Unit 9: Biodiversity Decline // Section 8: Invasion by Exotic Species
Throughout the history of commerce, humans have transported animals, plants, and other live organisms around the globe. In many cases these species were deliberately brought, either as food supplies for the journey or to create new crops and animal colonies on arrival. Some live cargo has come along uninvited—for example, termites embedded in the wood slats of shipping crates, or fingernail-sized zebra mussels that ships suck up in ballast water. Thousands of exotic species have been introduced to new habitats worldwide as a result of human trade and travel, but they all have something in common: the potential to thrive and multiply beyond all expectations in their new environments.
When species are moved to new locations that offers conditions for life similar to their native habitats, they may exploit vacant ecological niches and grow quickly, especially if they have no natural predators in their new settings. Figure 10 shows a shopping cart that was pulled from Great Lakes waters infested with exotic zebra mussels, which have covered nearly every inch of the cart's surface.
Figure 10. Colonization by zebra mussels, Great Lakes
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Source: © United States Environmental Protection Agency.
These invasive species are major threats to biodiversity because local species are not adapted to compete with them. One extreme case, the brown tree snake, was introduced to Guam after World War II (probably as a stowaway on military cargo planes) from its native range in the eastern Pacific. The snake has killed off nine of Guam's twelve forest bird species, half of its lizards, and possibly some of its bat species, and caused major damage to the island's poultry industry. In 2004, the U.S. Congress authorized up to $75 million over five years to prevent brown tree snakes, which have traveled as far as Texas in cargo shipments, from becoming established in Hawaii and the U.S. mainland and to control their presence in Guam.
Plants can also become invasive. Spotted knapweed, a perennial that probably came to the United States from Eastern Europe or Asia a century ago in imported hay or alfalfa seed, has become established across Montana. The plants, each of which can produce up to 18,000 seeds annually, compete for water and nutrients with native bunch grasses and produce a toxin that damages other plants. This technique, in which plants compete by poisoning other species, is called allelopathy. For example, garlic mustard, a weed found across 30 states and Canada, suppresses the growth of native trees by killing the fungi that help the trees take up nutrients from soil.
Invasive species flourish because they have left their normal predators behind, so one way to control them is to import those enemies. For example, biologists have introduced eight of spotted knapweed's natural insect predators to Montana with mixed success. This strategy assumes that the imported predator can survive in the new environment, and that it will not become invasive itself. Cane toads were imported to Australia in the 1930s to control beetles that fed on sugar cane, but although they had been used successfully for this purpose in Hawaii and the Caribbean, in Australia the toads did not breed at the right time of year to eat cane beetle larvae. However, they did spread across most of Australia, and have outcompeted many other local frog species. Because they produce toxins in their bodies, the toads are also poisonous to predators such as fish and snakes, although some Australian birds and rodents are learning to eat only the non-toxic parts of the toads (footnote 18).
Invasion by exotic species threatens nearly 50 percent of the endangered species in the United States (footnote 19). Scientists are using remote sensing and geographic information systems to detect and map land cover changes and the spread of exotic plants, and also use high-speed computation and modeling to project how populations will grow. The yellow areas in Figure 11 show invasive salt cedar along the Rio Grande River.
Figure 11. Landsat image of invasive salt cedar, 2002
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Source: © Center for Space Research, University of Texas.
It is important to note that many introduced species do not become invasive or have harmful impacts in their new settings. For example, major food crops and domestic animals have been traded worldwide and are rarely invasive. What makes a species likely to become invasive? The examples cited here point to several characteristics. Exotic species that reproduce quickly, can poison predators or competitors, and do not face natural predators in their new locales are well-positioned to spread and outcompete local species. As discussed in Unit 4, "Ecosystems," many r-adapted species have the capacity to become invasive pests because they flourish and reproduce quickly in unstable environments.