Each ecosystem on earth contains numerous integral parts which together function as a network and maintain the overall community. It is this community which humans are dependent on for survival. In more primitive societies, such as hunters and gatherers, this dependency is obvious as these peoples utilized plants and animals for shelter, clothing, and food. However, in societies that are more advanced, this dependency is less evident, but present. For example, oxygen given off by plants, as a by-product of photosynthesis, is in turn used by all respiring organisms.
Scientists are unsure as to how many species of plants and animals exist on the Earth.
Biologists support that there are at least 4 to 12 million different species. Some believe that the number of species could be as high as 100 million or more. There is a variation among these species of plants and animals, which is referred to as biological diversity. Biodiversity is one of Earth s most ecologically important natural resources. There are three types of diversity encompassed within the term biological diversity.