Southwest Australia woodlands

• Category: Forest Biomes.
• Geographic Location: Australia.
• Summary: This biodiverse region is the last refuge for some of Australia's most threatened marsupials.

Rising from the Swan Coastal Plain via the Darling Scarp, the Yilgarn Block is an inland plateau located 919 to 1,115 feet (280 to 340 meters) above sea level dominated by the giant jarrah (Eucalyptus marginata) and marri (Eucalyptus calophylla) trees of the Southwest Australia woodlands. These hardwoods reach their peak in the high-rainfall (55 inches or 1,400 millimeters annually) area of the western edge of the region, but slowly decline to stunted mallees at the opposite edge of the rainfall gradient, where only 25 inches (635 millimeters) of rain falls per year. Along this gradient, wandoo, Wandoo (E. wandoo), a culturally significant endemic eucalyptus plant to south-western Australia begins to dominate, and overall plant-species richness is noted.

Wildlife

The Southwest Australian woodlands is the last refuge for a multitude of Australian marsupial species that were decimated following the arrival of the European red fox (Vulpes vulpes) to the region in the early 1930s. While the fox drove up to twenty-two smaller Australian-native mammal species weighing approximately between 0.07 and 12 pounds (35 grams and 5.5 kilograms) to extinction throughout the rest of the Austrialian continent, several of these species persisted in the southwestern woodlands in places like the Dryandra Woodland and Perup Forest. These two conservation areas have high densities of native shrubs of the genus Gastrolobium, and these plants contain naturally high levels of the toxic substance sodium monofluoroacetate, or 1080. The native fauna have evolved to cope with this poison, and the herbivores that regularly eat the plants have a high tolerance to it. Introduced species, such as foxes and cats, have little tolerance to 1080, so their densities have been kept at low levels through secondary poisoning by feeding on the native species. This low density of predators has allowed species of native populations like numbats (Myrmecobius fasciatus) and brush-tailed bettongs (Bettongia penicillata) to persist, while other species perished throughout the rest of the continent.

Numbats are small, obligate termite feeders that are the sole members of their families. Numbats are active during the day, when their russet-colored coat with white stripes acts as effective camouflage from aerial predators like wedge-tailed eagles (Aquila audax) and other raptors. Brush-tailed bettongs are small kangaroos, weighing between two and four pounds (one and two kilograms), with a prehensile tail that carries nesting material that they deposit in a small depression beneath shrubs or logs.

In the trees, herbivores like western ringtail possums (Pseudochirus occidentalis) and insectivores like red-tailed phascogales (Phascogale calura) roam. Native predators like western quolls and chuditch (Dasyurus geoffroii) preyed on these species. However, they act as generalist predators, like foxes or cats, eating a variety of prey throughout its lifecycle, so the ecosystem remained in relative equilibrium.

The quokka (Setonix brachyurus) is another native kangaroo that survives in the upper reaches of watercourses in the higher-rainfall sections of the southwestern woodlands. Quokkas are also threatened by foxes, and their catastrophic decline also align with the arrival of the European red fox. The threat from foxes is exacerbated by habitat alterations due to changes in fire regimes since European colonization of Australia. For almost 50,000 years, quokkas were able to cope with the cool, high-frequency fires employed by the local Nyoongar people on a four- to eight-year rotation, reaching their peak when their habitat went ten to fifteen years without a fire. Beyond this period, however, the swampy habitat favored by the quokkas became too open, as it was dominated by a few plant species that offered little refuge from predation or diversity of forage.

Ecosystem Threats

As these examples of the plight of the southwestern woodland fauna illustrate, this ecosystem is threatened by several factors. The 1930s catastrophic species declines, attributed to the arrival of the fox, was certainly exacerbated by changes in fire regimes as Indigenous people left the land, and fire management entered a phase of control that reflected the prehuman fire regimes of large bushfires. This phase lasted until the 1960s, when more intensive fire management was employed to reduce the effect on humans. In the twenty-first century, fire managers seek to benefit biodiversity through their activities, while also reducing the risk to people and infrastructure.

Fire management in the Southwest Australia woodlands aims to balance biodiversity conservation, ecosystem health, and human safety. Prescribed burning is a key strategy used to reduce fuel loads and promote habitat diversity by creating a mosaic of vegetation in different growth stages. Fire-sensitive ecosystems, such as riparian zones and peat wetlands, are typically excluded from burns to protect their slow-recovering habitats. The integration of Indigenous Noongar fire practices, such as "cool burning," supports low-intensity fires that maintain ecosystem health and reduce wildfire risks.

Unplanned fires, caused by lightning or human activity, are addressed with rapid response plans in areas of high ecological or cultural value, while remote regions rely on natural barriers or strategic burns. Fire management strategies are guided by research on fire regimes and their ecological impacts, and emphasize the need to vary fire frequency, intensity, and timing. These adaptive plans aim to conserve biodiversity, protect fire-sensitive species, and address specific challenges like climate change and human encroachment.

Fire is a natural process throughout most of Australia and is not a threat to most species, as numerous plants have evolved to resprout via epicormic growth or from lignotubers. Some species need fire to open up the canopy and allow their seeds to drop and regenerate. Other species with short-lived seeds may be threatened if the fire return is too rapid, as they need time to allow their plants to reach seed-bearing age. Yet these threats are not occurring in isolation. A total of 90 percent of the eucalypt woodlands in Australia have been cleared, and the vast majority of the western side of the southwestern woodlands—an area known as the Wheatbelt—has been cleared for agriculture.

The Jarrah Forest is a multiple-use forest that has been extensively logged and that contains several bauxite mines and dams that supply water to the state capital, Perth. Many of the most important and common plant species in the southwestern woodlands are also threatened by root rot or dieback caused by Phytophthora cinnomoni, which was introduced from eastern Australia. The spores of this fungus are transported in moist soil, so large areas of vegetation in the southwestern woodlands, particularly along tracks, are dying.

Conservation Efforts

The Southwest Australia woodlands have several protected areas, including Dryandra Woodland Nature Reserve at 108 square miles (280 square kilometers), Perup Nature Reserve at 201 square miles (520 square kilometers), and Lane Poole Reserve at 212 square miles (550 square kilometers). Nevertheless, simply designating land as a conservation estate rarely solves conservation problems, unless land clearing is the sole threatening process.

In the 1990s, the Western Australian Department of Environment and Conservation instigated a broad-scale fox control project called Western Shield. This project involved aerially spreading dried meat baits injected with 1080 poison across large swaths of western Australia. This resulted in initial improvement in the status of numerous species, including the numbat, brush-tailed bettong, southern brown bandicoot (Isoodon obesulus), and quokka. Researchers noted beginning in the mid-2010s however, that mesopredators such as cats had ncreased in abundance and were having as much of an effect as foxes did previously. One study's results suggested that fox control programmes could change how cats interacted with prey that they shared with foxes, and that some prey may become more vulnerable to cat predation in open habitats after fox threat was removed. This would impact how the local ecosystem worked, not only affecting the predators and prey, but other animals in the local food web. An effective method of controlling feral cats is urgently needed, but it appears to be a long way off, given that the problem has been researched since the end of the twentieth century.

Bibliography

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