Ecosystems: What They Are. 1. There are two "sides" in every ecosystem, the organisms on one hand and the environmental factors on the other

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 (saltness) 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. We shall find that this difference in response to environmental factors determines which species may or may not oc­cupy a given region. In turn, which organisms do or don't survive de­termines the nature of a given ecosystem.

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 re­
quire bright sun; others do best in shade. Laboratory experiments clearly
bear this fact out. Plants may be grown in a series of chambers in which
all abiotic factors are controlled. 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

Ecosystems: What They Are

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. The point that supports the maximum growth is called the opti­mum. Actually, since maximum growth usually occurs over a range of several degrees, we speak of an optimal range. The entire span that allows any growth at all is called the range of tolerance. The points at the high and low ends of the range of tolerance are called the limits of tolerance. Between the optimal range and the high or low limit of tol­erance, there are zones of stress. That is, as temperature is raised or lowered from the optimal range, the plants experience increasing stress until, at either limit of tolerance, they cannot survive. Similar experi­ments have been run to test other factors, and the results invariably follow the same general pattern. Of course, not every species has been tested for every factor; however, the consistency of such observations leads us to conclude that this is a fundamental biological principle: Every species (both plant and animal) has an optimum range, tones of stress, and limits of tolerance with respect to every abiotic factor.

5. This line of experimentation also demonstrates that different spe­cies differ markedly with respect to the values at which the optimum and limits of tolerance occur. For instance, what may be an optimal amount of water for one species may stress a second and result in the death of a third. Some plants cannot tolerate any freezing temperatures, others can tolerate slight but not intense freezing, and some actually require several weeks of freezing temperatures in order to complete their life cycles. While optimums and limits of tolerance may differ from one species, however, there may be great overlap in the ranges of tolerance for various species. Thus many plants may grow under the same conditions, although these conditions may not be optimal for all of the plants.

6. 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

^аУ be stressed or killed by overwatering or overfertilizing as well as by

Unit Four

Ecosystems: What They Are

underwatering or underfertilizing, a common pitfall for amateur gar­deners. The factor that is limiting may change from one time to anoth­er. For example, in a single growing season, temperature may be limit­ing in the early spring, nutrients may be limiting later, and then water may be limiting if a drought occurs. Also, if one limiting factor is cor­rected, growth will increase only until another factor comes into play.

7. 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. Thus the limiting factor for one species may be competition or predation from another. This is certainly the case with our agricultural crops, where it is a constant struggle to keep them from being limited or even elimi­nated by weeds and "pests".

8. 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. Significantly, no­where in nature do we find a species restraining its own spread and influence in and of its own volition; restraint is always due to one or more limiting factors.

9. 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.