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Wednesday, November 26, 2014

More on Cane Toads in Australia

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.

REFERENCES

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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Comments

10 Responses to “More on Cane Toads in Australia”
  1. josiah says:

    Of course none of this will change anything. Evolutionists will complain that they haven’t been able to wave their eon-old magic wand to allow for true mutation based evolution, and so it’s no surprise that it isn’t observed.

  2. jlwile says:

    That’s certainly true, Josiah. To a fervent evolutionist, the magic wand of time cures all problems with the hypothesis. Nevertheless, I think the changes in the cane toad itself do address the time issue rather well. After all, it took almost no time (just 70) years for the cane toads to evolve legs that were long enough to start causing spinal issues. Thus, it took evolution just 70 years to run up against the limit of that aspect of biological change in the cane toad. So it seems to me that in this case, evolution occurred very quickly and then quickly “hit the wall.” This is also what Lenski’s bacteria experiment showed. This tells me that time isn’t necessarily an issue. What changes can occur seem to occur rather quickly.

  3. Jonathan says:

    Thank you for the post, Dr. Wile! This is very interesting. Would it seem that, in this case, the changes experienced by the populations NEED to change quickly, in order to preserve the population? It almost seems to me that time is the enemy in this case, since the toads have been invading so quickly! Although there is a lack of quantifiable data, this would appear to refute the hypothesis that evolutionary change is too slow to observe when it is happening.

  4. jlwile says:

    Thanks for your comment, Jonathan. You are right that the changes in the other animals have to occur quickly, otherwise, a population could go extinct. Thus, time is definitely the enemy for the other animals.

    I don’t think anyone would say that this kind of evolutionary change is too slow to observe. The kind of evolutionary change that is assumed to be too slow to be observed is the evolutionary change that produces a fundamentally different kind of creature. What these observations indicate to me is that such change cannot ever occur (regardless of the time allowed), because evolutionary change is so limited to begin with.

  5. Jonathan says:

    Yes, I agree that the most exciting part of this study is that it supports the idea that evolutionary change is limited(microevolution, if I’m not mistaken). I’m also surprised at how quickly those changes can take place.

    It seems it would be difficult to say that macroevolution is consistent with this study, since the evolution apparent here is simply a limited “adjustment” within a brief period of time. It almost seems like evolution might be most effective when it happens quickly! Anyhow, this article has certainly fueled my interest in this topic more, and I’ll be looking at some of your other posts on this subject!

  6. jlwile says:

    I am glad it has fueled your interest, Jonathan. I like your statement that “evolution might be most effective when it happens quickly.” The study at least shows that rapid evolution is effective. Interestingly enough, most young-earth creationists require rapid evolution after the Flood, and since the animals on the ark would be “invading” new territory, this kind of study has direct relevance.

  7. Kh says:

    Dr. Wile – you wrote:

    “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.”

    I must admit my knowledge of biology and evolution is limited, however, I do find this logic hard to accept. The fact that rapid, non-fundamental changes in a species is proven to occur, doesn’t really disprove that more fundamental changes, perhaps over time, also may occur – does it? Are the two mutually exclusive?

  8. jlwile says:

    Kh, I think you are missing the point. It’s not the fact that the observed changes are rapid and non-fundamental. It’s that the change itself is limited. Already, 10% of the toads on the front lines are showing severe spinal arthritis. This shows that the frogs can’t continue to get faster and faster. Instead, natural selection has pushed the toads to the limit of their physiology. So even in a short time, evolution seems to be running up against a wall. If there were no severe spinal arthritis (or other maladies associated with the increased speed), you could make the argument that given enough time, significant change in the toad’s speed would occur, perhaps producing a fundamental change in the toads. However, the very fact that the toads on the front lines are now having serious physiological problems indicates a strict limit to how much change can occur, at least when it comes to this characteristic.

  9. Kh says:

    No, but there is a logical fallacy here. Just because something does not occur in one instance, does not preclude that it may occur in other instances – does it? What I read from your conclusion in the blogpost is the following:
    - This study does not show that evolution brings forth new traits
    - This study does show that rapid, minor changes to the animals may occur
    - This study shows that some of these changes also cause trouble for the species involved.

    Up until this point, I completely agree with you.

    But from this point on, you conclude that since there are in fact rapid, minor changes to be seen, and since some of these might cause problems, evolution cannot produce more fundamental changes in a species. How does the one exclude the other? If the logic described above is correctly represented, I think your conclusion is very strange; especially given that this study obviously deals with a limited period of time.

  10. jlwile says:

    There is no logical fallacy. Kh. I am not saying that this study demonstrates that large-scale evolutionary change cannot occur. I am simply stating what this study says about evolutionary change. The study shows that in cane toads, the amount of evolutionary change when it comes to speed is limited. This is what we tend to see when we actually study evolutionary change. We see limited change that eventually “runs up against a wall.” Obviously, since we haven’t studied all possible cases of evolutionary change, this doesn’t preclude the fact that in some unknown situation, large-scale evolutionary change could occur. However, based on the studies that have actually been done, evolutionary change is limited.

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