The scourge of rabbits

The concept for Arid Recovery emerged during the rabbit plagues of the early 1990s. Hordes of rabbits decimated the sensitive vegetation throughout the arid and semi-arid zones. Some long-lived shrub and tree species were at risk of being lost from the landscape as rabbits ate seedlings and prevented new plants from establishing. Arid zone vegetation also suffers from grazing and trampling by hard-hooved cattle, sheep and goats, and browsing by feral camels. The release of calicivirus caused rabbit populations to fall by up to 90%, and Arid Recovery’s founders took the opportunity to fence rabbits out completely from 14 square km and allow native vegetation to regenerate.

The fenced exclosure also excludes introduced predators, feral cats and foxes, and allowed for the reintroduction of locally extinct native mammals, including three busy diggers: the burrowing bettong, greater bilby and western barred bandicoot. These native animals dig for grubs, roots, tubers and seeds. Importantly, they do it in a different way to rabbits. Where rabbits scratch the ground in shallow scrapes, bilbies, bettongs and bandicoots dig deeper more pointed holes. As these native diggers established in the reserve, the ground became pockmarked by their diggings, creating a myriad of mini composting holes that collect water, nutrients and seeds – the perfect environment for plants to germinate. Research has found that bilby digs contain more than twice the soil carbon compared to the surface and significantly more nitrogen. More exciting still, bilby digs have a plant germination rate triple that of rabbit digs or surface soil. Within 5-10 years of rabbit removal and bilby reintroduction, sensitive long-lived tree species such as Mulga had made a comeback on the reserve.

Too many native herbivores

Unfortunately, vegetation recovery was not as simple as removing rabbits. In the second decade of Arid Recovery, burrowing bettongs became increasingly abundant (link to bettong page). These generalist herbivores can eat most of the plants in the reserve and generally continue to survive and breed through long dry periods. Some of the more palatable plant species such as Ruby Saltbush (Enchylaena tomentosa) became increasingly rare within the reserve as the bettong population boomed. The recruitment of new seedlings of Mulga and Callitris stalled, and some other palatable plants such as the Native Plum (Santalum acuminatum) have suffered from overbrowsing.

Efforts to stabilise the overly successful bettong population (including introducing a native predator) have finally seen the number of bettongs fall to more sustainable levels. We hope that the ongoing monitoring program will start to measure positive changes in plant cover and diversity as vegetation recovery resumes again.

Native mammals reduce shrub encroachment

Removal of introduced predators from the reserve has had other flow-on impacts to the vegetation. Native rodents such as Spinifex Hopping Mice have boomed in the safety of the fence to more than 15 times the density of rodents outside the fence. All these rodents (and quite a few bettongs) busily dig through the sand for seed to eat, and have changed the structure of vegetation. The dunes outside the reserve are markedly more shrubby than inside, because the booming mammal populations inside the reserve have chewed their way through the seed bank and reduced shrub germination. Until recently, ants were thought to be the main ‘seed predators’ in Australia, but work by PhD student Charlotte Mills has shown that’s only the case in the absence of native small mammals which are sadly missing from much of the continent now.

Toughing it out

Arid Recovery is in the driest desert of the driest continent. So dry in fact, that there is not enough rainfall to support the iconic spiky spinifex grass (Triodia spp.). Instead, the desert here is dominated by chenopods (saltbushes) on the clay swales and wattles and hopbushes on the sandy dunes. Chenopods are particularly adapted to growing in saline soils and are the favourite food of Stick-nest Rats. The rats cope with the high salt content in their food by excreting it out in highly concentrated urine.

The low rainfall and extremes of heat and cold in this region mean that plants are adapted to grow very slowly. A nondescript little saltbush no more than 30cm high could be as old as 200 years or more. The largest trees in the area, Native Cypress (Callitris glaucophylla), can be as old as 270 years. These trees are truly on the edge of survival in this dry country. Prolific colonisers elsewhere, Callitris around Roxby Downs can only set seed successfully after successive years of above average rainfall. With good rain very infrequent, an adult tree may have as few as three opportunities in its 200+ year lifespan to set seed and replace itself. And if a rabbit eats those seedlings before they can grow, the Callitris might not be able to replace itself at all.

Over 250 plant species have been recorded on the Arid Recovery Reserve, including one threatened species – Atriplex kochiana. Many of these species are annuals that lie dormant in the seedbank until a rainfall event. Summer rains particularly stimulate annual grasses, while good winter rains cause a profusion of wildflowers, including the fields of Poached Egg Daisies and Sturt Desert Peas.