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Decline and recovery of Australian mammals: With particular emphasis on the burrowing bettong Bettongia lesueur

Short, Jeff (1999) Decline and recovery of Australian mammals: With particular emphasis on the burrowing bettong Bettongia lesueur. PhD thesis, Murdoch University.

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Australian mammals have fared badly over the past 200 years with 17 species extinct, 10 species surviving only on islands, and another 17 reduced to remnant populations of less than 10% of their range at the time of European settlement. Most extinct or threatened species fall within a critical weight range of 35 g to 5,500 g (CWR), as defined by Burbidge and McKenzie (1989). A disproportionate number come from arid and semi-arid Australia.

This thesis poses the questions: Why have these species proved so vulnerable and what can be done to redress the situation?

Section 2 describes a particular CWR species - the Burrowing Bettong Bettongia lesueur. The Burrowing Bettong is a small macropod that is extinct on the Australian mainland, but survives as remnant populations on offshore islands. Study of this species has been largely neglected due to the relative isolation of its surviving populations and the belief that they may be secure because of that isolation. I surveyed four islands on which Burrowing Bettongs occurred to establish their abundance and distribution; to assess the major factors impacting on their abundance; and to detail their ecology.

Section 3 briefly summarises the controversy surrounding the loss of CWR mammals. Is the primary cause of their decline and extinction due to a decline in habitat and habitat quality, to predation, or to some other factor or combination of factors? Previous major reviews have been comprehensive and inclusive, generally concluding that declines and extinctions of fauna are due to complex interactions of many factors. While the value of such reviews is acknowledged, my approach has been more pragmatic. I approached the issues from the view of a manager whose primary aim is an on-ground result: successful re-establishment of populations of threatened species after an absence of more than half a century. I took a hierarchical view of threatening processes seeking to identify and focus on the process most likely to limit successful reintroduction.

Section 3 includes three discrete bodies of research that each probe the cause or causes of mammal decline. The first examines the distribution, abundance and ecology of CWR mammals (including the Burrowing Bettong) on Barrow Island as influenced by vegetation mosaic. The second examines the historical record of bounty payments made on rat-kangaroos in New South Wales (the Burrowing Bettong is one of five species) to establish the spatial and temporal pattern of decline. This decline is juxtaposed against the major ecological events of the time: Sheep, Rabbits, Foxes and drought. The third collates and synthesises unpublished data on twenty five past reintroductions of macropods in an attempt to identify threatening processes and threatening practices. One such attempt was the unsuccessful introduction of Burrowing Bettong to Kangaroo Island in South Australia in the 1920s.

Section 4 details the reintroduction of the Burrowing Bettong from Dorre Island to Heirisson Prong at Shark Bay. This was the first reintroduction of this species to mainland Australia and was conducted against a background of past failure or limited success of reintroductions to arid and semi-arid Australia. This study pioneered or advanced a number of innovative techniques: the use of peninsulas to gain advantages in predator control; the use of a number of complementary barriers of defence against predators to minimise incursions, and in situ captive breeding of the endangered species in its natural habitat to provide a pool of animals for release over successive years (a buffer against demographic and environmental stochasticity and providing for adaptive development of predator control methods).

The reintroduced population of Burrowing Bettongs on Heirisson Prong represents the first mainland population for over 50 years. An initial group of twelve animals were transferred from Dorre Island in May 1992 and a first release to the wild was made in September 1993. The population has persisted in the wild for over five years and now exceeds 130 animals and is continuing to grow. Its continued survival depends on ongoing predator control.

Section 5 reviews the historical pattern of decline of mammals and the range of threatening processes that might be responsible. There appear to have been at least two major periods experiencing high loss of mammals in post-European times. The mainland extinction of the Burrowing Bettong forms part of the second phase - attributed primarily to predation by Foxes. The first period of loss coincided with, or immediately post-dated, European pastoral expansion. Typically, species lost during this period were smaller species within the CWR (< 350 g). These species may have been unable to cope with habitat alteration caused by stocking; or they may have been victims of the colonisation of the continent by another exotic predator - the feral Cat. The early loss of smaller species on both sides of the continent suggests the latter. Suggestions in the literature that Cats became established across Australia in pre- European times suggest otherwise. However, I provide some evidence to suggest Cats may have become established in the early pastoral phase of post-European Australia. Hence, predation by Cats provides a plausible explanation for the early loss of mammals.

The research within this thesis is firmly embedded in the “declining population paradigm” of Caughley (1994) and has made little or no use of the new tools of the “small population paradigm”. I have assumed that the cause of declines and extinctions is some external agent and I have sought to identify that agent and ameliorate its impact.

Caughley’s criticism of the “declining population paradigm” is that it represents a case-by-case zoological investigation to solve problems of particular species threatened with extinction. This does not appear to be true in this instance. The CWR mammals appear to have suffered broadly similar fates. Hence solutions generated for one species should have broad application to a suite of species. The conclusions may also generalise to extinctions and declines at many other insular situations that have been invaded by humans and their pest species in historic times.

The body of research within this thesis has advanced the general understanding of the processes of extinction of CWR mammals and provided the knowledge to successfully re-establish at least one species to the mainland from remaining relict populations on off-shore islands. It has led, also, to further related work on the control of predators; the behaviour of native species that make them vulnerable to predation by exotic predators; the behaviour of predators that led to a scale of impact greater than one might expect from predator-prey theory; and provides baseline data for modeling the interaction between predator and prey.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Bradley, Stuart and Wooller, Ron
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