Invasive species are non-native organisms that cause considerable damage when introduced to a new area. These species compete with native wildlife for resources, and thrive at the expense of the local ecosystem. The introduction of invasive species is often associated with human activity. Boats that travel between different bodies of water can carry hitchhikers, such as the zebra mussel. One of the most notorious invasive species in the United States, these rapidly reproducing mussels, clog pipes, and over take beaches in the Great Lakes. Some invasive species, however, are introduced intentionally. In the early 20th century, cane toads were brought to Australia as a form of pest control. Today, these poisonous amphibians number in the millions and have cause a decline in native predators on the island. It's not just animals. Bacteria, fungi, and plants can also become invasive. Brought to South Africa in the 19th century, the black wattle is an invasive tree often used for timber and firewood. This beautiful tree, and other thirsty invasives, are depleting the country's already record-low water supply. Because of their impact on human health, ecosystems, and infrastructure, invasive species cost the global economy over a trillion dollars each year.