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Invasive species and endangered ones are two sides of the same coin: When invasives move in, native species suffer — sometimes to the point of extinction.
“From prehistory to the present time, the mindless horsemen of the environmental apocalypse have been overkill [excessive hunting], habitat destruction, introduction of animals such as rats and goats, and diseases carried by these exotic animals,” eminent Harvard biologist E. O. Wilson wrote in his prize-winning 1992 book The Diversity of Life. In the modern era, Wilson argued, habitat destruction and introduction of exotic animals were the top threats to the world's biodiversity, and these two pressures often reinforced each other.
One notorious invader, the brown tree snake, is native to the South Pacific but was introduced to the island of Guam in the 1950s, probably as a stowaway on cargo planes. Predation by brown tree snakes has eliminated 10 of Guam's native forest bird species and reduced the last two to fewer than 200 birds apiece.
Predation also could alter the composition of Guam's forests, because forest birds play an important ecological function: When they eat fruits and eliminate the seeds, they scatter the seeds so that trees reproduce over broad areas. With fewer native birds, scientists say, Guam's forests could become less diverse because there will be less mixing of tree species.
Another endangered species, the Indiana bat, is threatened by habitat loss and white-nose syndrome, a disease that may have been introduced from Europe. The bats hibernate in winter in caves and abandoned mines in the Midwest and Southeast, then roost in wooded areas in summer. Habitat loss from human activities — development, cave exploration and blasting in old mines — is a serious threat to the bats, whose population has fallen by half since the late 1960s. Bats play many important ecological roles. For example, they eat huge quantities of insects and also pollinate plants.
White-nose syndrome is an infectious disease that has killed an estimated 5.7 to 6.7 million Indiana bats and other, more common species across 16 states over the past five years. One recent study projects that white-nose syndrome could drive several bat species — including a previously common type known as the little brown bat — to extinction in the Eastern United States as early as 2015.
Many invasive species are highly adaptable and can flourish in settings that have been disturbed — attributes that help them out-compete native species. Disturbances may be natural processes such as floods or fires, or may result from human actions, such as plowing up prairie grasslands, introducing cattle or sheep onto grazing lands or building roads, bridges, canals and other infrastructure.
Ecosystems often recover after invasions, although it may take many years, depending on how severely they have been affected. University of Michigan researchers have found that invasive zebra and quagga mussels are causing “astounding changes” in the food webs of Lakes Huron and Michigan by consuming microscopic algae, which form the base of the lakes' food webs. As levels of these algae drop, creatures such as small crustaceans that feed on them are affected. In turn, these organisms are prey for many types of fish.
The scientists called for Great Lakes management agencies to take urgent action against these shifts. “Ecological changes that formerly occurred over decades are now happening in just a few years,” said Donald Scavia, director of the University of Michigan's Graham Environmental Sustainability Institute.
Making ecosystems more resilient against invasions can be a long-term process. Last summer the U.S. Fish and Wildlife Service announced that the whitebark pine tree — an important species that stabilizes mountain slopes in Western states and provides food for animals and birds — was a candidate for listing under the Endangered Species Act, partly because of unprecedented predation by pine beetles over the past decade. The trees' natural range is also shifting with global climate change, which is raising average temperatures and decreasing rainfall across the West.
U.S. Forest Service researchers are conserving genetic material from whitebark pines and analyzing whether to plant them in new zones where they will be better able to grow. “Sometimes it's not how hard you get hit, it's how well you roll and what you can do for the next forest that grows back,” says David Cleaves, a senior adviser with the Forest Service. “The next forests will have to be more resilient in a different environment. That could mean new planting strategies, or supporting transitions to new types of forests.”
— Jennifer Weeks
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