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ECOSYSTEMS: WHAT THEY ARE
1. There are two "sides" in every ecosystem, the organisms on one 2. As it was noted before, the environment involves the interplay of many physical and chemical, or abiotic factors, the major ones being 3. In any study of the abiotic side of ecology, the key observation is that different species thrive under different conditions. This principle applies to all living things, both plants and animals. Some like it very wet; others like it relatively dry. Some like it very warm; others do best in cooler situations. Some tolerate freezing, others don't. Some require bright sun; others do best in shade. Thus, a single factor – temperature, say – can be varied in a systematic way, while all other factors are kept constant. Experiments show that, as temperature is raised from a low point that fails to support growth, plants grow increasingly well until they reach some maximum. Then, as temperature is raised still further, the plants become increasingly stressed; they do less well, suffer injury and die. 4. Since the results just described apply to any and all abiotic factors, we observe what is known as the Law of Limiting Factors: Any 5. The Law of Limiting Factors was first presented by Justus von Liebig in 1840 in connection with his observations regarding the effects of chemical nutrients on plant growth. He observed that restricting any one of the many different nutrients at any given time had the same effect: it limited growth. Thus, this law is also called Liebig's Law of Minimums. Observations since Liebig's time, however, show that his law has much broader application. Beyond its application to all abiotic factors, it may be applied to biotic factors as well. 6. In summary, the biosphere consists of a great variety of environments, both aquatic and terrestrial. In each environment we find plants, animal, and microbial species that are adapted to all the abiotic factors and also to each other in various feeding and nonfeeding relationships. Such environment supports a more or less unique grouping of organisms interacting with each other and with the environment in a way that perpetuates or sustains the entire group. That is, each environment with the species it supports is an ecosystem. Every ecosystem is interconnected with others through ecotones and through some species that cross from one system to another. At the same time, each species and, as a result, each ecosystem, is kept within certain bounds by limiting factors. That is, the spread of each species is at some point limited by its not being able to tolerate particular conditions, compete with other species, or cross some physical barrier. 7. A major concern of many environmentalists is how altering any factor, abiotic or biotic, may upset these limits and have far-reaching consequences through a ripple effect. Recognizing that everything in the biosphere is interconnected leads to the conclusion that nothing can be changed without affecting everything else to a greater or lesser degree. Obviously, humans have changed and are continuing to change things on a very large scale. What will these changes bring? Many environmentalists, including a number of scientists, promote the idea that we may be on the verge of a sudden and catastrophic "collapse of the biosphere" in which most if not all life may perish. On the other hand, cornucopians, also including a number of scientists, point out that there is no solid evidence to support such a "doomsday scenario"; much less that it is imminent. Indeed, they argue that all our experience to date should lead to the quite opposite conclusion: Humans have already caused the extinction of thousands of species and made manifold changes over most of the earth, but we and the biosphere are still doing well. Therefore, they maintain that continuing development as we have been does not pose an environmental threat.
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