4.2 'Biological control'
We are also guilty of importing exotic species, some of which, like the rhododendron (imported from Asia to Europe), have run riot in the absence of natural predators or primary consumers, and so have tended to out-compete native plants. Sometimes introductions have been accidental; rats and many disease-causing organisms have spread around the world via relatively modern transportation such as sailing ships. However, deliberate introductions, such as the rhododendron, have been made with worthy intentions. On a number of occasions animals have been introduced to new habitats in an attempt to limit the populations of pest species. This is known as biological control. In the first half of the 20th century, the American giant toad, Bufo marinus, was introduced into a number of Caribbean islands and then into agricultural land on Hawaii, the Philippines, Fiji and some other Polynesian Islands. In all these areas, it was reported to successfully limit the numbers of a variety of insect pests. As a result, when sugar cane farmers in Northern Australia faced heavy losses because two indigenous species of beetle had taken a great fancy to this relatively newly introduced crop, it seemed sensible to see whether these toads could eradicate the beetles. The first toads were brought into Australia in 1935 and a breeding colony was established in Queensland. Tadpoles and toadlets were given to farmers in beer bottles and jam jars from which they were released into suitable areas of the sugar cane fields. The results were disastrous. The toads took to their new country and spread like wild-fire but they didn't eat the beetle pests. They ate many harmless insects and even beneficial ones, like honey-bees. Their tough skin secretes toxins, protecting the toad from predation and often poisoning the would-be predator. Within five years of their introduction, the toads, which became known as the ‘cane toads’, were designated as vermin. However, they have been surprisingly difficult to eradicate. Although mammalian and avian (bird) predators have learnt to avoid them, reptiles and amphibians seem to have become rare in the areas where these toads are common. This could be a result of direct encounters but it might also be because of indirect competition for food. So here we have an example of the unexpected outcome of an attempt to manipulate a food web. Scientists had predicted, wrongly, that the toads would occupy the same niche in this new community as they had when introduced to other, apparently similar, habitats.