Since the highly poisonous South American cane toads were introduced to Australia as a form of natural pest control, their population has increased at an alarming rate. Now the race is on to find a way of keeping their numbers down as they spell danger for indigenous wildlife : although the amphibians were brought in to eat beetles threatening Queenland's sugar plantations, they have now infiltrated almost a fifth of the continent and even crocodiles have been found dead with the venomous creatu…