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Small mammal habitat protection -

what is needed? 

John Macris 
Convener of the Reserves Committee.

The last few years have seen the acquisition of some large new reserves in western NSW, totalling over 250,000 hectares.

Stock are being removed from these reserves after purchase, and artificial watering points progressively reduced. This, in conjunction with recent wet years and substantial reductions in rabbits by calicivirus, is producing some major areas where a more natural vegetation balance is returning. With grazing pressure alleviated, some plant species are springing up which have not been common in living memory. Thank heavens for a resilient soil seed bank; now – if we’re talking recovery, what about the fauna?

Animal recovery requires remnant populations. Many of the once common mammal species are entirely absent from these reserves, and indeed from most of mainland Australia. In the semi-arid to arid zone of the Western Division, faunal decline and extinctions have been appalling. Researchers estimate extinction of sixteen marsupial species in this region, three of these having disappeared nationally and twelve surviving only in a small number of refuges such as remote islands. (Ref 1,2)


Burrowing Bettong
Photo : Ken Johnson
Australian Photo Index

For native rodents the statistics are even more sobering, with 17 out of 23 once common species becoming extinct from the region since the 19th century and four of these now nationally extinct. (Ref 3)

Australia wide, the mammals most prone to decline and extinction since European settlement fall within the small to medium size and weight classes (35 g to 5.5 kg). (Ref 4)

An important question for the future management of the reserves in this region of high mammal decline is: what potential do they hold for reintroduction of locally extinct species? Signs of recovery of natural integrity have been promising. Hence, if the past impacts that led to decline and extinction of the small mammals are removed, the prospects for re-introductions should be good. Here lies the problem: some researchers believe that various forms of habitat modification were the main culprits in these extinctions, while others consider the impact of introduced predators to be fundamental.

Previously in this Journal (NPJ June 1995), Professor Peter Jarman provided a fascinating article on the small macropods known as bettongs. Burrowing, brush-tailed and rufous bettongs were once widespread in NSW. Now only the rufous bettong remains, in a fraction of its former range. It was Professor Jarman’s belief that while there had been many post-European impacts on bettongs, predation by foxes was the critical impact leading to extinction.

This question is now examined further, in light of other research.

Post-European impacts

Gross overstocking with sheep and the spread of rabbits in plague proportions coincided in the late 1800s. (Ref 5) These herbivores must have placed many native grazers under severe pressure. When drought took hold across NSW around the turn of the century, the standing vegetation was decimated by the millions of starving livestock and rabbits. Topsoil was blown in huge dust storms across the landscape. At the height of this drought, livestock numbers dropped by two-thirds in the Western Division (Ref 6) and one-third in the central and eastern regions. (Ref 7)

As most small mammal extinctions in NSW had occurred between the 1850s and 1920, the spread of grazing and introduction of the rabbit – in many ways a huge unplanned experiment in carrying capacity – are often identified as the causes of species losses.

Some more detailed research into the timing of species disappearances has, however, partly challenged this view.

Debating the timing and causes

For most species, the records of their last occurrence in a region are limited to a few accounts by scientists who were collecting for museums (such as Gould, 1863; Krefft, 1866), and so the most likely timing of extinction is ball-park. Many of the earliest declines were species at the smaller end of the range, such as rodents and bandicoots. At least some of these species were already very rare before the major phase of sheep grazing or invasion of rabbits. (Ref 8)

Change in fire regimes is one theory presented to account for an early habitat change. In brief, it was thought that the displacement of the western NSW Aboriginal inhabitants brought to an end a sophisticated practice of regular low-magnitude burns in mosaic patterns across the landscape. (Ref 9) Subsequent ecological changes and higher intensity bushfires are said to have impacted disproportionately on small mammals. However, studies of remnant populations of bettongs at Barrow Island WA(Ref10) tend not to support this theory on the ground.

Records indicate that extinctions of native mammals of around rabbit-body size occurred between the late 1800s and 1920s for most areas. Recently, a fascinating record of population dynamics of bettongs during this critical period has been analysed by CSIRO’s Dr Jeff Short.

Dr Short (1998) examined the records of NSW pasture protection boards in reference to annual bounties paid by these boards on scalps of certain vermin species listed under the Pastures and Stock Protection Act, 1880 and later equivalents. (Ref 11) Among species deemed as ‘vermin’ were the rat-kangaroos – the Potoroidae family of bettongs and potoroos. Some 2.85 million bounties were collected on rat-kangaroo scalps from 1886 to 1920. (Ref 12) and these are thought to be mostly for bettongs, except in moister coastal areas where the potoroo is found.

Bounty payment numbers represent a snapshot of species commonness. For example, payments numbering thousands at one point in time, then only dozens a few years later, suggest a major population decline. Hence the recording of payments according to specific pasture board regions on a yearly basis allows patterns of abundance of these small marsupials to be plotted and tracked over the 34-year period. Importantly, it also allows comparison of these changes with other records of possible impacts.

This analysis shows that populations of bettongs were not in fact in steady decline to extinction since the over-stocking and rabbit phase of the late 1800s, but rather experienced a number of major fluctuations in both downward AND upward directions until the early 20th century. As Jeff Short states: ‘in surviving to this point, rat-kangaroos had weathered major transformations to their habitat from intensifying human settlement, from invasion by rabbits, and from the interaction of these factors with drought’. (Ref13)

The very same vermin-bounty records also track the arrival and spread of the red fox (Vulpes vulpes) in NSW around the turn of the century. In all regions west of the moist coastal zone, as foxes moved into an area and proliferated, bettong bounty payments dried up from thousands and even tens of thousands per year to none. This collapse consistently occurred within 5 to 10 years of the arrival of foxes in a given area. (Ref14)

Getting serious about feral control

The evidence recounted here is supported by the fact that small native herbivores have persisted in the tropical north, Tasmania and on a number of small islands, all of which are free of foxes.

Several attempted reintroductions of bettongs on the mainland have failed, the only documented success being on a peninsula at Shark Bay in WA, where a barrier fence prevents fox penetration. In this setting, and in spite of the presence of rabbits and a few cats, the burrowing bettongs have multiplied in numbers since the early 1990s.

As the bounty payments example shows, small mammals are probably more resilient to human and grazing impacts than has often been thought, but can suffer sudden extinctions in the presence of introduced predators.

Therefore, to answer the question posed at the start of this article, if we want to contemplate re-introductions into the western parks – of species like bettongs, bilbies, small wallabies and so on – considerable investment in feral eradication is a clear and primary concern.

In future issues of this Journal, we will look at the various ways the problem of feral species is being approached, plus where injections of resources (or courage) could help the situation for our native species.

John Macris 
Convener of the Reserves Committee.

References

1 Dickman, C.R., Pressey, R.L., Lim, L. & Parnaby, H.E. (1993) ‘Mammals of particular conservation importance in the Western Division of NSW’. Biological Conservation 65:
219–48

2 The only species on this list still present elsewhere in NSW – the brush-tailed rock wallaby – persists in the eastern ranges, though its status is considered threatened

3 Dickman, C.R. (1994) ‘Native Mammals of western NSW: past neglect, future rehabilitation?’ in Lunney, D., Hand, S. Reed, P. & Butcher, D (eds) Future of the Fauna of Western New South Wales. Trans. Royal Zoological Society of NSW.
pp 81–91

4 Dickman, C.R. et al. (1993) op cit.

5 Denny, M. (1994) ‘Investigating the past: an approach to determining the changes in the fauna of Western NSW since the first explorers’, in Lunney, D. et al., op. cit., pp 53–63

6 ibid.

7 Short, J. (1998) ‘The extinction of rat-kangaroos (Marsupialia: Potoroidae) in New South Wales, Australia’. Biological Conservation 86: 365–77

8 Dickman, C.R. (1994) op. cit.

9 Short, J. and Turner, B. (1993) ‘A Test of the Vegetation Mosaic Hypothesis: A Hypothesis to Explain the Decline and Extinction of Australian Mammals’. Conservation Biology 8(2): 439–449

10 ibid.

11 Short, J. (1998) op. cit.

12 ibid.

13 ibid., p 373

          14 ibid., figure 4



Long-nosed bandicoot

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Long-nosed bandicoot