How much soil would a bandicoot dig if a bandicoot could dig soil? Quite a lot, it turns out. The southern brown bandicoot (Isoodon obesulus) weighs just 1.4 kilograms, but over the course of a year this tiny digging marsupial can excavate more than 3.9 metric tons of soil as it builds its nests and digs for food. This digging provides a critical role for Australian ecosystems by turning over soil, increasing nutrient cycling, creating avenues for water absorption and burying plant seeds.

Unfortunately, bandicoots—like most of Australia's native mammals—are in decline. In fact, of Australia's 29 species of digging marsupials and monotremes (egg-laying mammals), six have gone extinct, three are critically endangered and another nine are endangered or threatened. Almost all these species have lost much of their original ranges because of habitat destruction or invasive species and now exist in rapidly shrinking habitats, on remote islands or within walled-off, predator-free sanctuaries.

The effects of these mammalian extinctions and declines could be long lasting. According to research published September 24 in Mammal Review, the disappearance of these species has contributed to an overall decline in the health of the country's forest ecosystems. Forests and woodland ecosystems now have higher tree mortality rates and hold fewer varieties of plant species than they did a few decades ago. Although some previous research has linked this ecosystem health to climate change, this new study—by researchers at the Center of Excellence for Climate Change, Woodland and Forest Health at Murdoch University in Perth—says the forests would have likely been more resilient if not for the loss of the digging mammals.

Why exactly are these "ecosystem engineers" so valuable? They provide five key ecosystem functions, as detailed in this diagram from the study:

The effect varies by species, of course. Several species turn over soil when they forage for food; some of them only do that when other food isn't available. Other species burrow, and those—such as the critically endangered northern hairy-nosed wombat (Lasiorhinus krefftii)—turn over dramatically more soil (or they would if there more of them). Some species dig deep; others stay closer to the surface. Wombats burrow through compact soil, whereas boodies (Bettongia lesueur) prefer to build warrens inside hard, red soil. All of it matters, given that Australian soil is fairly low in nutrients and needs the turnover to absorb new organic material from falling leaves and other plants, fungi and animal feces.

Interestingly, some of the very invasive species that are responsible for Australia's extinction crisis do their own digging. The European rabbit (Oryctolagus cuniculus), which outcompetes native species for food, digs shallow pits, but those pits are not as deep as those made by native bilbies and bettongs, so they do not fulfill the same function.

The authors say this improved understanding of the role of digging mammals could help inform future ecosystem management decisions, such as species translocations or relocations, which could help enhance forest or woodland health.

Photos: Southern brown bandicoot by Steve Duke Photography. Baby wombat (note those digging claws) by Will Keightley. A massive wombat burrow, photographed by Geoffrey Rhodes. All via Flickr and used under Creative Commons license