Because the insects and competing plant species that control such plants in their native countries were left behind, some exotics have become highly invasive weeds in the United States and a serious threat to park lands. In response to an allocation of funds from the U. S. Congress, the National Park Service is accelerating its control of exotics. The long-standing dilemma of balancing the risks to native vegetation from the exotic weeds with the risks to native fauna and flora from an increasing use of herbicides is now greater than ever. To resolve this dilemma, the National Park Service uses its innovative Integrated Weed Management that consists of a combination of techniques such as mowing, hand pulling, releasing weed-eating insects, and applying herbicides. About 450 projects of Integrated Weed Management were identified to control these exotics.
Exotic weeds are a form of biological pollution.
They can replace the entire native flora of an ecosystem, cause a staggering loss of biological diversity, and severely reduce the number of already endangered plants. For example, a single melaleuca tree can replace dozens of native plant species, and collectively such trees can overtop and destroy natural wet meadows in the Florida Everglades. Similarly, leafy spurge, which has become common in some northern states, can crowd out all native grasses and forbs. Soil erosion increases drastically when exotic weeds replace native grasses. The harm from exotic plants is not confined to native flora but extends to native fauna.