If you think of biodiversity as encompassing the entire spectrum of life, from the largest mammals to the smallest microbes, an invasive species is one that lessens that diversity by throwing off the natural balance. An ecosystem has developed over millennia so that every part of the whole plays a role in the system’s survival. Trees produce oxygen for the animals to breathe. Fungi produce nutrients in the soil so the trees can grow, while insects carry pollen from plant to plant so they can reproduce.
Over the centuries, species come and go within an ecosystem. Some become extinct, while others migrate from different areas. The majority of new species that are introduced into a habitat do no harm [source: Spicer]. But an invasive species, by definition, is one that damages existing life forms, putting the entire ecosystem at risk. Predatory animals kill off smaller prey, while parasitic insects can eventually kill off their hosts. Some plants reproduce and spread at such a rapid rate that they crowd out other vegetation.
It is possible for one invasive species to devastate an entire ecosystem, as happened in Africa’s Lake Victoria. In the 1950s, British colonists introduced Nile perch to the lake’s waters to make for better fishing. Within a few decades, the perch had eaten most of the lake’s native fish. Because those small fish were no longer eating the algae off the lake’s aquatic plants, oxygen levels in the water plummeted, which meant that the rich marine life far below the surface could no longer be maintained. That, in turn, has left the perch with less to eat.
Today, Lake Victoria’s ecosystem is close to total destruction, with dangerous consequences for the people who live there. Before the introduction of the perch, residents of the Lake Victoria shoreline dried the small fish they caught before eating them. But the larger perch needed to be cooked, and as a result, huge stretches of surrounding forest were cut down to build fires. Now, with the fish population dying off, the lake’s residents are not only losing their major food source, but their surrounding landscape has been destroyed [source: Novacek].
Lake Victoria represents a worst-case scenario. But even if an invasive species does not wreak havoc on an entire ecosystem, it lessens biodiversity by taking over other, less hardy life forms.
Eldredge, Niles. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton University Press. 1998.
Novacek, Michael (ed.). The Biodiversity Crisis. The New Press. 2001.
Spicer, John. Biodiversity. Oneworld Publications. 2006.
Wilson, Edward O. The Diversity of Life. Harvard University Press. 1992.
Invasive species can affect biodiversity by either out-competing or hybridizing native species. If a species broaches a biogeographical boundary, for example, sometimes it has the capacity to breed with natives in its new habitat. It may even pass along a genetic edge to the resulting hybrid offspring. Then both of the genetically amped-up species -- the invasive and the hybrid -- can gang up on the original native, displacing the species or forcing it into extinction.
A classic example of an invasive species would be the brown tree snake, which is thought to have been introduced to Guam by accident as a stowaway on a military cargo ship. Over the next 30 years the snake established itself, all the while bringing to extinction nine forest birds, 50 percent of the lizards and likely some of the island's bat population. It's still considered a menace to native vertebrate populations [source: Hawaii Dept. of Land and Natural Resources].
Another example of an invasive species is the zebra mussel. It invaded the U.S. Great Lakes region in the late-1980s when it hitched a ride in a ship's ballast. It took less than a decade for the prolific mussel to spread to all the the Great Lakes, really doing quite a job of infesting the waters. Zebra mussels are resilient, fierce breeders and happy to live in all kinds of environments, so it was able to spread with ease. The trouble with them is they consume about a gallon of water per day, and that water contains live animals and algae, which is usually food for fish larvae and other small creatures. Naturally, then, those latter creatures begin to have trouble surviving, as their food supply is dented by the influx of mussels [source: USGS].
Not everyone thinks the worst about so-called invasive species, though. In June of 2011, more than a dozen ecologists argued in the journal "Nature" that it's less and less meaningful in today's world, with humans spread out everywhere, to even make the distinction between invasive and non-invasive species. They called for a re-evaluation of the standard biases against invasive species [source: Time].
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