Losing Australia’s diggers is hurting our ecosystems

The decline of digging mammals, such as this bilby, is threatening Australia’s ecosystems. AAP Image/National Parks and Wildlife

Despite once being described as common, mammals have been lost across the Australian landscape over the last 200 years. The impact has been particularly severe on Australia’s digging mammals, including iconic species like echidnas, bilbies and bandicoots. New research shows that the decline is not just bad for mammals, but for Australia’s ecosystems too.

Through introduction of predators, land clearing, and disease, six of the 29 digging mammal species that were present 200 years ago are now extinct. Nearly all the living species show massive range contractions – many are gone from the Australian mainland completely or exist only in predator-proof fenced reserves.

Our new study from Murdoch University, published today in Mammal Review, has highlighted the relationship between the loss of Australian digging mammals, and ecosystem decline.

Why do we need digging mammals?

Bettongs, potoroos, bilbies, bandicoots, and echidnas are important “ecosystem engineers”.

These mammals create disturbances in the form of nose pokes, scratchings, shallow and deep digs, long bull-dozing tracts and complex subterranean burrows.

They might be small, but these mammals punch above their weight. A digging mammal can shift around 1.8-3.6 tonnes of soil per kilogram of body mass in a year. A woylie - a bettong from Western Australia - creates between 20 and 100 diggings per night while foraging, while a southern brown bandicoot can excavate over 3.9 tonnes of soil per year.

This common wombat may not be threatened, but other species are critically endangered. Flickr/Timmy Toucan

Digging mammals improve soil health by turnover and mixing organic matter. Soil turnover brings deep soils and their nutrients to the surface. Their diggings also trap organic matter and other materials, increasing nutrient turnover.

They can break through hard soils, which would otherwise be impenetrable to plant seedlings. For example, wombats burrow through thick layers of very hard soils called calcrete.

They can also improve water infiltration which increases soil moisture. Digging by mammals can provide sites for water to enter soil. For example, echidna foraging pits have twice the water infiltration of undisturbed soils, while bandicoot diggings can be the only site of water infiltration in otherwise water-repellent soils.

Digging mammals spread important mycorrhizal fungi across the landscape. These fungi help plants to increase their absorption of nutrients and deal with our nutrient-poor Australian soils.

Many of these important symbiotic fungi produce fruiting bodies below ground, and probably rely on digging and mycophagous (fungus eating) animals to distribute their spores across the landscape. It is likely that the loss of these animals has therefore also indirectly led to loss of some of these critical fungi.

Digging mammals can also reduce the amount of combustible plant material within a landscape, possibly altering fire regimes.

Echidnas can help water soak into soils. Flickr/laubarnes

What do we do now?

Before European settlement, when digging mammals were numerous and widely distributed across the Australian landscape, the soil turnover would have been very considerable. The almost total removal of these mammals from the landscape has to be affecting ecosystem function.

Despite our study, we still know so little about the long-term effects of losing these significant animals. In Australia, we’re managing ecosystems that have recently seen a massive loss of ecosystem processes, but we don’t yet know the full impact.

For instance, we need to know more about how digging mammals influence fire, how translocating mammals could improve the environment, and the effect of mammals on fungi and plants.

We hope that the investigation will lead to recognition of the important role that these mammals play in shaping ecosystems, and therefore greater protection for those species that are still exist in the landscape.

Small mammals like this rufous bettong move large amounts of soil for their body size. Flickr/robstephaustralia