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