If you visit a national park this summer, you may not see the invasion underway. But look closely and you'll find invaders all around you -- aliens that stalk native plants and animals and do their best to undermine the very things that make our parks so special.
Some will be obvious -- the wild, bushy tamarisk tree that chokes the Rio Grande in Big Bend National Park and the streams in Death Valley, Calif. Others will be less visible but evident by their sound -- the tiny coqui frog with the screeching voice that is leapfrogging around Hawaii, including into Hawaii Volcanoes National Park.
Still others leave their mark in a changed landscape: a forest of wrecked trees, pitiful stumps where leafy branches once sprouted.
Invasive species, defined as non-native species that damage the environment or human health, are one of the biggest challenges facing the national parks, infesting 2.6 million park acres. The National Park Service says that at least 234 of the 390 national parks, monuments and waterways also are unwilling hosts to invasive animals.
They arrive in a number of ways, invariably related to human actions: Exotic plants are brought from other countries; illegal bugs hitchhike on nursery shipments; contraband reptiles, some procured as pets, are released or escape to the wild.
Alien species are by no means confined to park lands, but the parks face a special burden because it's the National Park Service's job to preserve the nation's most beloved landscapes and natural resources for future generations.
And if invasives have one specialty, it's damaging the natural environment.
EARTHWORMS, PIGS, FROGS AND MORE
Some make their presence known with a carpet of brilliant leaves or flowers, benign in appearance but voracious in crowding out vulnerable native species. Others quietly launch an ecosystem chain reaction. Some examples:
-- Non-native earthworms are changing the soil in Voyageurs National Park in Minnesota, which affects the trees growing there, which in turn changes the water flowing into thousands of lakes, and with it fish and plants that live in the lakes.
-- In the Florida Everglades, imported trees are driving out native mango and cypress trees and complicating a massive environmental restoration project aimed at remedying decades of water diversion.
-- In Eastern forests, a sap-sucking insect called the woolly adelgid has killed more than 90 percent of the hemlocks in Shenandoah National Park, and is working its way through Great Smoky Mountains National Park. In steep gorges where the hemlocks shade streams, the plant and creek species are losing their habitat.
-- The Hawaiian Islands are beset by the likes of wild pigs, rampaging Central American miconia plants, and reef-smothering algae. The dime-sized coqui frog, from Puerto Rico, not only annoys residents with its lawnmower-volume screeching but also gobbles up valuable insects and provides a tasty meal source for larger invaders like rats.
-- Cheatgrass overtakes streams in Zion National Park and exotic deer proliferate in California's Point Reyes National Seashore. Exotic trout interfere with the lake trout that Yellowstone grizzly bears dine on.
BATTLING THE SPREAD
Fighting off these encroaching armies is a daunting task. In recent years the government has elevated the issue, and the National Park Service has created special teams that travel the parks trying to make a dent. Although they have had some success, tight budgets prevent an all-out onslaught, especially with parks already thinning their ranger forces and cutting visitor center hours.
"If they don't do something more assertive, they will continue to have those plants and animals spreading to new areas," said Faith Campbell, invasive species expert for The Nature Conservancy. "There will be more and more organisms in the park that shouldn't be on the North American continent much less in the parks, and there will be fewer examples of the healthy organisms that should be there. The parks are no longer going to represent the natural resources of North America."