The long-term effects of toads on the Australian environment are difficult to determine, but some effects include "the depletion of native species that die eating cane toads; the poisoning of pets and humans; depletion of native fauna preyed on by cane toads; and reduced prey populations for native insectivores, such as
skinks." Precipitous declines in populations of the
northern quoll,
Mertens' water monitor, and
Mitchell's water monitor have been observed after toads have invaded an area, causing them to become endangered. A number of cases of declines in
goanna and
snake populations have been reported after the arrival of toads. For example, local populations of
yellow-spotted monitor dropped by up to 90% when their habitat was invaded by cane toads. The preliminary risk assessment of cane toads in
Kakadu National Park stated that the predation of the cane toad by native wildlife is the greatest risk to biodiversity. Other factors, such as competition with native wildlife for resources, and the predation of the cane toad on native wildlife, were considered much lower risk factors, but requiring further study. In the Northern Territory,
goanna deaths resulting from poisoning after predation on cane toads has been linked to a rise in the number of undamaged
saltwater crocodile eggs. Cane toads were present within a few days of the crocodiles hatching in April 2007. Numerous native species have been reported as successfully preying on toads. Some birds, such as the
black kite (
Milvus migrans), have learned to attack the toad's belly, avoiding the poison-producing glands on the back of the head. There are numerous reports of Ibis picking up cane toads in their beaks, flicking them about, washing them and then eating them. Anecdotal reports in the Northern Territory suggest that a native frog,
Dahl's aquatic frog (
Litoria dahlii), is able to eat the tadpoles and live young of the toad without being affected by the poison that often kills other predators. This may account for slower than expected infestations of toads in certain areas of the Northern Territory, although later research carried out jointly by several Australian Universities casts doubt on these reports. Some snake species have been reported to have adapted smaller jaws so that they are unable to swallow large cane toads, which have large quantities of poison. Another study, however, notes that the cane toad is adapting to a wider environmental range and may in the future be spreading into habitats currently not available. In 2009, the native
meat ant was found to be immune to the toad's poison and can successfully prey upon young cane toads. Whereas native frogs and toads have natural reflexes to avoid the meat ants, the cane toads do not tend to try to escape the ants, rather standing still when attacked waiting for the toxin to kill the attacker. New research has indicated that cane toads prey on
dung beetles by nestling in
cow pats and waiting for the beetles, eating up to 150 in one meal. In areas where cane toads have free access to water in dams, the dung beetles have been decimated. This indicates that the cane toad has the potential to economically affect the cattle industry through increased disease in cattle.
Predator effects with a juvenile cane toad. Predators in Australia are not adapted to the cane toad's toxin, which is its main defense mechanism. Because of this, toads do not tend to hide and are usually targeted by predators, which then expose themselves to the toxic effects. The larger the animal, or predator, is the better chance it has of survival, as its body weight effectively dilutes the concentration of the toxin in its body. There are reports of one native species, the
Torresian crow, eating cane toads without ingesting the poison by flipping it onto its back and eating its non-toxic innards via its underside. In 2014, researchers found evidence that the Australian freshwater crocodile (
Crocodylus johnstoni) had learned to eat just the hind legs of cane toads. One research study concluded that in less than 75 years, the
red-bellied black snake had evolved in toad-inhabited regions of Australia to have increased resistance to toad toxin and decreased preference for toads as prey. Evidence exists for native predator species
adapting to the presence of the cane toad, through learning or evolutionary selection, but the initial drop in population is often steep and can reduce biodiversity on a population level. One proposed solution is to use "teacher toads", or smaller toads that are less likely to kill predators. These toads would allow predators to learn not to eat the toads while mitigating mortality. Some have even proposed adding some chemical to make the toads distasteful to further discourage predation. These efforts have shown some promising results so far.
Richard Shine is using
behavioural conditioning techniques to teach the northern quoll to avoid cane toads. In 2019, the native
rakali or Australian water rats (
Hydromys chrysogaster) were found to have learned to make an incision to eat the cane toads' hearts and livers, while avoiding their lethal skin and glands. The water rats were able to adapt the hunting strategies within two years of the cane toads' introduction into their territory. It has been found that in Australia, cane toad tadpoles eat cane toad eggs.
Methods to control spread of cane toads in Australia Currently, most attempts to decrease the invasion of cane toads have been unsuccessful. Many of these strategies involve the physical trapping of toads, but these methods also capture unintended native species. Since the largest selective pressure on cane toads currently is intraspecies competition, these physical removals often only improve the conditions for untrapped toads. Also, since migration is high, any area purged of toads would most likely be reinvaded quickly. Many new ideas have been proposed to control the cane toad population. Some have suggested introducing a native viral or bacterial pest of the toads, but this has potential to once again invade native species. Two similar strategies have been proposed, both of which focus on
fecundity. One involves the release of sterile males into the population. These males would compete for resources with other males, while themselves not being able to reproduce. A second strategy would be to insert a gene in female toads, which would allow them to only create male offspring. In theory, this would limit the reproductive rates and control the population. Determining the efficacy and dangers of these approaches is difficult, as these methods have never been attempted, especially on a large scale. As of 2024, commercially available cane toad tadpole traps are being produced, with a bait using a lure consisting of a small
airstone coated with pheromone extracted from the parotoid gland of the toads, which attracts cane toad tadpoles but not other tadpole species. The traps are placed in the shallow edges of bodies of water, and are able to catch thousands of cane toad tadpoles within hours. Some local and state government bodies have been promoting the usage of such traps by residents to curtail spread of toads without impacting on the tadpoles of other frogs. In semiarid areas where water is scarce, the construction of toad-proof fences around dams can severely affect toad survival rates by denying them access to water. The RSPCA has guidelines for the humane culling of cane toads. Inhumane ways are illegal in most states and territories. Due to concerns over potential harm to other Australian wildlife species, the use of Dettol as
pest control was banned in Western Australia by the
Department of Environment and Conservation in 2011. Large predators, such as the
yellow-spotted monitors and
sand goannas, are being fed young, small, cane toads by researchers to create a "food poisoning"-like experience for the predators, in the hope that they will avoid eating adult toads, which might kill them. Poisonous sausages containing toad meat are being trialed in the Kimberley (Western Australia) to try to protect native animals from cane toads' deadly impact. The Western Australian Department of Environment and Conservation has been working with the University of Sydney to develop baits to train native animals not to eat the toads. By blending bits of toad with a nausea-inducing chemical, the baits train the animals to stay away from the amphibians. Researcher David Pearson says trials run in laboratories and in remote parts of the Kimberley region of WA are looking promising, although the baits will not solve the cane-toad problem altogether. By 2023, field research indicated that the taste aversion project had not been effective in reducing toad-induced northern quoll decline in the Kimberley. ==Novel uses==