Love them or Hate them, Cane Toads are Here to Stay( continued)
ARTICLES
Love them or Hate them, Cane Toads are Here to Stay( continued)
Native Animal Responses to Toads
As the highly toxic toads disperse across northern Australia, many native animals perish. Predators of frogs such as certain birds, quolls, goannas and most snakes, are often eliminated from an area after the arrival of toads. Even crocodiles are not immune to their toxins. Many parts of Queensland and the Northern territory have suffered local extinctions of species at the hands of toads. older crocodiles have become older because they have had a non-lethal encounter with toads and have remembered the experience.
Toads have been present in Queensland for almost 80 years and in that time some native animals have learned that toads are deadly and should be treated with care. Crows and currawongs have learnt how to eat toads by avoiding the most toxic parts and pecking out the belly parts of the toad( Figure 4). Other species have simply learned to avoid toads and not to eat them at all.
Figure 5 Young crocodiles still fall victim to the toads
Birds may be more tolerant to toad toxins than reptiles and marsupials. It has been suggested that this tolerance may be due to historic genetic exchange between Australian and Asian birds that have co-evolved with toads.
The Future
Figure 4 Torresian Crow preying on a Cane Toad. Rhinella marina, Lake Kurwongbah Qld. S. Wilson
The problem with developing a learned response to toads is that you have to survive your first( and perhaps second) encounter with toads. If you die as a result of the encounter, nothing is learned. However, if you survive( perhaps feeling sick in the tummy), you may be able to associate the sick feeling with toads and so learn to avoid them. If you are a species that has high parental care, you may be able to teach your offspring to avoid toads also.
But what if you don’ t have parental care of the young – how does the next generation learn of the dangers of eating toads? Consider freshwater crocodiles; they will eat a range of aquatic animals, including frogs. They also lay large eggs that are incubated in a nest that is guarded by the female crocodile. But the female does not teach the young about who or what to hunt. For a young crocodile it is trial and error. The result is that in areas where toads are present there is a high death rate of young crocodiles,
Toads are a highly adaptable species and they are here to stay. How our fauna ultimately respond to the presence of toads in unclear – some species will be lost, others will become more uncommon, others will be able to adapt and perhaps recover to former numbers. At present we seemed to have halted the spread of toads along the New South Wales coast through community vigilance and quick responses to the appearance of toads in a new area. Toads may still be able to spread southwards along the Darling River to Victoria. As there are fewer people and towns along the way, toads could easily go undetected and reach plague proportions.
In Western Australia, toads are currently raiding the Kimberleys. The invasion front will soon turn southwards where it will confront a dry barrier of about 100 kilometres before it can enter the Pilbarra( where toads will be able to survive and spread out). The WA government is considering a program of removing all dams and waterpoints in this 100 kilometre zone in an attempt to halt toads and keep the Pilbarra free of toads. Even if this tactic is adopted, stowaway toads are an ever-present threat to any area, and so surveys and checks will still be required to limit the spread of toads.
31 SCIENCE EDUCATIONAL NEWS VOL 67 NO 2