by

Associate Professor Mike Tyler

 There are three amphibians that are of concern in the assessment of feral populations: the Cane Toad Bufo marinus, The Asian Toad Bufo melanostictus, and the African Clawed Frog Xenopus laevis. 

Bufo marinus

The history of the 1935 release and artificial dispersal of B.marinus in eastern coastal Queensland is well documented.  Subsequently the species has made big inroads westwards into the Northern Territory and more slowly into northern New South Wales, but what is less well known is that as early as 1963 an isolated population was well established at Mt Isa. Two aspects of the toad’s presence in Australia remain uncertain.

Firstly, the extent of the continent that ultimately will be colonised and secondly, the impact that this species has upon the existing native fauna.

In terms of future colonisation, the current geographical limit in southern Queensland is approximately 100 km north of the headwaters of the Murray-Darling drainage system.  Given adequate time this species can be predicted to penetrate this system and proceed south through New South Wales and Victoria into South Australia.  This is not a wild prediction but based upon evidence of the habitats that exist there and the temperature tolerances of the toad, particularly in relation to water temperature suitable for breeding.  The billabongs along the River Murray are ideal breeding sites.

The question of the short and long-term impact of the Cane Toad upon native fauna is less readily answered.  The existing evidence is that a wide range of predators that feed upon native frogs are impacted seriously by the arrival of the Cane Toad.  These species include the Quoll and several goannas.

In the long term there is one observer who argues that individual predators who survive the arrival of the Cane Toad do so by possessing an intrinsic (and genetically linked) capacity to avoid the species.  It follows that the initial impact in any area will be one of almost total annihilation of all frog/toad predators, followed by the recovery of a new group of populations with the capacity to avoid (and therefore survive) the arrival of the toad.  The evidence in favour of this hypothesis is thin, and its few adherents will have to accept the responsibility for the long-term impacts of the species.

During the 1970s, when concern was expressed about the impact of the Cane Toad, statements were made that there was a disease that was bringing about the demise of the toads.  The observations were based upon evidence that toads in areas such as Townsville were emaciated and dying.  Experiments conducted by Tyler and Spears demonstrated that the emaciation was attributable to starvation: the toads and their ancestors in that area had eliminated the insects and other components of their food source.  It is this impact of the species that is rarely mentioned.  Effectively, Cane Toads are small vacuum cleaners that eat, and therefore eliminate all insect and other invertebrate animals that exist at ground level.

What does the future hold?  At present the only research being undertaken is by CSIRO via a Natural Heritage Trust National Feral Animal Control Program.  There are two components to this grant.  The first is seeking a gene that is specific for the development of the species, and then using that gene to interrupt the normal development process.

The second approach is to develop a viral vector that can carry a gene that will interrupt the normal metamorphosis of the tadpole.  Each of these programs has vague issues, but at least the search for a control agent continues.

One aspect of the presence of the Cane Toad that has not been considered is the sociological impact.  In 1974 when a number of toads were recaptured in Darwin after an accidental release, local inhabitants told me that one of the benefits of living in Darwin, as opposed to Queensland, was the absence of cane toads.  No one would consider Darwin a pristine environment but there is no doubt that many will consider it less attractive when it is infested with toads.

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Bufo marinus

Some quarters of government declare that Bufo marinus is not a pest, defining the latter as a species that has been responsible for the extinction of two or more native species and/or impacts upon primary production.

Bufo melanostictus

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Bufo melanostictus

Bufo melanostictus is a large toad widely distributed in Asia.  Its capacity to colonise new areas is demonstrated by the report by Menzies and Tapilatu of the presence of an isolated population near Manokwari in the far west New Guinea.  This population ranges over a distance of 80 km but the date of its arrival in the area is unknown.

On two occasions I have identified isolated individuals of B. melanostictus found at the dock in Darwin amongst shipments of timber from Malaysia.  It must be considered a species of concern because of its colonisation capacity and the fact that coastal areas of northern Australia provide suitable habitat.  Produce from numerous Asian parts will need to be closely inspected to prevent the introduction of this species.

Xenopus laevis

 
Being totally aquatic, Xenopus laevis is readily maintained in captivity, and is by far the most popular laboratory amphibian in the world.  It is present in several laboratories in Australia and is maintained there under strict quarantine controls.

The impact of feral populations is substantial because Xenopus is a carnivore, and predates upon native species of frogs and fish.  In many countries feral populations have become established and eradication attempts have been unsuccessful.    The species poses a major threat if released in Australia.

Michael J. Tyler

October 2001

University of Adelaide

This article is reprinted with kind permission of the author.