When an invasive species threatens a local population through competition for resources or direct predation, the results for the locals can be devastating. There have been several examples of organisms that have been directly endangered or pushed to extinction by introduced species, often with cascading consequences for the ecosystem. According to the National Wildlife Federation, 42 percent of all endangered species are threatened primarily due to an invasive species.
Invasive vs. Native Organisms
An invasive species is an organism that is introduced into an ecosystem where it didn't originally evolve. Often, the introduced organism flourishes in this unfamiliar environment, since there are few threats, if any, to its growth and propagation. An invader can be a mammal, an insect, a plant, or even a microbe like bacteria. When an invasive species begins eliminating local species, controlling the growth of the invasive organism and the subjugation of the local populations can be difficult or impossible.
Guam and the Brown Tree Snake
One case of an invasive species endangering local populations on a large scale occurred on the island of Guam, which saw an invasion of the brown tree snake in the 1950s. The snake was likely a stowaway from Papua New Guinea, and it quickly rose to dominance as the only large snake on the island. (The only native snake was a small blind wormlike creature.) By 1968, the tree snake population had expanded to every part of the island, threatening local populations of birds and mammals. By the time the U.S. Fish and Wildlife Service surveyed the island in 1984, populations of rodents and birds were all virtually extinct, and to this day these populations are significantly rarer than in other forest environments. The tree snake population, meanwhile, retains a species density of over 13,000 per square mile.
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Zebra Mussels in the United States
Invasive species often choke multiple native species out of an environment simultaneously. The zebra mussel, a native of the Balkans, Poland and Russia, hitched a ride to the United States in the ballast water of a cargo ship and muscled out the local populations of mollusks from the Great Lakes region. These mussels can produce up to 1 million eggs in a season, 2 percent of which will reach adulthood. This incredible growth rate becomes a problem when the mussels clog water intake pipes and otherwise damage man-made structures. They also coat indigenous organisms like clams to such an extent, they prohibit the clam from feeding. Other organisms like turtles and crayfish are also susceptible to having their movement, reproduction, respiration, or food supply threatened by the invasive zebra mussel. Once zebra mussels establish themselves, they are impossible to eradicate, and they can cost industrial facilities millions of dollars per year in efforts to control them.
The American Chestnut
An invasive fungus or pathogen can be just as threatening as a more complex organism. The American chestnut, a towering hardwood that once populated 200 million acres of the eastern United States with a population of around 4 billion individual trees, was devastated by a fungus known as the chestnut blight. This fungus originated from an Asian cousin, the Chinese chestnut, imported to the U.S. in the late 1890s. It only took a few decades for the blight to girdle almost every single living chestnut, effectively eliminating the tree from the United States. The species persists, since the root system survives the blight, but an adult tree cannot grow. This makes the native chestnut species "effectively extinct" once the current generation dies off, since no new seeds can be produced.