The cane toad may be the poster animal for invasive species. It is a native of South America and has been introduced into many ecosystems to feed on pests. The toad is now a problem, especially in Australia. The toad is now a pest, free from all the parasites and predators that it used to live in. However, its poison glands are a danger to most other species who try to eat it.
However, it is not immune to predation. Australian canetoad tadpoles were observed eating canetoad offspring. Cannibalism appears to be an evolutionary reaction to the absence of other species within its invasive range. This causes cane toads turn against their last competition, each other. The toad already has an evolutionary reaction to reduce the risk of cannibalism.
Cannibalism may make evolutionary sense because it can limit competition from other species. But the research team at the University of Sydney that has tracked the cane toad’s cannibalism suggests that the species’ successful invasion of Australia has accentuated this evolutionary pressure–something that may also occur with other invasive predators. One of the marks of an invasive species is its abundance in its new range, at which point competition for limited resources becomes more likely. This competition is limited, but cannibalism also provides nutrition resources.
There is plenty of potential for inter-toad rivalry with the Australian population, which has a density about 10x that of its native Australian range. This competition was documented in early toad development. Recent hatched toads can spend several days becoming tadpoles. They are often eaten by more experienced tadpoles. If there are many eggs in a body of water that is heavily populated, the clutches may become completely extinct before they reach their hatchling stage.
In South America, tadpoles may eat tadpoles. It is more common in Australia. The researchers set out to determine if there were biological differences in cannibalism between native and invasive populations.
They obtained toads of both native and invasive species and monitored the development of their offspring. The researchers started by placing fertilized eggs into a small container and one tadpole. The researchers found that Australian cane toads were becoming aggressive cannibals by placing fertilized eggs in a container with one tadpole.
Although many things can make a difference in this regard, researchers found that Australian tadpoles are more inclined to hunt for recently hatchling cane toads. The invasive Australian toads chose to move into containers with no hatchlings or one that contained hatchlings of cane toad cane toads. They were almost 30 percent more likely to choose the container with the hatchlings.
The hatchlings are no longer able to feed themselves and become too big for their peers. Some evidence suggests that the attraction to the eggs earlier is due to toxins from the mother.
Cannibalism, which is a high level of predation, tends to cause evolutionary responses that limit vulnerability. Researchers found that Australian toads spent less time at the hatchling stage to reduce the potential impact of cannibalism.
Two mechanisms were involved in this. The presence of tadpoles was a key factor in one of these mechanisms. This means that development accelerates when there is a threat. However, there was an additional acceleration regardless of the presence or absence of tadpoles. The hatchling stage took South American cane-toads approximately five days, while Australian populations spent only three days. The pressures of cannibalism reduced hatchling development by almost half.
So why don’t cane toads rush through hatchling stage if they can do this so quickly? Researchers found that Australian tadpoles developed slower than those from South American countries. Rushing through hatchling is costly and will pay off later in development and growth.
Many species have seen these kinds of changes caused by predator/prey interactions. It’s unclear if they have been documented so clearly in species where predator and prey can be the same species. The researchers make convincing arguments that this type of interaction is possible because the invasive species inhabits a different environment. Unfortunately, cane toads in Australia are also winners because of the competition for cannibalism.
Original publication: Ars Technica.
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Publiated at Sun 29 August 2021, 13:43:17 +0000