Summer 2003
A Rocky Road To
Re-introduction

 

The history of small macropod reintroductions should remind us of the risky nature of the bettong project: all but a few previous attempts have failed.    Bettongs were not the only small mammal species whose number declined as a result of Europeans' changes to the Australian environment.    At least eighteen Australian mammal species have become extinct since 1788, half of all mammal extinctions worldwide in recent time.    In 1991 a further 113 Australian animal species were classified by the I.U.C.N. (International Union for the Conservation of Nature and Natural Resources) as being endangered or threatened.    These species only survive in small numbers, in a few highly vulnerable locations.

 

Endangered macropods have been reintroduced to parts of their former ranges at least 25 times in the last few decades.    Most attempts failed within two years, but a few of the populations introduced to islands have survived.    Overall, 11% of reintroductions to mainland sites have succeeded, and 60% of reintroductions to islands.

 

The reason for the difference becomes clear when sites are compared in terms of the presence of predators.    Only 8% of reintroductions to mainland and island sites with predators succeeded, compared with an 80% success rate for islands without predators.

 

The effects of predators on their prey change depending on the prey species' density.    Dr. Roger Pech and colleagues in the CSIRO Division of Wildlife and Ecology demonstrated that the predator-prey relationship can fall into two possible steady states: predators may have no regulatory effect on a prey population, killing that might have died anyway; or they can have profound effect, maintaining prey abundance at low levels, at times causing the populations of prey species to "crash".

 

Each state is fairly stable, but dramatic natural or human induced changes to environment conditions may cause prey species to switch from one to the other.    The birth rate of a predator regulated population might increase dramatically following a year of particularly good rainfalls and food abundance, or if predators fall suddenly.    This may lead to what Dr. Pech calls an outbreak: a change to the unregulated state, where prey species escape the control of predators.    In an outbreak, predators do not limit the prey population size in that state.    Rather, the abundance of prey rises and falls according to the carrying capacity of the ecosystem.    In contrast, when an unregulated population is exposed to a drought or disease (for example, myxomatosis among rabbits) the birth rate can decline, leading to a change to a predator regulated state.

 

Food Availability

 

The populations of unregulated prey species oscillate depending on climatic factors, with predator numbers following changes in prey numbers after a slight lag.    Predators in these systems do not exert a controlling effect on prey; the usual reason for a decline in prey abundance is food availability.

 

This up-down pattern can continue for centuries, but if any extreme environmental changes occur that allow prey numbers to fall below the threshold where they can be regulated by predators, prey numbers can be maintained at low levels, with the risk of falling to zero.

 

Bettongs, like most of Australia's small endangered mammals, are always vulnerable to predators because of their small populations.    It's hard for us to influence the predator-prey relationship to change to an unregulated state, because foxes and cats are so difficult to control in large areas for any length of time.

 

Despite Heirisson Prong's geography, which helps exclude predators, and the extensive baiting and other precautions that have been taken there, we can't assume the bettongs are safe.    A break in the fence, combined with a decline in the baiting program's effectiveness, could see predators infiltrate the Prong once more.    We will never really be able to label the bettong project a 'success' while the peril of predators continues.
 


 

Acknowledgements for this article goes to “Ecos”, the CSIRO and the Useless Loop Community Biosphere Project Group.       Line drawing with thanks to Sue Stranger

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