Friday, 31 July 2015

Boomerang Effect

I have not followed the Cane Toads in Australia story for quite a while .... last time I looked, they were still expanding their range, but it was hoped that natural obstacles such as deserts, and mountains, would mean that even if man couldn't halt them, natural geography would. This would mean that the south and western parts of the continent would (barring accidental introductions via cars, trucks, trains, or crate packages), remain free of the pest.

There were also programs on-going to:
  • Cull the toads.
  • Harvest and sell the toads to China and Vietnam as delicacy (strangely if they were native to China they would likely be endangered, given that countries propensity to eat anything that walks or crawls or swims).
  • Find some commercial use for them in medicine.
  • Stop them breeding via neutralised females.
  • Protect the native species that their presence endangers.

..... and even some talk of toad leather for shoes etc ..... in fact anything at all, that might put the brake on this creatures expansion. But none of the suggestions or actions seemed to be a definitive solution, and their numbers just kept growing.

No, Not This Common Toad .....

Personally, if I was the Aussies, I would set aside Aus $500m, and offer a $1 bounty per head for every dead cane toad brought in. It would be an incentive that small boys wouldn't be able to resist, and the toads numbers would drop considerably (at least in urban and semi-urban areas), and very quickly, if you could earn several hundred dollars a week by capturing and killing these creatures. Animal rights protesters would no doubt complain, but hey, what about the snakes and lizards?

Anyway that's where the story was on my last post .... However my attention was drawn back to this subject again, when the latest update came from the New Scientist magazine in March 2015, suggested that although the introduction of the toxic cane toad in the 1930's, has caused massive ripples through the ecosystem in ways rarely seen before, this seems to have allowed other prey species numbers, that were also under threat, to bounce back.

Its This Monster Of A Toad ...

It is apparently a rare piece of solid evidence, for an invasive species causing what is called a “trophic cascade” in the wild, where the killing off of the top predators has unexpected effects throughout an ecosystem. E.g. When American settlers wiped out wolves in Yellowstone National Park, this caused an increase in herbivores, which in turn led to a decrease in plants, which then caused a loss of birds, and a degradation of the ecosystem .... so wolves had to be reintroduced to reset the balance in the park, but this led to the wolves numbers growing (because there was so much prey), and now cattle farmers outside the park are demanding that they be culled again as they start taking their stock as well.

So the poisoning of vast numbers of the toads potential predators, has in effect also protected all the predators other prey ..... for example, when three species of predatory water monitor lizards (that try to eat the toads), were wiped out in one area, the numbers of Crimson finch (Neochmia phaeton), who were also prey to these monitors rose, with survival rates amongst their chicks going from 55 per cent, to 81 per cent, and presumably the same for other bird prey of the lizards and snakes that have now been wiped out.

At first glance that might seem to be a good thing, but just like in Britain, where the removal of the top predators and the curtailing of hunting, means that certain species such as rabbits, deer and even now, wild pigs, exhibit population explosions and dangerous genetic issues as well (survival of the fittest no longer applying), its not necessarily so. One obvious potential trophic cascade example, would be that the higher number of crimson finch and other prey birds birds, could affect plants that the birds feed on, as well as other animals that eat those plants, and so on, and so on, down the food chain.

This means that man has to start culling them himself, if the populations are to remain healthy and in numbers controlled in manner similar to that of the natural world.

So in Australia, while the initial rebound of some creatures numbers will seem to be a happy side effect, the removal of the predators will also mean that there is no natural check on those species numbers, and at some point, they also will need controlling .... and you will still have the toads!

There are hundreds of millions of them, stretching from coast-to-coast and in places, they form seething masses of up to 2,000 toads per hectare. They have even evolved to become better invaders by growing longer legs, and being more aggressive, driving some species to extinction in local areas. As one researcher said ... “Who would have thought that invasive cane toads would affect a granivorous bird? Our study can reveal that the cane toad invasion has exposed previously unknown trophic links, thereby underscoring our ignorance of the complex interactions in animal communities.”

“The point is that the loss of top-down regulation due to loss of keystone reptilian predators is throwing the whole ecosystem out of balance ...... through the process of trophic cascades that is potentially far-reaching and currently not well understood.”

So far this issue has no resolution in sight, and it only needs one eco-terrorist to transfer Cane toads into the South and West of the continent, and the issue would suddenly go from 'very bad' to eco-disaster.


  1. Do you know if there are other examples of other introduced species having as bad an affect but outside Australia?

    1. To be honest, although there must be some, I don't really know of them. However I do recall reading that the American Crayfish is devastating UK rivers and canals.Thanks for the comment Dodo.

    2. Not forgetting fauna ; Japanese Knotweed is an invasive herbaceous perennial which causes serious damage to buildings, hard surfaces and infrastructure and threatens native plants and animals.

    3. Japanese Knotweed is a good example Vroomfondel, as its treated as a 'bio-hazard' when discovered.


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