An ecosystem's diversity or "biodiversity" can be important to its health and continued existence. The web of relationships in a complex environment such as a rainforest means that many species depend on each other. Genetic diversity among individuals in populations better equips organisms to deal with disaster or disease. Human impact on biodiversity has largely been negative, although some types of human activity may have had a beneficial effect.
Habitat Destruction & Hunting
As the human population has grown, so too has the amount of land required for food. The increase in the amount of cropland from 1950 to 1980 was greater than the growth in cropland from 1700 to 1850. A growing population also requires more land for transportation and housing. As environments such as the rain forests of the Amazon are converted to cropland or paved over, habitat is destroyed, and many species may be left without any place to live or they may even be driven to extinction.
Some species have also been driven to extinction by hunting or overharvesting. Some species of fish, for example, have been overharvested and their populations are in rapid decline.
Modern agriculture also damages biodiversity in another way. In many cases, farmers around the world have adopted the same standardized varieties of crops such bananas and rice. As local varieties are replaced by the new standard varieties, the genetic diversity of these species decreases, and some useful genes may eventually disappear altogether from the population. Ultimately, the species is less adaptable at fighting disease or withstanding changes in the environment because of a lack of genetic diversity.
On many occasions, humans have transferred species from one continent or island to another -- sometimes intentionally and on other occasions by accident. Sometimes the newcomers rapidly outcompete the species that already live there and drive them to extinction, thereby reducing biodiversity of an area. The brown treesnake, for example, was accidentally introduced to the island of Guam during WWII and, ever since then, the number of bird and reptile species on the island has rapidly plummeted.
For the most part, human impact on biodiversity has been negative; over the past few centuries, the extinction rate has climbed to as much as a thousand times the estimated natural rate. Nonetheless, there are a few examples of positive human impacts on biodiversity. Creating protected areas such as nature preserves, for example, has helped protect biodiversity or at least slow its loss. Managing fisheries and logging operations so that resources are only removed at a sustainable rate has also helped conserve some biodiversity.