The tiny marsupials who spend their nights digging for truffles on the forest floor could be holding together ecosystems in ways scientists are only beginning to understand.
TAKING AN EARLY MORNING walk through the scrubby forest in Tasmania’s south-east, Professor Chris Johnson from the University of Tasmania can tell if the local bandicoots and bettongs have been hard at work the night before.
These little marsupials, looking a bit like kangaroos in miniature, spend all night digging for dinner on the forest floor. But it is not the foraging pits that these little diggers leave that gives them away, but a lack of crunching dead leaves underfoot.
Johnson, an ecologist with over 30 years experience studying mammals, says that these little critters are so prolific with their digging activities that they can bury large amounts of leaf litter in a single night.
And it is this influence on leaf litter loads that has him suspecting that bandicoots and bettongs, together with other mammals that dig for food or burrow for shelter, can influence how a bushfire burns through a landscape.
“Fires tend to start down on the ground in the dry litter and a fire will spread if the litter layer is continuous,” Johnson says.
“These animals are in there digging holes and throwing soil up into heaps and that creates a lot of micro-firebreaks at a very, very small scale. This diminishes the likelihood that a small fire will spread and turn into a large intense fire.”
While the actions of digging mammals would do little on those high fire-danger days when catastrophic, high-intensity fires can occur, Johnson believes that overall they can have a significant influence on the patterns of low-intensity fires.
“When you’ve got those mammals going about their business in the woodland ecosystems, the number of days you have when low intensity fires escalate into high intensity fires is going to be fewer, so on the whole there should be less fire impact.”
At the moment the evidence is anecdotal and Johnson is now gathering the data to see if his suspicions are correct. But he says the impact of bettongs and bandicoots on reducing fuel loads are so obvious that anybody could notice the difference, well, that is if they actually had the opportunity to see it.
“Most people can’t see it because they’ve got no experience of it. Very few Australians have ever walked through a woodland that has got the full complement of digging mammals because most of them went locally extinct in the early twentieth century.”
It is a sad fact that the last 200 years has seen Australia gain the mantle for the highest rate of mammal extinctions in the world, and those mammal species that dig or burrow are no exception to this statistic.
Of the 29 mammal species that are considered to be diggers, six are known to be extinct and a further seven species are endangered. Another five species are vulnerable or near threatened, while most of the remaining species have suffered huge restrictions in their range.
But it is only now that scientists are discovering just how important these animals are to Australian ecosystems, and their benefits are not just limited to influencing the way a fire burns.
According to Dr Mathew McDowell, a post-doctoral researcher with Flinders University, so significant are the large-scale losses of these animals that it could jeopardise the survival of whole ecosystems. It is a concept he calls ‘extinction debt’.
“When we lose those burrowing and digging animals the effect is of such an impact that probably all of the other species within the area are in trouble… thirty to fifty years down the track they may just suddenly disappear as well.”
It is this situation that Professor Giles Hardy and Associate Professor Trish Fleming are hoping does not eventuate in the renowned Jarrah and Wandoo forests in Western Australia’s south-west.
Hardy and Fleming, both from Murdoch University’s Centre of Excellence for Climate Change, Woodland and Forest Health, have been examining the impact of the losses of once-common digging mammals such as boodies and woylies on the health of the forests. And they are seeing some alarming results.
“In certain parts of these forests we are losing trees of all age classes and a lot of trees have changed quite significantly in canopy structure and health, with a quarter to an eighth of the canopy of a healthy tree,” explains Hardy.
“When we start to tie all these aspects together, we start to see there is a potential link between tree declines and native mammal loss.”
And that link comes in the form of a symbiotic relationship between the roots of the trees and native Australian truffles.
The special type of fungi, known as mycorrhiza, helps the trees gain greater access to water and nutrients. They are spread about the forest floor by mammals that dig up and eat the truffles. With less digging mammals around, the fungi do not spread as readily and the trees become more susceptible to stress.
“Trees with no or fewer mycorrhizal fungi are much more susceptible to diseases because the fungal sheaths that cover the roots provide a lot of protection,” Hardy says.
“Also trees with mycorrhizal fungi are much more resilient to drought-stress because the fungi explore much more soil than a root does &mdsh; many thousands times more than what a root can do, so they can source a lot more water and nutrients.”
Fleming adds that by digging into the soil the mammals also have a positive effect on soil quality, which would also benefit the trees.
“When they dig, these mammals turn over the soil and change the composition of the soil. They also affect the nature of the soil. Mammal diggings form indentations in hydrophobic soils (where water just runs off) and so increase water infiltration.”
But where numbers of digging mammals have dropped away, the abundance of rabbits has dramatically risen. And rabbits dig as well, so why aren’t we seeing the same benefits to ecosystem health conferred by them?
It all comes down to the way they dig, according to Professor David Eldridge from the University of New South Wales.
“Rabbits dig pits that are a lot smaller, they are a lot shallower, and they don’t last as long. They are not digging enough of them, they are not digging them deep enough, and they are not digging them the right way.”
Interestingly, Eldridge has found that rabbit diggings are associated with a greater number of exotic plants, whereas those of bilbies and bettongs favour locally native plants. Seeds are also easier for ants to take from rabbit pits, meaning they are eaten before the have a chance to sprout.
But Eldridge also points out that it is not just mammals that dig. Some reptiles and birds, such as goannas and bush turkeys, also dig, and these animals are likely to also play an important role in maintaining our ecosystems as well.
Whether or not they can be a replacement for the large numbers of digging mammals we have already lost, however, is another story.
For Professor Johnson, it all provides a compelling case for greater control of feral cats and foxes, which continue to be the biggest threat to the dwindling numbers of digging mammals that remain.
Until then, however, your best bet for seeing the grand work of Australia’s little diggers on mainland Australia may just be from behind a predator-exclusion fence.
Christopher Doyle ABC Environment
Originally Published at http://www.abc.net.au/environment/articles/2014/06/30/4029166.htm#comments