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AUSTRALIA'S DESERTS, DESERT WILDLIFE OF AUSTRALIA
S12. Spinifex hummock grassland on rocky range habitat in central Australia, photograph by Catherine Nano.
S13. Spinifex hummock grassland on sand plain habitat in central Australia, photograph by Catherine Nano.
Other low woodlands
Other low woodlands characterise the eastern edges of the arid zone. Prominent examples include gidyea (Acacia georginae and A. cambagei); brigalow (Acacia harpophylla); ironwood (A. excelsa); and Casuarina cristata. In all cases, the understorey is highly variable, ranging from a dominance of short-lived grasses in some areas to a greater abundance of chenopod herbs and shrubs in others. Witchetty bush (Acacia kempeana) is another prominent central Australian shrubland species, known especially for the grubs it houses that are a prized food-source for Indigenous peoples. It is widespread throughout central Australia, occurring on calcareous- and skeletal-soils on igneous rocks and on well-drained red earth soils.
River Red Gum (Eucalyptus camaldulensis) occurs along usually-dry arid-land channels. In the major channels, this species occurs as the single dominant tree, while in the smaller channels it forms mixed stands with a range of species including ironwood (A. estrophiolata), mulga and coolibah (E. coolibah). Characteristic associated native grass species include curly windmill grass, cane grass, silky browntop and kerosene grass. These habitats are of critical importance for hollow-dependent animals (e.g. parrots, bats). They are, however, badly infested with the highly invasive couch and buffel grasses, both of which are known to significantly alter habitat quality and fire frequency.
Chenopod shrublands are virtually absent from the northern half of the arid zone. In the south, they are mainly restricted to areas characterised by alkaline, salty clay soils and are a highly characteristic feature of gibber plains of the south-east portion. These shrublands are dominated by drought- and salt-tolerant low shrubs, these being mainly saltbush (Atriplex), copper burr (Scleroleana); bluebush and cottonbush (Maireana) and samphire (Halosarcia). All communities also characteristically support a grass-dominated ground flora. Most chenopod species are palatable to stock and many shrublands have been subjected to heavy grazing over the last 100 years and are now highly degraded. Many of the species belonging to this habitat are now listed as threatened.
Permanent and semi-permanent freshwater habitats
Permanent and semi-permanent freshwater habitats are extremely rare throughout the arid zone. Prominent examples include spring-fed rock pools in sandstone gorges; temporary lagoons and waterholes along watercourses and persistent deep rock pools. These wetlands act as refuges for drought-intolerant plants such as ferns and sedges and they support a disproportionately high number of rare and threatened species.
Desert claypans form shallow temporary lakes after rainfall. These lakes are highly variable, both spatially and temporally in terms of their salt concentration - this in turn has a pronounced influence over the type and amount of vegetation present at any one site. Although some individual lakes are themselves extensive (e.g. Lake Eyre in South Australia), inland salt lakes together comprise only a very small proportion of the Australian desert landscape. The ability to tolerate both high salt concentrations and desiccation are critical requirements for any plant to survive salt lake habitats.
S14. Flowering of annual and ephemeral daisies and other herbs in central Australia, photograph by Catherine Nano.
S15. Mulga shrubland in central Australia, photograph by Catherine Nano.
Natural springs form when highly mineralised artesian water reaches the surface through fault lines in overlying rock. Mounds occur when the minerals concentrate at the ground surface through evaporation. These habitats are themselves extremely rare and support a unique plant community comprised of reeds, grasses, green algae and various salt-tolerant shrubs. They also support various rare plant species (e.g. button grass) and a great number of aquatic invertebrates and fishes that occur nowhere else. Many springs have completely disappeared over the last 100 years, due to excessive levels of underground water extraction for farming purposes.
RICHNESS OF PLANT AND ANIMAL GROUPS
The Australian desert flora consists of a relatively small number of species compared with the flora of more temperate regions of the continent.
Ferns are poorly represented among Australian desert plants. Where present, desert ferns mainly occur in refuge areas that are fire protected and where moisture levels are relatively high. Examples of such sites include rock pools in the MacDonnell Ranges of central Australia.
Gymnosperms (seed plants that lack ovaries) are likewise extremely limited in Australian deserts. Cypress pines (genus Callitris) and the Macdonnell Ranges cycad are among the few gymnosperms in Australian desert landscapes.
Angiosperm (flowering plant) families with the highest representation in arid Australia are Myrtaceae (e.g. the eucalypts); Asteraceae (daisies); Mimosaceae (mainly wattles); Poaceae (grasses); Fabaceae (peas); and Chenopodiaceae (e.g. saltbush, bluebush and cottonbush). Families that are notably poorly represented (c.f. more temperate areas of the continent) are Orchidaceae (orchids); Epacridaceae (heath family) and Rutaceae.
The genus Acacia dominates the desert flora in terms of species richness. Other prominent groups include Eremophila (native fushias) - most species of which occur only in the arid zone; Eucalyptus and Goodenia.
About 50 Australian desert genera also occur in the other hot desert regions of the world. Prominent examples of highly ubiquitous desert-dwelling families include: Amaranthaceae (includes the showy mulla mulla wild flowers); Boraginaceae (the borage family); Caesalpiniaceae (the Senna family); Chenopodiaceae (the salt bushes); Euphorbiaceae (spurge family) and Brassicaceae (the Brassica family).
Richness of most animal groups is lower in the Australian desert region than in coastal areas (table S16). However, most families of Australian animals are found in the deserts.
Knowledge of richness and diversity of the invertebrate fauna is poor across Australia, but this is especially the case in Australian deserts. However, insects are clearly the largest group of desert animals in Australia in terms of number of species and biomass. Some groups, such as termites and ants, are abundant and play an important role in ecosystem functioning. Groups of invertebrates that rely on freshwater do occur in the arid zone but generally there are significantly less species than in coastal regions. Aquatic molluscs and crustaceans are present where there is permanent or semi-permanent water. The aquatic insect fauna includes species that rely on water for the entire life cycle (such as water scorpions and water striders) and others that have aquatic larvae (e.g. dragonflies).
Freshwater fish occur in streams, waterholes, and free-standing water associated with artesian springs and bores in the Australian deserts. The number of species present is often surprisingly high. For instance, 34 native species occur in the Lake Eyre catchment and some species have a very restricted distribution (e.g. the Dalhousie hardyhead occurs in seven streams in the vicinity of Dalhousie Springs, in the far north of South Australia).
Frogs are not commonly encountered in Australia's deserts except after significant rainfall. However, over 40 species have been recorded. Some species occur in vegetation surrounding permanent or semi-permanent freshwater. Others occupy claypans and sand plain environments and are active only for brief periods following rain.
The reptile fauna of the Australian deserts is very rich and contains some unique species such as the thorny devil. Goannas, skinks, dragons and geckoes are prominent groups in the desert fauna. The desert lizard fauna of Australia is the richest of any desert area in the world. Some sites in central Australia have over 40 lizard species at a density of over 400 individuals per hectare.
Over 200 bird species are recorded from Australia's deserts. Many of these species are widespread across the continent being found in moister areas closer to the coast. About 40 species can be considered to be largely restricted to desert habitat.
About 95 mammal species occurred in the Australian deserts at the time of European settlement. However, the desert mammal fauna has suffered a very high extinction rate (table S17). Among the surviving species, rodents, insect-eating bats, carnivorous marsupials, and macropods (kangaroos and wallabies) are prominent.
ADAPTATIONS OF DESERT WILDLIFE
Among the extreme environmental conditions experienced by plants and animals in Australia's deserts are aridity, heat and salinity. Plants and animals have evolved a range of morphological, behavioural and physiological adaptations to enable persistence and reproduction in desert environments.
The response of plants to the unpredictability of rainfall in Australian deserts can be broadly classed as drought tolerance or drought avoidance.
Drought tolerance is achieved by several means. Certain perennial plants (known as xerophytes) have a physical structure that promotes water storage and conservative water use, enabling them to thrive in areas where water is usually scarce. Classic features of xerophytic plants include: sclerophyllous (hard) leaves with a waxy or hairy surface and few and sunken stomata or greatly-reduced leaves (cladodes, e.g. in desert oak) to minimise water loss; succulent leaves and stems or fleshy underground tubers for water storage (e.g. parakeelya); and a deep root system for access to subterranean water supply or a shallow root system to enable rapid uptake of moisture when it suddenly becomes available after rainfall. In desert Acacias, the leaflets are suppressed and phyllodes (modified leaf-stems) perform the task of leaves. These structures are vertically flattened and oriented towards the ground, reducing the amount of light interception and hence water loss. All spinifex species are likewise classified as xerophytic, given their tough, pungent-pointed, sclerophyllous and narrow leaves.
Other species achieve drought tolerance by carrying out photosynthesis using a metabolic pathway that involves production of a four-carbon molecule. This metabolic pathway is known as C4 photosynthesis and the plants that use it are referred to as C4 plants. In contrast, the great majority of plants (approximately 99% of all plant species) are C3 plants that produce a three-carbon molecule during photosynthesis. C4 photosynthesis offers a competitive advantage over C3 photosynthesis under conditions of drought, high temperatures and nitrogen limitation. In simple terms, the C4 cycle leads to an increased concentration of CO2 within the plant leaves which in turn increases the amount of photosynthesis and decreases the chances of organic material and energy loss from the plant.
Desert perennials can avoid death through moisture stress by remaining dormant during dry periods then ‘springing to life’ when water is suddenly available. This ‘strategy’ is prominent in many perennial tussock grass species that have a very short growing season (e.g. woollybutt, neverfail, Michell grass and kerosene grass). Certain arid zone tree species (e.g. kurrajong and bats wing coral tree) likewise exhibit this ‘deciduous’ drought response.
For the short-lived (ephemerals, annuals and biennials) species, drought avoidance is achieved by remaining in seed-form throughout dry periods and undergoing rapid vegetative growth and reproduction only under relatively non-arid conditions following major rainfall events. For these drought evaders, population persistence is dependent on the successful transition between the most resistant life stage (the dormant seed) and the most sensitive stage (the seedling). For this reason, ‘bet-hedging strategies’ can be of critical importance. Such strategies may include long-term seed bank establishment, staggered seed bank release, and a delayed response to moisture availability, meaning that germination is restricted to large rainfall events only.
Mechanisms for coping with saline environments likewise take the form of tolerance (halophytes) or avoidance (facultative halophytes) strategies. Certain species are able to excrete excess salts through their leaves by way of maintaining a normal internal salt concentration. For other species such as bladder saltbush, it has been shown that growth is actually stimulated by the presence of sodium chloride salts. Avoidance of the negative effects of high salt environments is achieved by short-lived plants that quickly complete their life cycle during periods immediately following major rainfall events when the salt concentration is low.
Most of Australia’s desert fauna is active at night. Among vertebrates, only birds do not contain a significant portion of nocturnal species. Almost all mammals are nocturnal and almost 50% of reptile species. Nocturnal activity enables species to avoid daytime extremes in temperature.
Many desert animals in Australia shelter in burrows during the day. Although the soil surface is the hottest place in desert environments, areas below the surface are significantly cooler, have higher humidity, experience no sunlight, and also experience lower levels of infrared radiation. Although heat does penetrate the soil surface it does so slowly such that soil below the surface is at its warmest at night and at its coolest during the day.
Burrow size and structure is highly variable ranging from the single entrance burrows of sand goannas to elaborate communal structures built by species such as the great desert skink. This lizard builds large communal burrow systems that consist of a network of connected burrows to a depth of over 1 metre (m) and 10 m in diameter with five to ten entrances. The slit spider from the Simpson Desert builds a horizontal slit (3 mm high by 39 mm wide) in consolidated sand especially on dune crests. This species is the only spider known to construct its own crevices without the use of silk.
Although many desert animals burrow underground, most forage above the surface. Among mammals, the marsupial-mole is an extreme and unique animal which spends its life entirely underground literally swimming through the sand (image S18). This small marsupial (14 centimetres (cm) body length) is tubular in appearance and has a host of adaptations for coping with a life spent in sand including being blind, possessing tiny ears, spade-like front claws, a backward opening pouch and short fur. Very little is known about it because those individuals that appear periodically above the ground are likely to be sick and not displaying typical behaviour. Nonetheless, it may well be a common desert animal given that Australian deserts contain almost 2 million sq km of sand habitat.
Behavioural control of temperature
Each animal group has a narrow range of temperatures within which individuals can survive. For example, birds maintain body temperatures at between 40-42°C and are likely to perish once the temperature rises above 46°C. Animals must maintain energy budgets so that heat gain and loss (through metabolism, conduction, convection, infrared radiation, evaporation and sunlight) is balanced and body temperatures are maintained at near optimum levels. Behavioural selection of suitable microclimates is used by many day-active lizards to avoid heat stress. Dragon lizards change body position relative to the sun during the course of the day, decreasing the body area receiving sunlight when air temperatures are high. Climbing species of dragon lizards increase perch height when air temperatures increase whereas non-climbing species move to the shade of vegetation.
S18 Like many desert animals, the marsupial mole (Notoryctestyphiops) survives extremes of temperature by living underground. Photograph by Mike Gilliam.
Water from food, concentrated urine and sale tolerance
The majority of desert animals are able to obtain sufficient water from food and metabolic water (i.e. water produced as a by-product of digestion of food). Species that feed on foliage, nectar, or other animals can generally readily obtain enough water for survival and reproduction. However, some animals such as the spinifex hopping-mouse and the sandy inland mouse are able to subsist and obtain sufficient moisture on a diet entirely of seeds. Animals lose moisture when metabolic waste is released from the body as faeces or urine. The ability to concentrate urine reduces the amount of water that is lost from the animal when metabolic waste is released. Among vertebrates, mammals are the most efficient at producing concentrated urine and desert mammals in particular are highly effective at doing so. The spinifex hopping-mouse and the sandy inland mouse produce the most concentrated urine known for any mammals and also very dry faeces.
Aestivation, torpor and hibernation
Aestivation, torpor, and hibernation are physiological adaptations that enable animals to survive extreme climatic conditions by reducing their body temperature, energy expenditure and water loss. Hibernation and aestivation are broadly similar processes that involve long-term (weeks or months) inactivity and reduced metabolic rate. Aestivation usually occurs in aquatic or semi-aquatic species such as snails, frogs or lungfish, and is a response to drying of the environment. Torpor is a response to reduced food availability but typically lasts only a few hours. All three of these processes are a common feature of desert life.
Aestivation is a response to aridity exhibited by some Australian desert frogs. These species undergo aestivation in burrows dug into the soil of sandy watercourses or claypans to a depth ranging from 30 cm to 170 cm (depending on the species). At the onset of aestivation a frog assumes a water-conserving posture and becomes inactive. Within a week, a thin, transparent cocoon begins to form over it which covers the entire body surface and consists of multiple layers of sheets of cells. The cocoon reduces water loss. The frogs remain in these cocoons for months or years and do not become active again until sufficient rain falls. Then the frogs break through the cocoon and make their way to the surface to feed and mate in temporary pools of water.
The unpredictable nature of rainfall in the Australian deserts means that many desert animals possess life histories that enable them to exploit the peaks in primary productivity (plant growth, flowering and seeding) that occur soon after heavy rain. Population densities of some Australian desert rodents increase markedly after heavy rainfall. Following increases in primary productivity many rodents increase in abundance following a time-lag and move into areas of plentiful food. At these times rodents are a prominent feature of the desert landscape. Once conditions begin to dry and primary productivity declines, these rodents then show a sharp drop in numbers, again after a lag in time. Within 12-24 months, numbers may return to the original low levels. These cycles are referred to as 'boom-bust' cycles and they are a feature of species such as the long-haired rat and spinifex hopping-mouse.
The temporary waterbodies that form after rain, such as when saltpans and claypans become filled with water, attract a wide variety of animals. Crustacea, such as tadpole and shield shrimps, appear in pools within a few days of rain. Desert crustaceans have a rapid life cycle and when conditions dry out, eggs become dessicated and remain dehydrated and inactive on the desert floor for years until the next rains arrive. The increase in numbers of aquatic invertebrates can in turn attract large numbers of waterbirds that quickly establish at temporary wetlands and begin breeding. Some lake systems can support massive numbers of waterbirds at these times (e.g. Lake Eyre may have 325,000 waterbirds of 44 species at such a time).
THREATS TO AUSTRALIA'S DESERTS
The impact of European settlement on Australia’s deserts has been profound. The establishment of significant human settlement and extensive sheep and cattle grazing, the removal of Indigenous peoples and their alienation from their ancestral lands, and the introduction of plants and animals have all had a profound influence on desert wildlife. In Australia, unlike other arid regions, there has not been a process of desertification (i.e. spread of the desert) and much of the desert landscape remains intact. The most obvious signs of recent human impacts on Australian deserts have been the extinction of native plants and animals, the invasion of introduced species, and changes in the timing and extent of natural processes such as fire.
Desert rock and desert sand
Australia is the world’s lowest and flattest continent, therefore, people are often surprised to learn that mountain ranges are a prominent feature of some of Australia’s deserts. Extensive mountain ranges include the MacDonnell Ranges (surrounding Alice Springs), Hammersley Ranges (part of the Western Plateau), and Musgrave Ranges. Mountain ranges are significant not only from a geographical perspective but also because of the influence they exert on climatic conditions. Compared with surrounding areas, desert mountain ranges may experience increased rainfall, more stable temperatures, and often support permanent or semi-permanent waterholes.
From the perspective of a plant or animal, desert rock habitat offers different opportunities and increased stability compared to other desert environments, particularly the sand environments that dominate much of Australia. Rock surfaces do not warm during the day to the same extent as sand surfaces and food resource availability is also not as patchy. Rock habitat has an abundance of shelter sites such as caves, crevices and rock overhangs that provide stable microclimates. For example, temperature within a cave in the MacDonnell Ranges measured every four hours for a 12-month period varied only from 16-30.5°C whereas outside temperature ranged from -6-40.8°C.
Mammal and bird species that occupy desert range habitats tend to have smaller and more stable home ranges and turnover of individuals is lower than those in sand environments. Range environments also function as refuge sites for some plant species that evolved during wetter periods.
Mulga-spinifex mosaics in arid central Australia
Mulga and spinifex habitats together characterise a large part of central Australia. Abrupt boundaries often form between these two vegetation types, giving rise to a mosaic pattern of contrasting shrub-grass alterations across the landscape (image S19). Concerns for the long-term stability of these mosaics result from ongoing claims that fire-intolerant mulga shrubland is being converted into fire-encouraged spinifex grassland. It is thought that the shift relates directly to a modern-day increase in the frequency of wildfire incursion into mulga habitat due to the near cessation of the ‘patch-burning’ practice of Indigenous Australians. Actual evidence for such a change is, however, scant and somewhat contradictory, reflecting the ongoing difficulties associated with the separation of cause and effect in the absence of long-term data sets. Recent work on mulga-spinifex mosaics on central Australian mountain ranges and dune fields examined the relative importance of physical habitat characteristics (e.g. soil, slope, aspect) from fire effects for shrub-grass boundary positioning. This work showed that mulga contraction in these areas has in fact been minimal over the last half-century, meaning that boundaries are highly stable. It demonstrated further, that patterning in these mosaics results from the combined influence of gradients in site physical characteristics (especially soil type), firing, as well as a range of biological factors such as competition, animal-mediated dispersal and facilitation (shrub ‘island’ effects). Overall, the study emphasised that although mulga is more resistant to fire-mediated contraction than previously recognised, the quality and integrity of mulga habitat can be undermined by too-frequent firing.
S19. Mulga-spinifex mosaic in central Australia, photograph by Catherine Nano.
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