Bilbies as ‘Eco-Engineers’
Bilby (Macrotis lagotis) diggings are important in the restoration of the soil and the regrowth of the vegetation of arid Australia. By creating deep holes in the sand, this enables plant material to fall in and decompose. Each bilby essentially creates numerous compost pits every night.
These become high in carbon and nitrogen; an important nutrient for the growth of plants and the germination of seeds. The graph on the left shows just how much greater the percentage of carbon found in the soil inside bilby diggings is, compared to the soil on the surface. This in turn creates many more opportunities for seedlings to grow, as you can see in the graph on the right (see here).
Range and abundance
One of the many bilby burrows on the reserve. Photo by Hugh McGregor
Bilbies are very powerful diggers, constructing spiral-shaped burrows which may be three meters long and up to two meters deep. The entrance to a Bilby burrow is often under a small shrub and at Arid Recovery is usually left open. At other sites Bilbies often backfill their burrow entrance, possibly to protect it from predators or to regulate the burrow temperatures. Bilbies frequently dig new burrows, and may visit up to ten in a night.
Bilbies have an excellent sense of smell and hearing, which they use to find food such as seeds, insects, bulbs, fruit and fungi. They can detect food up to a meter underground. It uses its long sticky tongue to lick up seeds and insects. This feeding technique means Bilbies ingest a lot of sand during foraging, so that up to 20 – 90% of their faeces may be composed of sand! The Bilby does not need to drink, obtaining all its water from its food.
Bilbies can breed all year round and have a gestation period of 12 – 14 days. The joeys spend the first eighty days of their lives in their mother’s pouch. Two weeks after emerging from the pouch, the young Bilbies become independent of their mothers. Female Bilbies can begin breeding from six months of age. A Bilby’s pouch contains eight teats although there are rarely more than two young occupying the pouch at one time. The pouch is backward opening so when the mother Bilby digs, the pouch does not fill with sand.
The rapid contraction in the Bilbies’ range since Europeans settlement is largely due to predation by feral cats and foxes. Other factors in their decline include grazing by rabbits and livestock, the fragmentation of habitat by land clearing, and changed fire regimes. Now classified nationally as Vulnerable, the fate of the Greater Bilby is hanging in the balance. A relative of the Greater Bilby, the Lesser Bilby (Macrotis leucura) is now completely extinct. At the beginning of the 1900s it was considered to be common in its range.
What is Arid Recovery doing?
In 2000, we released Greater Bilbies into the Arid Recovery Reserve. It is now estimated that there are between 800 and 1500 Bilbies within the reserve (see here). In 2004, we attempted to release Bilbies outside the fence. This was accompanied with substantial predator control, with a lot of extra trapping and shooting over many months. We also tried to train bilbies to become more predator savvy, by chasing them with model cats. These efforts were ultimately unsuccessfully.
In the future, it is hoped that we will develop more effective feral cat control and fox control techniques, and have better methods for making bilbies more wary of cats and foxes. The University of New South Wales in collaboration with Arid Recovery has developed a project ‘Tackling Prey-Naivety’ to try and improve the anti-predator responses of the Greater Bilby to cats and foxes. The project has seen positive results already, with Bilbies being able to survive and breed whilst living alongside low densities of cats in one of Arid Recoveries experimental enclosures.
Written by Rachael Loneragan (March 2017)