The best answer for this is as below, copied from http://in.answers.yahoo.com/question/index?qid=20090119035333AAao1yP
Vegetation or natural vegetation supports critical functions in the biosphere, at all possible spatial scales. First, vegetation regulates the flow of numerous biogeochemical cycles, most critically those of water, carbon, and nitrogen; it is also of great importance in local and global energy balances. Such cycles are important not only for global patterns of vegetation but also for those of climate.
Second, vegetation strongly affects soil characteristics, including soil volume, chemistry and texture, which feed back to affect various vegetational characteristics, including productivity and structure.
Third, vegetation serves as wildlife habitat and the energy source for the vast array of animal species on the planet and, ultimately, to those that feed on these. Vegetation is also critically important to the world economy, particularly in the use of fossil fuels as an energy source, but also in the global production of food, wood, fuel and other materials.
Perhaps most importantly, and often overlooked, global vegetation including algal communities has been the primary source of oxygen in the atmosphere, enabling the aerobic metabolism systems to evolve and persist.
Lastly, vegetation is psychologically important to humans, who evolved in direct contact with, and dependence on, vegetation, for food, shelter, and medicine.
Wildlife includes all non-domesticated plants, animals, and other organisms. Domesticating wild plant and animal species for human benefit has occurred many times all over the planet, and has a major impact on the environment, both positive and negative.
Wildlife can be found in all ecosystems, Deserts, rain forests, plains, and other areas-including the most developed urban sites-all have distinct forms of wildlife. While the term in popular culture usually refers to animals that are untouched by human factors, most scientists agree that wildlife around the world is impacted by human activities.
Anthropologists believe that the Stone Age peoples and hunter-gatherers relied on wildlife, both plant and animal, for their food. In fact, some species may have been hunted to extinction by early human hunters.
Many wildlife species have spiritual significance in different cultures around the world, and they and their products may be used as sacred objects in religious rituals. For example, eagles, hawks and their feathers have great cultural and spiritual value to Native Americans as religious objects
All wild populations of living things have many complex intertwining links with other living things around them. Large herbivorous animals such as the hippopotamus have populations of insectivorous birds that feed off the many parasitic insects that grow on the hippo. Should the hippo die out so to will these groups of birds, leading to further destruction as other species dependent on the birds are affected. Also referred to as a Domino effect, this series of chain reactions is by far the most destructive process that can occur in any ecological community.
Today, hunting, fishing, or gathering wildlife is still a significant food source in some parts of the world. In other areas, hunting and non-commercial fishing are mainly seen as a sport or recreation, with the edible meat as mostly a side benefit.
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