I had to cut the story of one of my favorite invasive species from last week’s post, as it didn’t exactly fit into the story I was trying to tell. The species — the cane toad, one of the most invasive species in Australia — has such a storied history that it deserves its own post.
To start off, I cannot recommend the documentary “Cane Toads: An Unnatural History” nearly enough. Not only is it informative, it is outright insane from beginning to end, filled with frankly bizarre editing decisions and interviews such that it’s been described as “if Monty Python made a documentary”. It’s just over 45 minutes and well worth every second:
But anyway, the story goes roughly like this:
In the early 1900s, cane sugar was a big business for Australian farmers. The crop had a massive global market that the new country broke into with force; to this day, Australia is the second largest exporter of sugar.
But the crop had a problem in the form of beetle grubs. These grubs would destroy harvests, eating and breaking down plants before they could be sold at market and flying so thick as to block out the sun. As the sugar business swelled, farmers put more and more pressure on researchers to find a way for them to control these pests.
So when Raquel Dexter proposed introducing a toad species from the Americas to try and control the grubs, farmers jumped at the idea. In 1935, 102 toads were introduced to a single stream to try and curb beetle populations. And they didn’t stop there.
A timeline of cane toad invasion. Source.
Now, this uncontrolled spread could actually be seen as a positive thing if the animals were effective at controlling beetle populations. Unfortunately, they actually don’t eat those grubs at all. One of the problematic species of grubs lives its entire life in canes, far above the reach of the ground-dwelling toad; the other emerges in grub form when there are no canes in the field, leaving the toad no shelter for habitat. There was absolutely no overlap between these species at any point in time. There was no reason to think the biocontrol would work.
And as it happens, the grubs were one of the only species the toad wouldn’t destroy. The toads are fiercely poisonous, capable of killing almost any animal that punctures their poison-laden skin — which has included countless birds, pets, and even several children. They also make a diet of small animals, including rodents and snakes; they also eat so many insects that other predator species have been dying off due to a lack of prey.
So due to the huge danger posed to Australian species, conservationists have been focused on ways to attempt to stop the spread. Unfortunately, so far we haven’t been particularly good at it. Attempts to trap toads have also trapped native species, reducing competition for the toads that remain; attempts to cull the population have been thwarted by the incredible number of offspring the toads can produce. Recently, research has suggested that cane toad tadpoles can be lead to egg masses, which they’ll then eat; these tadpoles can then be removed from the pond to try and reduce the population size. We’re still working on it.
Also working on it are the rest of Australia’s fauna. Crows, for instance, have learned to flip toads over before eating only their insides, avoiding the poison-filled skin. Other predators are beginning to develop resistance to the toad’s toxin, a very rapid adaptation to the introduced species.
This is an example of the dynamism I mentioned last week, that the ecosystem is already adapting to this new invader. At the moment, the cane toad is still clearly harmful to the surrounding ecosystem; animal populations decrease dramatically when cane toads arrive in an area. But given enough time and adaptation, future Australians might see the cane toad as just another feature of their landscape, a commonplace — and completely natural — creature that they live alongside.
So should Australians keep working on trying to control the toads? At the moment, these toads still are a clear and present danger to the existing ecosystem, so by that logic they should still be removed. But there might come a point in the future where we view these invaders as a normal part of a functioning ecosystem.
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