The cane toad (Rhinella marina) is a large amphibian native to Central and South America. Introduced to Australia in 1935 to control cane beetles, the toads quickly spread, becoming an invasive species across much of northern Australia and parts of Florida. They pose significant ecological problems due to their toxicity at all life stages, voracious appetite, and rapid reproductive rate. Cane toads poison native predators, compete with native wildlife for food and habitat, and alter ecosystems. Understanding the various factors that lead to their mortality, both natural and human-influenced, is important for managing their populations.
Natural Mortality Factors
Cane toads, despite their resilience, face several natural mortality factors in the wild. Some native Australian animals have adapted to tolerate their toxins or developed strategies to consume them safely. The Keelback Snake, for instance, can eat cane toads without lethal effects, a trait potentially linked to its Asian ancestry where similar toxic toads exist. Birds such as the Torresian Crow and Australian White Ibis have learned to flip toads over and consume less toxic parts. Native rodents, including the dusky rat and water rat, appear to tolerate cane toad toxins and prey on them.
Diseases and parasites also contribute to cane toad mortality, though they rarely control populations broadly. Cane toads in Australia often host lungworms, small thread-like worms living in their lungs. While these parasites can be fatal in severe cases, the toads brought this specific lungworm species with them from South America. Environmental stressors also play a role; toads require constant moisture and can die from water loss or excessive absorption in floods. Extreme temperatures outside their 5–37°C range also impact survival.
Cannibalism among tadpoles is another factor limiting cane toad numbers. Tadpoles consume the eggs and hatchlings of their own species, a behavior pronounced in invasive Australian populations. This self-regulation helps control their abundance by eliminating competitors. Scientists are even exploring gene-edited “super-cannibal” tadpoles as a potential control strategy.
Human-Led Control Strategies
Human intervention is crucial for managing invasive cane toad populations, with a strong emphasis on humane methods. The most widely recommended humane euthanasia method involves stepped hypothermia: placing the toad in a refrigerator for at least 12-24 hours to induce torpor, an amphibian state similar to hibernation that switches off pain receptors. After this cooling period, the toad is then transferred to a freezer at -20°C for at least 24-48 hours to ensure death. This method is considered best practice and is suitable for all life stages, including eggs and tadpoles if placed in water.
Another humane method involves specific anesthetic sprays. HopStop, an aerosol spray containing chloroxylenol, is designed to be sprayed directly onto toads, anesthetizing them within seconds and causing death humanely within 30-60 minutes. This product is approved by organizations like the RSPCA for humane cane toad control. When using such sprays, it is important to apply sufficient quantity, typically two sprays for larger toads, and to confirm death before disposal.
Physical removal techniques are also employed. Hand capture, particularly at night when toads are active, is a common method for individuals and community “toad busting” events. While manual collection can significantly reduce toad numbers in localized areas, it is most effective when integrated with other strategies, such as targeting breeding sites.
Exclusion fencing, made of smooth material at least 50 cm high and secured into the ground, can create barriers to keep toads out of specific areas like yards, as they are not adept climbers or jumpers. Proper disposal of euthanized toads is also important, as their toxins remain potent after death. They can be composted in a hot system, placed in household bins, or buried at least 50 cm deep to prevent access by other animals.
Ineffective or Harmful Methods
Certain methods for controlling cane toads are ineffective, inhumane, or harmful to the environment and other species, and should be avoided. Physical trauma, such as hitting toads with bats or golf clubs, is inhumane and often does not result in a quick or painless death. Such actions cause prolonged suffering and are ethically unacceptable.
The use of unapproved household chemicals or poisons is highly discouraged due to the significant risk of harm to non-target species, pets, and the environment. For example, spraying toads with Dettol is inhumane, causing extreme pain and tissue damage. Other chemicals like methylated spirits are not reliably fatal and lead to prolonged suffering. Applying these substances can also contaminate soil and water, negatively impacting native wildlife and pets.
Introducing non-native diseases or biological control agents without thorough research can have unforeseen consequences on ecosystems. Deliberately spreading diseases could inadvertently affect native amphibian populations or other species. Early attempts at biological control, such as engineering a virus, were discontinued due to concerns about their broader ecological impacts and the finding that managing local impacts was a more effective long-term approach. Only scientifically validated and humane approaches should be used for cane toad management to protect both the target species and the surrounding ecosystem.