As an eco-tourism destination, crescent-shaped and finely forested Lord Howe Island takes some beating.
The remnant of a now-extinct shield volcano, dating back 7 million years, the heavily eroded island is dominated by basalt peaks and surrounded by the world's southernmost coral reef.
With only a small band of residents, and controlled tourism, Lord Howe – which lies about 600 kilometres off the east Australian coast – has remained largely pristine since being settled in 1834.
Most of its forest remains untouched and the sheltered lagoon teems with wildlife. Alas, the same cannot be said of the endemic wildlife, which has been all but devastated by introduced animals.
Among these is a small and harmless-sounding amphibian – the bleating tree frog.
Introduced accidentally in the 1990s from eastern Australia, probably along with a boat-load of supplies, the light brown frog, with irregular bands on each side of its body, is now widespread across the island, including forested areas, cattle pastures and buildings.
To investigate how the creature got to the island, biological scientists at Monash University and the South Australian Museum took genetic samples from the Lord Howe population of frogs and compared them with the genetic information of those on the mainland.
"We wanted to find their closest relatives," says Lynette Plenderleith, who is completing a doctorate on the frogs. "We were able to determine that the Lord Howe Island population originated in northern coastal New South Wales."
Due to their intolerance of seawater, amphibians are rare on volcanic islands such as Lord Howe. "They must be introduced by humans to be able to colonise such isolated places," Ms Plenderleith says.
Although scientists are unsure of whether the frog will spell disaster for the island's wildlife community, they are calling for greater biosecurity controls and wider public awareness to help prevent more animals and plants slipping through the net and destroying native systems.
As yet, the bleating tree frog is the only one to have succeeded in invading the island. "But, without careful consideration for the future, other animals may follow suit," she warns.
Next up
Without stricter biosecurity measures, another potential invader is the cane toad, Rhinella marina.
The large, ground-dwelling amphibian was introduced from Hawaii in 1935 to control the grey-back beetle in north Queensland sugarcane plantations. The toads are now a pest of near-epidemic proportions.
Two decades of research and millions of dollars in federal funding have failed to slow their relentless progress. They have swept across northern Australia and have frog-marched, at the rate of about 50 kilometres a year, into the Kimberley and as far south as northern New South Wales.
They are now heading further southwards towards Sydney – and possibly beyond.
"The story of the cane toad is one of many from history that involves the misguided introduction of exotic animals to vulnerable areas," Ms Plenderleith says.
When Europeans first arrived in Australia, they missed the sights and sounds of their homeland. "So they brought over many animals for familiarity and comfort, some of which are now the most destructive in Australia," she says.
Combat strategies
Might they one day infest Victoria? "They'll keep turning up in trucks but are unlikely to establish themselves," says Sydney University biologist Rick Shine. "Of course, we thought that about Sydney, as well, until toads starting breeding in one of the suburbs a few years ago."
To tackle the problem, Professor Shine and his colleagues are pioneering novel techniques for combating the pest, now found everywhere except Antarctica and Greenland.
"Australia has no native toads, and so cane toads, and their associated bufotoxins, are a new and deadly food for many Australian animals if consumed," says one of Professor Shine's doctoral students, Georgia Ward-Fear.
Large predators, including quolls, monitor lizards (better known as goannas) and freshwater crocodiles, are at particular risk because they eat the poisonous toads, she explains. "Removing these large predators may cause huge imbalances in ecosystems and could lead to chaos for our native wildlife."
Thanks to a behavioural phenomenon known as "conditioned taste aversion", some animal species have the capacity to learn not to eat toads.
"If a native predator eats a small toad, it gets some toxins and will feel ill – but it is not killed by the experience," Ms Ward-Fear says. "They learn from such experiences and are not tempted to eat other toads they encounter."
Work on native quolls in the Northern Territory has found that would-be predators avoid toads after being subjected to "taste aversion therapy". This entails feeding a small sausage of minced toad with a nausea-inducing but non-toxic chemical.
When released into areas occupied by toads, the trained quolls and their offspring survive better than untrained quolls, Ms Ward-Fear says. Similar work on flood-plain goannas and blue-tongue lizards suggests that these species may learn the survival trick.
Lethal
"The problem for naive native predators is that the first toads they are likely to encounter are the large toads packed full of bufotoxins," she notes.
"If they attempt to eat these toads, the experience is likely to be lethal. This has resulted in large numbers of native predators being found dead during toad invasions. If these predators had encountered small toads, they may have had an opportunity to learn."
The idea of using "teacher toads" to save native predator populations was suggested by Professor Shine in 2008. This relies on the ability of native predators to learn not to eat toads if they are not killed during their first encounter.
"If we were to expose native predators to small toads as the invasion was arriving, predators could eat them and feel ill – but learn from the experience and not eat larger lethal toads," Professor Shine explains. "This method cannot save all the predators in an area but it can save enough of them to allow populations to recover."
The Sydney scientists are working with the WA Department of Parks and Wildlife and the Balanggarra Corporation, the land's traditional owners, at a remote flood plain in the East Kimberley, training local goannas in the art of taste aversion to prevent them from eating adult toads.
"If we succeed, our method of training will be modified for landscape-scale," Ms Ward-Fear notes. "We can then target other populations of goannas ahead of a cane-toad invasion."
Location
The scientists use a radio-tracking device, in the form of a specially designed VHF radio transmitter fitted to the tail of each goanna, in order to find them again after goannas have been captured and trained.
"We track the survival of 'trained' and 'untrained' goannas once the cane-toad invasion moves through," Ms Ward-Fear says.
The small trackers emit a pulse, each at different frequencies. The signals are picked up by a handheld radio receiver and antennae. "It's as if the goannas each have their own radio stations that we can hear when we become the aerials that tune into their frequency."
The scientists locate the goannas every few days to ensure the training is working. "We also collect important information on the goannas' rates of growth, habitat use and general ecology," Ms Ward-Fear says.
"We are getting lots of new and interesting information about the goannas as a by-product of this project. We also find that, after a goanna has eaten a small toad, it is unlikely to eat another one in subsequent training trials. Whether this translates into an aversion to the bigger toads has yet to be demonstrated."
So far, about 80 goannas have been fitted with tail transmitters. Cameras have also been set up around burrows to monitor their activities.
Breeding
Alternative approaches to dealing with toads also show promise. One involves interfering with their breeding.
Toads take between six and 18 months to reach sexual maturity. Breeding, generally in still or slow-flowing water, occurs twice a year. Toad spawn consists of long, gelatinous strings with double rows of jet-black eggs. Tadpoles emerge wriggling and eager to eat after about six to eight weeks.
The biggest obstacle to getting rid of cane toads is that one clutch of eggs, laid by a single female, can contain more than 30,000 eggs.
"Even if you catch and kill 99 per cent of adult toads in an area, the few remaining can produce so many offspring that, before you know it, you are back to where you started," Professor Shine points out.
The solution, he says, is to stop toads from reproducing: "A chemical 'bait' created from the toads' poison is a real magnet for toad tadpoles."
Secretions from the shoulder glands of dead toads can be used as bait. This is relatively cheap, easy to obtain and irresistible to toad tadpoles. "But it repels the tadpoles of native frogs," Professor Shine says.
"This is perfect for catching toad tadpoles in funnel-traps in ponds. Other native fauna such as fishes and insects aren't attracted to this chemical – but toad tadpoles are incredibly good at detecting it and they readily search for its source."
When toad exterminators use this chemical as bait in funnel-traps, they catch thousands of toad tadpoles and almost nothing else.
"In one natural pond, we collected more than 40,000 toad tadpoles in less than a week," Professor Shine recalls. "I think we got them all – over the next few weeks not one toad emerged from that pond."
The researchers plan to train "toad-busting" community groups in the proper methods of collection. (Toad poison is dangerous to humans, as well as many native species, and so must be carefully collected by trained people wearing protective equipment.)
"In continuing work with our collaborators at the University of Queensland, we are developing stronger, safer and easier-to-use bait," Professor Shine says. "To do this, we'll isolate the active agent in the toads' secretion and use it in pure form."
Ant tactics
Having evolved in Central and South America, cane toads are not adept at dealing with native Australian species and conditions.
"Evolution hasn't had a chance to adjust them to their new neighbours as yet," Professor Shine says. "For example, young native frogs are not at risk from meat ants because the frogs avoid places where the ants forage. If one approaches, they hop away."
Young toads, in contrast, ignore them even if they approach. The result is that millions of young toads are eaten by ants.
The scientists have shown that, by encouraging local ants to forage where young toads are gathered – by putting out cat food, for example – they can increase the number of toads that perish.
Warning
Unlike the tadpoles of native frogs, cane-toad tadpoles produce communication chemicals, called pheromones. One of these is a so-called alarm pheromone that warns other toad tadpoles of danger.
If scientists regularly add this chemical to ponds, the toad tadpoles get stressed and roughly one-third of them are wiped out.
"Native tadpoles don't detect the chemical, as it only affects toads," Professor Shine says.
The scientists have still other strategies up their sleeves. But, for the moment, "we need to keep working on it," Professor Shine says.
Links
Read the latest research on the bleating tree frog in PLoS ONE: 10(5): e0126287. doi:10.1371/journal. pone.0126287
Learn more about frogs in the textbook Oxford Big Ideas Australian Curriculum Science 8 (Oxford University Press)
VCAA link
Please send bright ideas for new topics to pspinks@fairfaxmedia.com.au
