whole, the archaeological evidence is more consistent with a pattern of widespread ‘cryptic’ refugia than with a geographic division into refuges, corridors and barriers. People appear to have survived across much of the desert, but as scattered occurrences and at low densities—in effect, in pockets of microhabitat. Some regions may have been abandoned, including some areas of sandy desert and parts of the Pleistocene coast, but direct evidence for abandonment of large parts of the interior is more limited than once thought. ‘Each desert has its own barriers, corridors and refuges,’ says Cane, ‘and one should look to this inner variability in order to understand the true nature of desert colonization and settlement’ (1995:49)”.

We hope that our review and the hypotheses we outline stimulate further studies of arid zone biogeography. In particular, we look forward to the ways in which future biological collections will open the vast “dead heart” of the continent, to further our understanding of how life has been able to persist and flourish amidst the formation of the largest desert in the Southern Hemisphere.

The quintessential Australian arid zone lizard, Moloch horridus
(photo credit: Damien Esquerré)