Before I delve into why the cane toads might carry an important message today, it’s important to go over a bit of their history. They were originally introduced into Queensland, Australia from Hawaii (a place where they are also an introduced species) to combat a problem with cane beetles infesting sugar cane crops (Fig. 2: arrow shows site of introduction). While the initial introduction contained only about 100 toads, in the subsequent years over 60,000 toads were introduced into northwest Australia. However, not only did these toads become the poster organism for invasive species, they did little if not nothing to control the sugar cane pests.
Cane toads are a great example of all the traits that make “good” invasive species. The females lay tens of thousands of eggs a year and are able to lay these eggs in almost any body of water. Not only are these eggs not susceptible to the viruses or predators present in their native habitat, but the tadpoles possess a toxin that kills many of the animals that try to eat them. However, it isn’t just the tadpoles that possess this deadly defense mechanism, the toxin coats every life stage of the toad and provides a huge obstacle for any potential predators. Finally cane toads will eat just about anything both dead and alive, which makes them a menace to any native insects and small animals species.
These traits combined with their ability to survive in a wide range of climates have made them especially successful in Australia. While once just 60,000 toads on a small section of the coast, there are now more than 200 million toads spread over almost all of Queensland and into the Northern Territory. Figure 3 shows the spread of cane toads between 1940 and 1980. Due to their toxic coating, the toads have led to large decreases in many reptile, bird, mammal and rodent populations. In addition, their strong competitive edge has led to reductions in native toad species. Overall, the toads have had a huge impact on the biodiversity in their introduced region and are hypothesized to spread further into currently toad-free areas of west, and perhaps even south Australia.
While people have tried and proposed many methods for controlling the cane toad populations in Australia, nothing has been successful so far. Physical removal of toads merely provides the remaining toads with an even further competitive edge by removing intraspecific competition. Scientists have proposed the idea of genetically engineering a cane toad specific virus and introducing it into the population, but a method like that requires extensive funding, research and time. Professor Peter Koopman’s work on the sex chromosomes of the toad has provided an interesting idea for biological control. He has proposed a method by which genetically modified male cane toads that produce only male offspring could be introduced into the population and over time skew the sex ratio such that the cane toads would no longer be an issue. While ideas such as Koopman’s are inspired and quite creative, they too require large amounts of time and money in order to put them into action.
However, there might be hope. Scientists are starting to see changes in the way native Australian species interact with their new toad neighbor. Birds that once died from ingesting the cane toads are now selectively preying on the less toxic parts of the toad like the stomach. In addition, some native snake species have taken to consuming smaller cane toads and avoiding larger ones, which by extension have more toxin covering their bodies. Even more interestingly, young cane toads are more susceptible to the native meat ants as they do not possess the adaptive traits that allow for escape from these predators like their native toad counterparts.
I find this trend in adaptive behaviors in native species very interesting. After all, cane toads have been present in Australia for over 85 years. While I don’t think anyone would argue that they have not been a destructive force, over time perhaps their effect on the ecosystem is becoming more complicated. As time passes and native species adapt to the presence of the cane toad, is the toad considered part of the local habitat, or is it still a target for removal? Could removing an invasive species long after it was first introduced be potentially detrimental?
Imagine the Pacific Coast, with its large dunes rooted by European beach grass (Fig. 4) This grass, introduced on the west coast in the late 19th century has allowed for the stabilization of the dunes and the ability for other plant species to colonize closer to the coast line. While an invasive species, the eradication of this grass that has existed on our beach for over 100 years could have unforeseen effects not only on our man made infrastructure, but also on the ecosystems both in and around the dunes themselves.
I think the same might be true for the cane toads. They have not only become part of Australia to some of the people who live there (as seen in Cane Toads: An Unnatural History), but they have caused both known and possibly unknown adaptive changes in the species around them. Not only is any method to remove these toads going to be costly in terms of time and resources, but it could also have unforeseen ecological effects.
Personally, I think that invasive species are a huge problem and that there are plenty of new, or relatively new harmful species that we could be focusing our time, money and energy to eradicate and control. In addition, we should be trying to find ways to avoid spreading more potentially invasive species by monitoring trade and rethinking some classic ideas around pest management. However, after 85 or 100 or even 200 years of unsuccessful management methods, perhaps devoting more time and research on an established invader will do more harm than good. By trying to control cane toads in Australia, we could be missing opportunities to combat invasive species that haven’t fully actualized their destructive potential.
Wikipedia: Cane Toads in Australia
Cane Toads: An Unnatural History: Youtube
Figure 1: Photo taken by Duncan Mclean
Figure 2: Shanmuganathan et. al. 2007 (see references)
Figure 3: Wikipedia
Figure 4: Photo from Oregon State University. Link
Brown, Gregory P., Matthew J. Greenlees, Richard Shine and Georgia Ward-Fear (2009): “Maladaptive Traits in Invasive Species: in Australia, Cane Toads Are More Vulnerable to Predatory Ants than Are Native Frogs.” Functional Ecology, Vol. 23,. pp. 559-568
Koopman, P. (2006, June). Daughterless cane toads. Paper presented at Invasive animals CRC/CSIRO/Qld NRM&W cane toad workshop, Brisbane, Australia.
Phillips, B. L., & Shine, R. (2006). Adapting to an invasive species: Toxic cane toads induce mophological change in Australian snakes. Proceedings of the National Academy of the Sciences, 101(49), 17150-17155.
Shanmuganathan, T., Pallister, J., Doody, S., McCallum, H., Robinson, T., Sheppard, A., Hardy, C., & Halliday, D. (2010). Biological control of the cane toad in Australia: A review. Animal conservation, 13(s1), 16-23.
Tyler, M. J. (1998). Australian frogs a natural history. Ithaca, NY: Cornell University Press.