Synergistic Effects and Conclusions

Any one of these threats to biodiversity might not be enough to drive a species to extinction, but combined they may. Around the world, temperate estuaries have been permanently changed by humans. Estuaries, such as the Chesapeake Bay off the coast of Virginia, are severely affected by nutrient pollution from agriculture and sewage runoff. These excess nutrients cause phytoplankton blooms, some of them toxic, which in turn are decreasing or eliminating oxygen from the bottom sediments, making them uninhabitable to marine life. Historical analysis of the sediments reveals that as early as the late eighteenth century, human settlement in the watershed was affecting nutrient loads to the estuary and consequently the type of phytoplankton that was growing. However, sediments were still not experiencing low oxygen conditions, because the bay also had acres of oysters. Oysters could filter the entire bay in a matter of days, removing the excess phytoplankton and maintaining oxygen levels. But humans then began harvesting oysters at increasing rates, until the bay was nearly depleted of oysters by the 1930s. Without oysters to control the impact of excess nutrients from the land, the system collapsed, and the Chesapeake is now substantially and possibly irreversibly altered. That is not the only place where multiple disturbances brought about the collapse of an entire ecosystem. For instance, the Hawaiian Islands harbor one of the earth's most spectacular biotas, but also one of the most fragile and endangered. Introductions of exotic species, in combination with habitat disturbance by humans, have transformed more than 90 percent of the natural areas in Hawaii and led to countless extinctions. In the Amazon region, the water lost from plants through evapotranspiration is believed to contribute 50 percent of the annual rainfall. Deforestation reduces evapotranspiration rates, leading to decreased rainfall, and subsequently increases the area's vulnerability to fire (Laurance and Williamson, 2001). Fire can quickly burn acres of forest. Deforestation thus leads to additional forest loss through its indirect affect on the climate.

Now more than ever, we are realizing that all life on earth is interconnected. Humans are affecting not just the species that will go extinct today but also what will evolve in the future. The future of earth's biodiversity depends on us. As we begin to understand our impact on biodiversity and its importance to human survival, we are also discovering how to save biodiversity.

This material is based upon work supported by the National Science Foundation under Grant No. 0127506. Any opinions, findings and conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

—Melina F. Laverty and Eleanor J. Sterling

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