BELOVED BURROWERS: DIGGING THEIR WAY INTO OUR HEARTS

Written by Leonie Valentine, Species Conservation Manager, WWF-Australia.

Today on World Wombat Day, we celebrate Australia’s most iconic burrowing mammal and all the other diggers that help keep our ecosystems healthy.

A long-nosed bandicoot delicately scents the ground using her lengthy, slender schnozzle. Her extraordinary nose, with its enhanced olfactory perception, assists in pinpointing the locations of subterranean insects, earthworms and fungi. Suddenly she stills, and with intense determination she uses her elongated claws to dig a hole in the ground, about 8cm deep, discarding the ejected soil in a heap beneath her. With abrupt precision, she plunges her snout into the hole and extracts a protesting earthworm for her dinner. Slouching slightly on her haunches, she daintily devours this tasty morsel before bobbing away, leaving a conical pit and spoil heap behind her.

From robust hairy-nosed wombats to the tiny subterranean marsupial mole, our beloved burrowers once occurred right across the Australian mainland and on many islands, too. Around the world, there are many kinds of digging mammals – badgers and prairie dogs being some well-known examples. In Australia, approximately 22% of our >300 non-flying, land-dwelling mammals create burrows for shelter, spend their life ploughing through soil, or excavate foraging pits while searching for their dinner and can be considered digging species.

Digging mammals come in many shapes and sizes, including some of the rollicking rodents (e.g. pebble mound mouse and hopping mouse), macropods to the max (e.g. woylie and Gilbert’s potoroo) and the rockstars (e.g. echidnas). But here, we focus on some of the quintessential digging marsupials that capture the diversity and uniqueness of our beloved burrowers: the bilby, bandicoots, marsupial moles and wombats.

By quarrying for food or creating burrows, digging mammals move and rework soils via a process known as bioturbation - the disturbance of sedimentary deposits by living organisms. They are often considered ecosystem engineers. These hard-working excavators may enhance many essential ecosystem functions through the mixing of soils. Their digging actions can increase water infiltration, alter nutrient concentrations and fungal diversity and improve seedling recruitment, all of which can influence vegetation and structure of landscapes.

This means our beloved burrowers include some incredibly important keystone species. Their ancestral association as diggers has led to some amazing evolutionary features. All of our burrowers have developed specialised digging features – such as elongated claws – and they all have back-ward facing pouches – to avoid inadvertently covering their babies in quarried dirt!

The beloved burrowers can be separated into three groups based on the type of digging they perform: the sand swimmers, the architects, and the gardeners.

The mysterious marsupial moles are strictly subterranean sand swimmers who live entirely underground, preying omnivorously on invertebrate larvae, centipedes and the occasional lizard. Due to their secretive nature, very little is known about these particularly bizarre animals. However, if the fused cervical vertebrae – that supports their neck while freestyling through the desert sands – is anything to go by, they are likely to be headstrong little critters.

Unlike the marsupial moles, whose tunnels through sand collapse immediately behind them, the architecturally inclined wombats and bilbies create warrens that are structurally intact underground labyrinths, persisting in the environment for decades (or even longer). Wombat warrens are impressive – super impressive – they can even be seen from space. Although most of the tunnelling (up to 30 metres in any direction) for warrens is 1-3 metres below ground, on the surface they often include multiple entrances with substantial mounds of ejected soil. Both wombats and bilbies turn over prodigious amounts of soil, with a single bilby estimated to excavate up to 30 tonnes of soil per year (though both are burrowing and digging for food). These constructed dwellings also host a thermally stable environment, providing relief from both the heat and cold extremes encountered above ground, and providing a critical refuge for many species during fire events. Indeed, the wombats and bilbies are generous hosts, maintaining an open-home policy, allowing many other species, including numbats, goannas and kangaroos, to board within their abode.

In contrast to the underground lifestyles of the marsupial moles, wombats and bilbies, the bandicoots are surface-dwelling gentle gardeners, digging multiple pits while fossicking for their subterranean dinner dishes of insect larvae, earthworms, fungi and tubers. As they unearth their prey, they create shallow pits (5 – 20cm deep), bringing buried soil to the surface and leaving an associated spoil heap of ejected earth. Although the digging activities of bandicoots may appear small at a local scale, their cumulative impact can be surprising. Weighing less than a bag of flour, the delicate eastern-barred bandicoot can turn over up to 13kg of soil per night.

Similarly, the quenda (initially considered a subspecies of the southern brown bandicoot) is predicted to exhume nearly 4 tonnes of soil each year. This combination of digging and discarding soil disrupts the microhabitat layer by exposing soil at the digging site and burying organic matter and litter under the spoil heap. Collectively, the enormous numbers of digs in an area with a healthy population of bandicoots can be important for broader-scale landscape processes. By breaking the soil crust when they dig, bandicoots can reduce soil compaction, increasing soil-water infiltration and mixing soil horizons. By covering surface litter in soil, bandicoots create a conducive environment for litter decomposition, leading to altered nutrient concentrations and microbial and fungal diversity, which can lead to enhanced seedling germination and growth. In addition, by disrupting the amount of litter on the surface and covering it with soil, bandicoots may also reduce the amount of surface litter, potentially influencing fire behaviour.

Tragically, many of the world’s digging mammals are threatened. Compared to other global regions, Oceania (especially Australia) has the highest proportion of threatened digging mammals, with nearly a third of the species considered Vulnerable or Extinct. If we just focus on the digging marsupials and monotremes – this figure jumps to nearly 50%. What this means is that our country is not only lacking these spectacular and unique animals, but it is also deficient in the ecosystem services they historically provided. Although conservation efforts for this diverse group of mammals are underway – more work is needed to protect and restore our unique digging mammal species. So, when considering what mammal to vote for, think of the mysterious marsupial moles, wise wombats and big-eared bilbies or the bashful bandicoots and vote for one of our beloved burrowers!

Want to help protect our beloved burrowers? Here’s how you can get involved.

• Sign the petition and call on the Australian Government to commit to stronger protections for our wildlife and the wild places they call home.
• Discover if threatened animals need protection in your local area by using WWF’s My Backyard tool, and find out how well they’re being cared for.
• Tune in to Scat Chat with WWF to learn about the weird and wonderful ways that animal scat is being used to help wildlife conservation.
• Find out more about how you can get involved to help regenerate Australia’s wildlife.