Sunday, March 9, 2014

How do invasive species affect biodiversity?

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 millennial 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. 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 1950's, 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.

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.

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