Ecological, Health and Human Impacts

Invasive species are one of the most critical problems facing our natural communities. In fact, nearly half of the species currently listed under the Endangered Species Act are threatened by invasive species, and invasives are now considered the second most serious threat to biological diversity. Some weedy plants form dense stands, excluding a host of native vegetation from where it previously lived. Compared to the species they replace, many invasive plants like tamarisk and melaleuca do not provide food or habitat for birds and animals. Thus, where once a region might have been home to a variety of communities, each with its own vegetation and associated creatures, there might only be a few species left after an invasive species takes hold.

Some invasive species are predators, devouring native species that have developed no defense to the newcomer. The introduction of rats and snakes on islands has driven birds to extinction, like the Guam Flycatcher, Tahitian Sandpiper and Hawaiian Black Mamo. The same thing has happened to native fish and amphibians when predatory fish are introduced to isolated ponds whose organisms were never before subjected to such predators.

Other invasive organisms spread diseases, and some are diseases themselves. Unchecked, they can change the very landscape. Several of the trees that our great-grandparents would have described as common – American chestnut and elm – are virtually unknown in forests today. And if nothing is done to stop them, invasive species will ensure that many of the trees in our forests today – like maples, hemlocks and whitebark pine – will be unknown to our grandchildren.

Exotic, invasive species directly harm human health as well. Historically, transportation of smallpox, measles, malaria and influenza has devastated populations that lacked prior exposure and immunity to these diseases.

  • European bubonic plague moved across the Atlantic and kills several people in the U.S. per year.
  • The Asian tiger mosquito entered the U.S. in 1985 and has since expanded to most of the continental U.S., carrying dengue fever, yellow fever, eastern equine encephalitis, and LaCrosse encephalitis.
  • West Nile virus, which also causes encephalitis and can be fatal to the elderly, arrived in the northeastern United States in 1999 and has been positively identified in almost 1,500 cases and over 60 deaths.
  • Cholera bacteria have been found in the Chesapeake Bay.

Coupled with climate change scenarios that suggest that parts of the U.S. will become more favorable for tropical species over the next century, the entry and spread of tropical diseases threatens to become a serious public health issue in coming years.

Collateral Damage – Economic Impacts

In addition to impacts on human health and natural communities, invasive species do billions of dollars in damage to agriculture, forestry, fisheries and infrastructure. Expenditures to combat invasives in the U.S. total approximately $137 billion annually. Estimates for the damage caused to the cotton industry by a single organism, the boll weevil, range from $6 to $50 billion.

  • Leafy spurge, Scotch broom, and cheatgrass have decreased the forage value of rangelands. Cattle refuse to eat leafy spurge, and cheatgrass invasion results in more frequent, widespread, and more destructive fires than occurred previously.
  • Non-native viruses, fungi, weevils, mites, whiteflies and other insects have devastated wheat, cotton, vegetables and honeybees.
  • Chestnut blight, Dutch elm disease, the balsam wooly adelgid, gypsy moths, Asian long-horned beetles and pear thrips cause millions of dollars in damage to street trees, timber and the maple syrup industry.
  • The sea lamprey, left unchecked, could cause $500 million in damage to the Great Lakes fisheries and related industries.
  • Zebra mussels and Asian clams clog utility pipes, irrigation pipes and boat engines. A recent calculation estimated zebra mussel damage at $5 billion annually.
  • Control of residential pests such as cockroaches and rats costs about $6 billion annually.

Damage to ecosystem services is more difficult to estimate economically, because no one can put a price on the loss of a species or the destruction of a water source. However, an evaluation of the cost of tamarisk invasion (in losses to water provision, flood control and wildlife habitat) in the southwest estimated losses of $3.8 to $11.2 trillion dollars over a 55-year period.