Invasive Species

When a non-native species is introduced to a new ecosystem, whether accidentally or deliberately, it does not always establish a persistent population; many introduced organisms fail to survive or reproduce in their new environment. Those that do succeed, however, can become invasive, spreading rapidly because they often lack the natural predators, parasites, or competitors that kept their populations in check in their native range. The zebra mussel, introduced to North America's Great Lakes in the late 1980s via the ballast water of transoceanic ships, illustrates this pattern clearly. Free of the predators that limit its numbers in its native Black Sea region, the species multiplied so rapidly that it clogged water intake pipes, altered lake food webs by filtering out plankton that native species depended on, and cost affected industries hundreds of millions of dollars in cleanup and prevention. Efforts to control invasive species once they are established have proven difficult and expensive, which is why many ecologists now argue that preventing introductions in the first place, through measures such as ballast water treatment requirements, is a far more cost-effective strategy than attempting eradication after the fact.