Australia’s ecosystems are a shadow of their pre-human state. Between 80,000-40,000 years ago, as our species spread across the continent, we eradicated dozens of gigantic mammals and birds. Following the second major wave of human migration into Australia, beginning in 1788, humans again eradicated numerous species. Today, our continent has lost almost all of its megafauna, and many of the ecosystem services they provided have degenerated. Yet some of the intricate interactions which maintain the rich web of life across our continent have been preserved by the fortunate species that avoided extinction. Among these is the Swamp Wallaby (Wallabia bicolor), a small macropod with a large impact.
1. Nutrient cycling
Many of our lost megafauna were browsers, such as the car-sized wombat Diprotodont and the gigantic Short-faced Kangaroo, which specialised in eating the leaves of shrubs and trees. However, most of our modern herbivorous megafauna, like the Eastern Grey Kangaroo (Macropus giganteus), are grazers that specialise in eating grasses. While our extant grazers help to cycle nutrients and carbon through the ecosystem (from their grassy diet) the loss of browsing megafauna means that the cycling of nutrients and carbon from shrubs and trees, via herbivores, has degenerated. However, the Swamp Wallaby is one of the only remaining marsupials which primarily browses. Interestingly, some of the most commonly observed plants that Swamp Wallabies are seen eating are gum leaves (Eucalyptus spp.) and wattles (Acacia spp.). Both of these groups are known for their often toxic and unpalatable foliage, particularly wattles which are renowned for their poisonous nitrogen-containing secondary metabolites. Interestingly, wattles are some of the major nitrogen fixers in Australia, and pull inorganic nitrogen out of the atmosphere and fix it into a form that animals and plants can use for growth. The Swamp Wallabies then consume these leaves and pass the precious nitrogen into the soil for plants to use, as well as the carbon which the plants have trapped in their leaves. Essentially, Swamp Wallabies are fertilising our forests and locking up carbon in our soils.
2. Spreading spores and seeds
While the first wave of human expansion across Australia caused many megafauna species to vanish, the second wave of human expansion, following 1788, resulted in the loss of much of the mesofauna. Our mesofauna includes small to medium-sized mammals such as bandicoots, potoroos, and bettongs. When the second major wave of humans arrived on the continent they brought a range of damaging alien species, most notably the Domestic Cat and Red Fox. These alien invaders wiped out numerous mesofauna species, many of which were important dispersers of fungi, such as native truffles. The dry and nutrient-poor soils of Australia present a challenge for plants. To solve this, many plants have developed intricate mycorrhizal associations with fungi which grow alongside their roots and help them gather nutrients and water. However, these fungi need to be dispersed for their offspring to find new plant hosts to form mutualistic partnerships with. Formerly, the bandicoots, potoroos, and bettongs did most of this dispersal, but when they vanished from much of Australia, the task was left to the versatile Swamp Wallaby, which not only browses on leaves but also consumes fungi. In essence, Swamp Wallabies eat the fruiting bodies of fungi, which can be underground truffles or above-ground mushrooms, and then spread the spores in their dung. A recent study even showed that Swamp Wallabies can spread these spores over a kilometre from their source! Although far less studied, Swamp Wallabies also carry the seeds of native plants such as Bidgee-widgee (Acaena novae-zelandiae) which catch in their thick fur. Swamp wallabies act as a ‘marsupial taxi service’ for native fungi and plants by helping spread their spores and seeds throughout the landscape.
3. Dung-diners, flesh feeders, and blood-drinkers
Swamp Wallabies not only serve to maintain the wider ecosystem, but also act as an ecosystem in their own right! Their nutrient-rich dung provides a substrate for fungi, such as Podospora austrohemisphaerica, to grow on. Their dung also provides food for specialist dung beetles such as Coptodactyla, Onthophagus, and Temnoplectron, which are known to consume the scats of kangaroos and wallabies. However, little is yet known of the fungi and insects which use Swamp Wallaby dung, but future studies will undoubtedly illuminate this hidden world. More than 40 species of parasitic worms have been recorded in the Swamp Wallaby as well as two species of louse flies (Hippoboscidae), three species of lice, including the Wallaby Louse (Boopia notafusca), and several mites and ticks including the Kangaroo Tick (Amblyomma triguttatum). In some cases, a single Swamp Wallaby hopping around a forest can support a greater number of parasite species than the total number of mammal species in that same forest! Even in death Swamp Wallabies support biodiversity. Their carrion provides an ephemeral ecosystem for more than a dozen different necrophagous insect species, from Carrion Beetles (Ptomaphila lacrymosa) to blow flies (Calliphora spp.). These insects are then eaten by other animals and the remains of the carrion is cycled back into the soil where plants and fungi can use the nutrients to fuel their growth.
A friend of the forest
Swamp Wallabies fertilise our forests, help lock up carbon, spread native plants and fungi through the landscape, and support dozens of insects and other invertebrates. Their versatile ecology has also helped to buffer our ecosystems from the effects of megafauna and mesofauna extinctions. Swamp Wallabies have a disproportionately positive impact on our continent and could perhaps be regarded as keystone species. Quite a big achievement for such a small marsupial!
Banner image is courtesy of Brisbane City Council [CC BY 2.0], via Flickr.