Crocs Chow Down on Invasive Toads, Instantly Regret It

By Andrew Moseman | June 6, 2008 4:28 pm

A freshwater pygmy crocodile from northern AustraliaAnother week, another story right out of “There Was an Old Lady Who Swallowed a Fly.”

First, U.S. Forest Service scientists in Hawaii, beset by the invasive strawberry guava tree that has taken over native Hawaiian ecosystems, proposed introducing a Brazilian insect that harms guava and slows its growth. The Forest Service representatives claimed that the Brazilian scales, as they’re called, will attack only guava and spare the native species. But they might want to take a word of caution from Australian scientists, who are now seeing scores of crocodiles die as a result of similar tactics.

The croc-killing culprits are cane toads. Australia had no native toads, but in 1935 they imported these amphibians to battle beetles that were damaging the sugar cane crops. The invasive amphibians quickly spread out of control, however, multiplying like rabbits across the Australian continent. In the last few years, they have entered the habitats of Australia’s rare freshwater crocodiles, which saw the toads as something good to eat. Unfortunately, the toads’ poisonous skin spells death for any unlucky croc that consumes them.

Today The Guardian reported that once the toads moved into their territory, freshwater crocs dropped in numbers from more than 600 in 2006 to less than 400 last year. British researcher Adam Britton says captive breeding might be the only way to save them, but he hasn’t found the money for it.

The crocodiles’ plight should serve as a warning against importing one species to kill another. Invasive species can spread like wildfire, so bringing them in as mercenaries seems like a dubious plan loaded with possible unintended consequences. Cane toads don’t know that they’re wrecking the Australian ecosystem—they’re just trying to survive and multiply, and doing a heck of a job of it.

Image: Guillaume Blanchard

MORE ABOUT: invasive species
  • Curtis Mills

    This sounds like a cautionary tale for GMOs, (genetically modified food).

  • Carole McIntyre

    Back in the Fifties, there was an “innovation” in livestock fencing. It was multiflora rose, said to be “horse-high, bull strong, and hog-tight.” Plant this along fencerows, and forget about setting replacement posts and repairing wire.

    Except. . . it seeds through pips that pass through birds’ digestive tracts unchanged, and the thorny exuberant bushes will sprout anywhere: in the middle of pastures, in hayfields. It’s a bear to get rid of, too. Even goats won’t eat it. Fifty years after its introduction it’s an outright problem, and has been for most of those fifty years.

    I’m sorry for the crocs, although they’re not my favorite species. But the lesson to be learned covers both the cane toads and the multiflora: unintended consequences of introducing strange species will get you, every time.

  • Alyson Irvin

    Hawaii should know better. They also imported the cane toads, they imported the mongoose, and they had a rabbit problem from imported rabbits. You would think they would have learned by now how badly things can go when one tries to introduce one species to control another. Frankly, in light of their historical failure in that regard, I am shocked they would even be considering it.

  • Jay Warner

    Many of today’s “invasive species” came from consciously imported stock. Think Kudzu vines. Think buckthorn & honeysuckle, as ones I fight weekly. English sparrows drive blue birds from my front yard, just when I need all the insect eating birds I can find. I understand that gray squirrels do a lot of damage in England, Scotland and Wales.

    When are we going to realize that importing a species from one area into another, for _any_ reason, is simply asking for trouble? In Wiconsin we are getting into promoting purple loose strife beetles to cut back the growth of this (very pretty but) nasty wetland destruter. Remains to be seen yet if we got it right, or what else the beetles will decide to eat. Some people think that they understnd the interactions in nature today. But it’s a pretty complicated ecosystem world out there.


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  • Franny

    This article is confusing “introduced species” with “biocontrol”. Biocontrols are used regularly around the world to control harmful invasive species. Modern biocontrols are carefully investigated by scientists with backgrounds in entomology, botany, ecology, and other related fields of expertise. For instance, the proposed biocontrol insect for strawberry guava in Hawaii has undergone 15 years of research in Brazil and Hawaii, and has been tested against 81 species of plants found in Hawaii. The insect is considered highly specialized – meaning it has evolved such a close relationship with its host plant that even when infesting a strawberry guava right next to its cousin, the common guava, it will not infect the common guava. The insect does not even eat the plant, rather, it causes the plant to form galls, requiring the plant to put energy into making the galls and thus less energy into fruiting and spreading. Laws in Hawaii, as in other places that employ biocontrols, have required extensive federal and state reviews and no less than 6 permits are needed to allow this release.

    Cane toads, mongoose and other notorious introductions have given biocontrol an undeserved bad name. These animals are considered “generalists” (meaning they eat a lot of stuff, and done’t have that close individual relationship. Such animals would never now be considered for biocontrol. (Mongooses were introduced onto private land by cane farmers, without any research or testing.) Since 1970, when these strict regulatory practices were adopted in Hawaii, over 100 biocontrols were released. None have attacked non-target species, and some had spectacular success in combatting otherwise impossible invasives.

    The old stories about bad-news introduction give biocontrol a black eye, and take away a valuable resource from conservation groups, land managers, and farmers. South Africa, New Zealand, and Australia all have records of very successful use of modern biocontrol.

  • Darr Sandberg

    If the word went out that cane toad liver was “Nature’s viagra – ten times better than rhino horn” – they’d be hunted to extinction in a year or two.

    Bad joke aside, given how good we are as a species at driving anything useful to extinction – finding a real use for cane toads or strawberry guava wood is worth looking into.

    And if scientists could/would prove that climate change is going to wipe out hops – driving up the cost of beer, middle america would be so Stop-Global-Warming-Now in a week, it’d be scary.

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  • Gabriela Chiran

    If cane toads are killing crocs than maybe they should be fed with meat. I’m sorry for those crocs who die.


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