The case of the Cane toad invasion

The misplaced

General description

The cane toad (Rhinella marina, formerly Bufo marinus) is an amphibian vertebrate. Female toads average about 10 to 15 cm, and are larger than male toads. The largest toad on record is 24 cm long. They can weigh up to 2.5 kg. In adults, the skin is dry and warty in appearance, and  usually mottled shades of brown and yellow. Adult toads are toxic to most predators as the produce a milky poisonous substance from their parotoid glands, which are located at the back of their heads.

R. marina is indigenous in many South America countries, Central America and southern parts of Texas, USA. They spend most of their time on land but do require ponds for water replenishment and for egg laying. They can tolerate a range of temperatures in their native habitats, which enables them to thrive in many novel environments as does their ability to use brackish water for breeding if necessary.

Feeding

Cane toads will try to eat anything that they can fit into their mouths.

The cane toad is opportunistic in its feeding habits and will consume almost anything that it is able to catch. Terrestrial arthropods make up the bulk of the diet, but snails, crabs, small vertebrates (mammals, birds, lizards and frogs), pet food and human faeces may also be consumed. Cane toads will gorge themselves if food is in abundance.¹

In their juvenile form, cane toads are highly cannibalistic and feed largely on their smaller siblings.

Cane toad reproduction

Cane toads mate on land, but the females lay strings of jelly encased eggs in water.  There maybe as many as 25,000 eggs in a batch. The eggs hatch into tadpoles, which then develop limbs and reabsorb their tail tissue, turning into toadlets. The R. marina toadlets are darker in colour than their eventual adult form and their skin is smoother. The eggs are poisonous but the toadlets are not very toxic initially. As they grow older, they will start producing their own toxin.

Toads are thought to have a life span of about 10 years and females breed twice a year.

The displaced

Cane toads have been transported to a number of countries as a potential candidate for biocontrol of pests and it has mostly been a disaster in terms of a) the biocontrol not working as planned and b) the cane toad's ability to displace endemic species.

The toads have

• a high tolerance for a range of abiotic conditions
• the ability to breed rapidly
• voracious appetites that can encompass many types of raw material (An adult cane toad can consume up to 200 food items in one night)
• poison production to kill off potential predators
• poison sacs which makes them unpalatable to potential predators
• the ability to pass on fungal and microbial diseases which are dangerous to local amphibians

With these characteristics, R. marina can make itself at home in novel environments by killing native inhabitants and by out competing organisms which have  food requirements that overlap with the toads' diet range. This severely threatens local ecosystem stability through marked losses in biodiversity.

The consequences

Where is the invasion?

The cane toad, native to South America, has been deliberately introduced to many regions of the world—particularly the Pacific—for the purpose of biological control of agricultural pests.

The cane toad was taken Jamaica to  reduce the rat population - unsuccessfully. Despite its failure to control the rodents, the cane toad was introduced to Puerto Rico in the early 20th century to counter a beetle infestation ravaging the sugarcane plantations - successfully.

It was then introduced populations to Australia, Florida, Papua New Guinea, the Philippines, to several islands of Japan, to Taiwan, many Caribbean islands, Fiji and many other Pacific islands, including Hawaii.

Because cane toads have introduced to many parts of the world, there are many different stories of the consequences of their arrival in new ecosystems. Their impact on Australia has been studied extensively and is well documented, so it will be used here to document the invasion. It does not cover the entire range of possible effects, but it does give a good feel for the damage these invaders can bring.

The cane toad was introduced in the 1930s to coastal areas of Queensland. It was thought that they would eat native beetle species which were attacking sugar can crops in the area. Almost 90 years later, the cane toad population is estimated to be about 200,000,000  and the toads have spread from a relatively small area to an area of about  1.2 million km², with substantial populations in Queensland, Northern Territory, northern New South Wales and north parts of Western Australia.
In Australia, native animals directly affected by cane toads include:

• frogs
• reptiles such as goannas, crocodiles, blue-tongue lizards, snakes
• fish
• mammals such as quolls and bats
• ground dwelling birds

Wherever it has gained a foothold , the cane toad has brought with it declines  in native species. Over time, can , cane toads in Australia are overcoming what were thought to be hard barriers to their spread, presumably through natural selection and evolution. Some natives have learnt to avoid cane toads as prey, and some others have learnt how to make a non-lethal meal of toad flesh. The rakali (formerly known as the Australian Water Rat) is one such predator which has learnt to modify its behaviour to only eat non-lethal parts of the toads.  Some species of Melomys (e.g. M. burtoni ) will also happily eat Rhinella marina. Another is the Keelback Snake, which is native to Australia but shares some genes with Asian snakes which can eat the toads with no ill effects.

It is thought that some very fast evolutionary changes have occurred as a result of the invasion e.g. snakes with smaller mouths that prevent them from eating cane toads, cane toads with longer legs to assist them in travelling faster.

Eradication and remediation strategies

A number of measures to eradicate cane toads have been tried.

Manual removal - because cane toads can increase number so quickly, manual removal strategies have been largely unsuccessful. It does produce some useful results where water resources are reduced to a discrete number of small pools where the toads must visit to meet their water needs.

Capture by trapping  has not been very successful and has resulted in some native animals being caught in traps.

Aversion conditioning has had some success. It involves "teaching" natives that cane toads are not good to eat by making young cane toads available and the natives finding them unpalatable without actually dying.

Various forms of biocontrol- Ironically, our best chance of eliminating cane toads is going to be some form of biocontrol. Various scientific groups have been trying viral pathogens, pathogenic worms and pheromone possibilities.

Cane toads have been spotted hitching a ride on a large snake to escape rising flood waters.

Dig Deeper

• Cane toads in Oz
• The impact of cane-toads on the trophic cascades of freshwater ecosystems in the Northern Territory
• Cane toads on the march: invasive species finds 'ideal niche' in Australia
• Invasive species-driven trophic cascades: Are cane toads indirectly contributing to small mammal collapses across tropical Australia?
• Cane Toad Rhinella marina
• Darwin trial catches 22 thousand cane toad tadpoles in a week
• Shifts in the foraging tactics of crocodiles following invasion by toxic prey