Invasive Species Cause Havoc
Invasive species are notorious for wreaking ecological havoc: introduced organisms can disrupt community dynamics, alter ecosystem structure and function, and sometimes drive native flora and fauna to extinction. The brown tree snake’s invasion of Guam and the subsequent annihilation of the island’s native bird populations is a prime example. As always with nature, however, the issue of whether invasive species are always harmful may be more nuanced than it first appears. A new study in The American Naturalist (Shine et al. 2012) offers an excellent case in point: a situation in which one invasive species appears to inoculate a native animal from the effects of another ecological intruder.
One of the Worst
The cane toad (Bufo marinus) is one of the world’s more notorious invasive species. Native to Central and South America, it was initially introduced to several island systems in the Caribbean and Oceania as a way to control pests on sugar cane plantations. Unfortunately, it became apparent that the toad was more than plantation managers had bargained for. Cane toads eat nearly any plant, invertebrate, or vertebrate they can cram into their mouths—which, considering that the toads have been documented to weigh almost 6 pounds (2.71 kg) in some cases, is means they can consume everything from plants to rodents and birds.
In addition to their voracious appetites, cane toads have highly effective defenses against predators. They possess glands that secrete a special fluid, bufotoxin, which is fatally poisonous when ingested. The dual advantages of being a voracious consumer and possessing potent chemical defenses have made the cane toad a devastatingly successful invasive species.
An Alien Invasion
One of the key toxic compounds found in bufotoxin—bufadienolide—also happens to be found in species of plants belonging to the genus Bryophyllum, which are native to Madagascar. In one of history’s more intriguing coincidences, Bryophyllum was also introduced to eastern Australia in the 20th century. It was originally used as an ornamental addition to gardens, but by mid-century it had begun to infest the landscape. This occurred within a decade of the first introductions of cane toads to eastern Australia. The linkage between this double-whammy of invasive introductions may not have been immediately apparent, but nature has a way of twining phenomena together into intricate webs and cascades.
And the Winner is…
And finally, a native species enters the story. The Australian blue-tongued skink (Tiliqua scincoides) is a large lizard that is typically noted for two primary reasons: it is endemic to Australian (with the exception of one subspecies also being found on a smattering of Indonesian islands), and it possesses a tongue that is a brilliant, Crayola-worthy shade of blue.
Tiliqua is an omnivore and chows down on items ranging from leaves and fruits to small animals. Two subspecies of the lizard, T. scincoides scincoides and T. scincoides intermedia, occur in areas of Australia that have been progressively encroached upon by the cane toad over the last few decades. In some areas of the country, Tiliqua populations have dropped precipitously, due to an epidemic of deaths resulting from lizards trying to eat the toxic toads.
An important note regarding the lethality of bufotoxin, however, is that predators that have historically co-existed with cane toads are able to develop a resistance to the poison, with successive generations being more and more resistant to its effects. This adaptation allows many predators, ranging from snakes to birds to rats, to consume the toads in their native Central-South American habitats.
This caveat is of critical significance in evaluating patterns of toad-induced Tiliqua population crashes throughout Australia. A research group from the University of Sydney, led by Richard Shine, was surprised to find that some populations of Tiliqua appeared to be highly susceptible to the fatal effects of bufotoxins, while others were highly resistant to the poison—even in places where they had never been exposed to the toads.
This surprising pattern begs the question of how the lizards managed to adapt to the chemical defenses of an animal they had not yet encountered. In the mythology of Australia’s Aboriginal Warlpiri people, the blue-tongued lizard is a sorcerer that often plays wily tricks on people. Still, this prescience about a future invasive amphibian seemed a bit of a stretch even for the craftiest of lizards, and Shine’s group sought to offer a mechanistic explanation.
The researchers hypothesized that, since 1) Bryophyllum produces a toxin nearly identical to that of the cane toad, and 2) the skinks will eat plants as well as animals, that the plants had actually indirectly helped the lizards to defend themselves from bufotoxin. The presence of the poison-bearing plants put selective pressure on some populations of Tiliqua to adapt to the toxin before the toads ever appeared on the scene.
Testing the Theory
To test this hypothesis, the University of Sydney team collected lizards from localities that had been invaded by Bryophyllum, as well as from places that remained free of the invasive plants. They then injected each of these lizards with toxin from cane toads. The effect of the bufotoxin on each lizard was assessed by having the hapless reptiles swim laps in a trial pool both before and after the dose of poison, and comparing pre-injection to post-injection performance.
The results supported the hypothesis that exposure to Bryophyllum increased resistance to toad toxins: the toads from localities known to have the plants has a significantly smaller decrease in muscle performance after being injected with bufotoxin relative to “naïve” toads from areas that were free from both plants and toads. Given that the generation time for Tiliqua is only 2-4 years, this means that the lizards were able to show substantial adaptation to the poisonous compound in only 20-40 generations—the blink of an eye on an evolutionary timescale.
Even though invasive species are definitely an issue of critical concern, the complexities of their effects on ecosystems should never be underestimated. The introductions of both Bryophyllum and the cane toad to Australia were probably mistakes, yet it appears that the former at least served to inoculate Tiliqua populations against the latter. This shouldn’t be surprising, as the explosion of any invasive species population usually results from a complex chain of unforeseen events.
The implications of this study are important: now researchers know how to predict which populations of Tiliqua are most vulnerable to declines when exposed to the cane toad, and they can focus their conservation efforts accordingly, instead of spending scarce resources on populations that have already developed endogenous defenses to the toxic amphibians.
Price-Reese, S. J., G. P. Brown, and R. Shine. 2012. Interacting impacts of invasive plants and invasive toads on native lizards. The American Naturalist 179(3): 413-422.