Posted by jlwile on May 27, 2011
In my previous post, I discussed the cane toad invasion of Australia. While studies of the invasion have shown a new mechanism of selection that is distinct from classic natural selection, they have also shown how limited the range of evolutionary change in cane toads really is. This is consistent with the creationist view and quite contrary to the evolutionist view.
In this post, I want to discuss the changes that the cane toads have produced in other Australian animals. As you might expect, as a foreign species spreads across an ecosystem, it is going to have an effect on the already-established species there. In general, one expects the effects to be negative, but that doesn’t always seem to be the case. Indeed, a large study designed to assess the damage that the cane toad invasion has done to the already-established animals in Australia says:1
Overall, some Australian native species (mostly large predators) have declined due to cane toads; others (especially species formerly consumed by those predators) have benefited; and for yet others, effects are minor or are mediated indirectly rather than through direct interactions with the invasive toads.
So in the end, it’s a bit of a mixed bag. However, what I find interesting are some of the details of how these animals have changed in response to the cane toad invasion.
Cane toads, like many other toads, are toxic to many predators. The level of toxicity varies depending on the predator. Some predators just get sick when they eat cane toads, others die. Many snakes, for example, tend to die when they eat cane toads. This, then, can be a serious problem for the snakes of Australia. What’s interesting, however, is an entire population of snakes can change in order to adapt to this problem.
Such was the case for two different species of toad-eating snakes in Australia. The two species in question are fond of eating toads and are big enough to eat cane toads. However, they both are very sensitive to the poisons produced by cane toads, so the snakes tend to die after eating them. These species, then, are considered “toad vulnerable” species. Two other species that also inhabit areas overrun by the cane toads are not considered vulnerable. One species is too small to eat them, and the other species was already quite resistant to their poisons before the cane toads showed up. A team studied how the two “toad vulnerable” species changed compared to the “not vulnerable to toads” species, and the results were fascinating.
The team took several measurements of preserved snakes from all four species as found in museums. They then went into the field and took the same measurements in the snakes that were exposed to the toads. They also noted the geographical location so they could correlate any changes to the length of time the snakes had been exposed to the toads. They found that in the two “not vulnerable to toads” species, nothing (on average) had changed. However, in the two “toad vulnerable” species, the snakes in the field were (on average) longer than the ones in the museums, and they had smaller heads! The longer the cane toads had been in the area, the more significant the differences were.2
How do the authors explain these changes? Well, the bigger the head, the easier it is for the snake to eat large toads. Cane toads are pretty large, so only the big-headed snakes can eat them. That, of course, resulted in a lot of dead big-headed snakes. The small-headed snakes, however, couldn’t eat the cane toads easily, so they didn’t. As most of the big-headed snakes got killed off, then, the small-headed snakes had less competition and flourished. As a result, the populations of those two species of snakes has, on average, changed in head and body size due to the invading cane toads.
Now of course this explanation makes complete sense, and it shows the value to having a population with many different individual characteristics. If all the snakes had been big-headed, the population might have been severely threatened. Instead, because there were a lot of different head sizes in the population, the population is seemingly not threatened by the invasive species.
Other species just learn to avoid the toads. A small marsupial called the planigale likes to eat toads, and when the cane toads moved in, planigales started eating them. Rather than dying, however, they just got sick. Rather quickly, they learned that it was best to avoid the cane toads, and that’s what they did. As a result, they still go after other toads – just not the cane toad.3
In other species, there seemed to be individual preferences on whether or not to eat toads. In a laboratory study of death adders, for example, some individuals would happily go for a toad if one was presented, and others would ignore the toad. When those snakes were then equipped with radio transmitters and followed in the field, the ones that ignored toads in the lab were more likely to survive than the ones that were happy to eat toads in the lab. The authors suggest that the individual behavior of ignoring toads will be naturally selected and passed on to future generations, making the death adder population less threatened by the cane toad invasion.4
Now note what is happening in each of these cases. The animals aren’t producing new traits to deal with the cane toad invasion. Instead, natural selection is just selecting traits that already exist among the individuals. Those traits that make the animals less vulnerable to the cane toads become the dominant traits in the population. Like the analysis of how the cane toad is changing, then, it shows that evolutionary change is quite limited. Evolution can produce snakes with smaller heads or predators that tend to ignore an invasive species. However, it cannot fundamentally change the snakes or other predators. That’s the “take home” message I get from these interesting studies on the cane toad invasion of Australia.
1. Richard Shine, “The Ecological Impact Of Invasive Cane Toads (Bufo marinus) In Australia,” Quarterly Review of Biology 85 (3):253-291, 2010.
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2. Ben L. Phillips and Richard Shine, “Adapting to an invasive species: Toxic cane toads induce morphological change in Australian snakes,” Proceedings of the National Academy of Sciences USA 101:17150-17155, 2004.
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3. Webb, J. K., G. P. Brown, T. Child, M. J. Greenlees, B. L. Phillips, and R. Shine, “A native dasyurid predator (common planigale, Planigale maculata) rapidly learns to avoid toxic cane toads,” Australian Ecology 33:821-829, 2008.
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4. B. L. Phillips, M. J. Greenlees, G. P. Brown, and R. Shine, “Predator behaviour and morphology mediates the impact of an invasive species: cane toads and death adders in Australia,” Animal Conservation 13 (1):53-59, 2010.
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