Washington, D.C. -- While the booming global economy promises greater prosperity in the next century, it poses a real threat to the country's native plants and animals as well as to its productive croplands, says an ecologist from the University of California, Berkeley.
"International trade has a really big cost associated with it," said Carla M. D'Antonio, associate professor of integrative biology and an expert on invasive plants. "Most harmful non-indigenous insect pests and plant pathogens arrive in the U.S. as stowaways on nursery stock, raw logs or cargo containers. And many invasive exotic plants are purposely introduced through the horticultural market."
Alien invaders already cost the country $136 billion every year in lost and damaged products and the expense of controlling them, according to a January article in the journal Bioscience. With a predicted exponential rise in international trade, D'Antonio predicts at least a 50 percent rise in this cost in the next 20 years, not including any increased costs from battling invaders already established in this country.
"We obviously can't get rid of trade, but we have to make a commitment to inspection, quarantine and control to accompany the rise -- and that is just to maintain things as they are today," she said. "The first place to stop these exotic species is at the dock, or even better, before they leave their native country."
D'Antonio will lay out her predictions in a talk on Sunday, Feb. 20, at this week's meeting of the American Association for the Advancement of Science in Washington, D.C., at a session on "Ecological Forecasting: How Will Human Domination Impact Ecosystems This Century?"
She estimates that some 1,500 new insects and 50 new pathogens -- fungi or bacteria -- will make their way into the United States in the next 20 years, with perhaps 15 percent causing serious ecological and/or economic harm.
The economic effects come primarily from invaders, like the Russian wheat aphid, that attack crops; parasitic plants like witch weed, which now affect corn farmers; or weeds, such as thistles, that invade cropland. Alien weeds like spotted knapweed, star thistle and cheat grass reduce the quality of rangeland for grazing cattle, while fungi and insects plague both croplands and forest trees. A recent arrival is the Asian long-horned beetle, first detected only in the past three years, now feasting on hardwood trees in the Northeast.
"This beetle came over from China in wood packaging material and is destroying street trees in several cities in the East," she said. "It has a very wide host range and, while it is currently feasting on a variety of maple species, it has the potential to expand to many other types of trees."
Crop weeds accounted for $26 billion in loss and control last year, while agricultural pests cost the country $14.4 billion, according to the report in Bioscience. Forest pests, on the other hand, cost $2.1 billion, counting control costs and lost production. Fire ants alone accounted for a billion dollars last year in control costs and the loss of rangeland in the Southeast and Texas.
Apart from economic damage, however, these invaders upset the ecosystem, preying on native species, outcompeting them and spreading disease. They also can alter ecosystem processes to the point that the systems can no longer support native species. Non-indigenous species are second only to land use change in causing species extinction, D'Antonio said.
"I don't think any native communities are entirely resistant to invasion," she said. "With repeated introduction of new propagules, you increase the probability that a few of them will establish themselves, and eventually some will cause the extinction of native species."
Propagules are larvae, seeds or pieces of plants or fungi capable of developing into mature adults.
One of the major sources of alien pathogens such as fungi are nursery plants imported for the horticultural market. These come into the country carrying foreign contaminants in the soil around their roots or in the roots themselves. Though all nursery stock must be quarantined and irradiated, this is not sufficient to prevent the entry of many harmful pathogens, D'Antonio said.
"Nursery stock has really been bad news for U.S. forests," she said.
The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) is responsible for inspecting all imports for hitchhiking invaders, but huge container ships are difficult to inspect thoroughly. And some imports, such as seeds, are immune from inspection. More than 100,000 different seeds are involved in the uncontrolled seed trade today, with at least 59,000 species available in U.S. seed catalogs alone, she said.
In addition, many species introduced as ornamentals have become highly invasive and ecologically damaging plants. These range from kudzu and the tree melaleuca in the South to giant reed grass and saltcedar in the West and Japanese honeysuckle in the East.
Though the USDA is now studying its future needs, partly as a result of President Clinton's decree several years ago to do everything possible to control the introduction of new species, it is unclear whether APHIS can keep up with the increase in imports.
"Our inspection and quarantine and eradication efforts must be commensurate with the increase in trade," D'Antonio said.
D'Antonio's specialty is invasive plants in the Western U.S. and Hawaii. Among the plants she has studied are ice plant, a South African succulent that invades dune and shrub communities in coastal areas; French broom, an abundant invasive shrub along the Pacific coast; African pasture grasses that have invaded Hawaiian dry forests and caused their almost complete demise; and grasses introduced long ago, such as velvet grass, canary grass and tall fescue, that force out native bunch grasses in the continental U.S. Of primary interest are the ways through which exotic plants outcompete native plants.
One of her current new concerns is African buffel grass, planted by Mexican cattle ranchers and now invading desert areas in southern Arizona.
"The plant has the potential to enhance the occurrence of fire -- a disturbance to which the native species are not adapted," she said.
Materials provided by University Of California, Berkeley. Note: Content may be edited for style and length.
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