ECOSYSTEMS: WHAT THEY ARE

1. There are two "sides" in every ecosystem, the organisms on one
hand and the environmental factors on the other. All the organisms – plants, animals, and microbes – in the ecosystem are referred to as the
biota (bio, "life"). The way the categories of organisms fit together is
referred to as the biotic structure. The nonliving chemical and physi­cal factors of the environment (climate, soil quality and so forth) are
referred to as abiotic (a, "non") factors.

2. As it was noted before, the environment involves the interplay of many physical and chemical, or abiotic factors, the major ones being
rainfall (amount and distribution over the year), temperature (extremes
of heat and cold as well as average), light, water, wind, chemical nutrients, pH (acidity), salinity (saltiness) and fire. The degree to which each
is present or absent, high or low, profoundly affects the ability of organisms to survive. However, different species may be affected differently by each factor. This difference in response to environmental factors determines which species may or may not occupy a given region.

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
one factor being outside its optimal range at any given time will cause
stress and limit the growth of an organism. The factor that is limiting
the growth is called the limiting factor. It may be any factor that affects
the organism. The Law of Limiting Factors includes the problem of
"too much" as well as the problem of "too little". For example, plants
may be stressed or killed by overwatering or overfertilizing as well as by underwatering or underfertilizing, a common pitfall for amateur gar­deners. The factor that is limiting may change from one time to anoth­er.

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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