Ecosystems: What They Are

1. Photographs of the earth taken from the moon made it clear as never before that the Earth is a sphere in the void of space like a space­ship on an everlasting journey. "Spaceship Earth" became a popular term and concept. Spaceship Earth is unique among all the planets we know. In addition to its rock base, it has an oxygen-rich atmosphere, an abundance of liquid water, and, most conspicuous, millions of kinds of living things of which we, Homo sapiens, are but one. The layer around the Earth where air (atmosphere), water (hydrosphere), and minerals (lithosphere) interact with living things is called the biosphere.

2. No living organism exists or can exist as an entity unto itself. Each organism is but one member of a particular species. The word "species" refers to the total population of a specific kind of plant, an­imal, or microbe. But no species is an entity unto itself either. All spe­cies depend on air, water, and nutritive elements from the earth's min­erals. But plants, animals, and microbes also play important roles in removing pollutants from air and replenishing its oxygen and carbon dioxide, in purifying water, and in recycling mineral elements. There­fore, we can say that plants support animals; animal activities support plants; and all organisms help to support and maintain air, water, and soil quality.

3. Thus, the biosphere consists of a virtually infinite number of mutually dependent and mutually supportive interrelationships among all living organisms, air, water, and minerals. Humans are no less a Part of and no less dependent on these interrelationships than any oth-^er species, although we frequently delude ourselves into thinking that ^we are. Our very term "natural resources" implies that all these things ~ air, water, soil, minerals, grasslands, forests, wildlife - are there simply tor our taking with no prices to be paid for their use, misuse, or pollu-^tlQn, and no need to put anything back in return. However, there are

prices to be paid, and some of the prices, such as potential global warm­ing, depletion of the ozone shield, and loss of tropical forests, may be steeper than we can afford.

4. It should be clear that sustainability of human society as well as sustainability of other species will depend on maintaining the integrity of the biosphere. To do this, we need to know how the biosphere works, how the biosphere functions to support all life. Thus, it is important to examine ecosystems. An ecosystem may be defined as a grouping of var­ious species of plants, animals, and microbes interacting with each oth­er and with their environment. The environment includes temperature, precipitation, amount of moisture, and all other chemical and physical factors to which organisms are exposed. Furthermore, the interrelation­ships are such that the entire grouping may perpetuate itself, perhaps indefinitely. This definition is a very condensed description of what is observed in nature and can be best understood by considering some ex­amples of ecosystems with which you are probably already familiar.

5. A quick tour across the United States shows deciduous (leaf-shed­ding) forests in the East, which turn brilliant colors in the fall before the leaves drop; prairies or grasslands in the Central States; deserts with dis­tinctive cacti in the South-west; and coniferous (evergreen) forests in the northern and western Mountain States. Across northern Canada and on the upper slopes of mountains, there are treeless expanses called tun­dra, and in equatorial regions we find tropical rainforests. These differ­ent ecosystems are the result of different climates, that is, different envi­ronmental conditions of temperature and rainfall.

6. Looking at these examples more closely, we note that each eco­system is characterized by a distinctive plant community, which is de­fined as grouping of particular plants. Each plant community supports a particular array of animals. An array of microbes will be found in each ecosystem, feeding on dead plant and animal material. Each of these examples represents a distinctive grouping of plants, animals, and microbes interacting with one another and with the environment. Thus, each grouping in association with the environment that supports it is an ecosystem. Furthermore, we know that these groupings existed long before humans came on the scene and, if not disturbed by humans, would continue to exist for a very long time and perhaps even indefi­nitely. That is, ecosystems perpetuate themselves.

7. Major terrestrial ecosystems, such as forest or prairie, are not entirely uniform; each consists of a number of more or less distinct but related ecosystems. Thus, each is also called a biome, a term that re­fers to a number of closely related ecosystems. Within a biome, an eco­system may include any more or less distinctive grouping of organisms interacting with each other in a particular environment. For instance, a patch of woods, an open field, a pond, a stream, each may be con­sidered and studied as an ecosystem. While it is convenient to divide the biosphere into biomes and ecosystems for study and discussion, it is important to recognize that they seldom have distinct boundaries, and they are definitely not independent of one another. Rather, one ecosystem tends to blend into next through a transitional region called an ecotone, which is a region that contains many of the species and characteristics of the two adjacent systems.

8. The ecotone between adjacent systems may include unique en­vironments that support distinctive plants and animals as well as those that are common to the adjoining ecosystems. Consider a marshy area, which often occurs between the open water of a lake and dry land. Whether they contain distinctive species or not, ecotones may be stud­ied as ecosystems in their own right. Furthermore, some animal spe­cies - migratory birds, for instance, - may inhabit different ecosys­tems at different times of the year, and what happens in one ecosystem used by the migrators will definitely affect any other ecosystems used during other seasons. For example, losses and fragmentation of forests have disrupted migration lanes_and resulted in drastic declines in the populations of certain North American songbirds. How the loss of all these birds will affect various ecosystems is a question we just cannot answer at this time.

9. Oceans include a variety of environments depending on tem­perature, water depth, nature of the bottom, and concentrations of nu­trients and sediments. Each of these environments, called marine en­vironments, supports a more or less distinctive array of seaweeds, Plankton, fish, shellfish, and other marine organisms. Thus, different areas of oceans — reefs, continental shelves, deep oceans — may be studied as separate ecosystems, even though their interconnections and mterdependencies are obvious. There are conspicuous ecotones be-

^ween ocean and freshwater systems in the form of estuaries, and be-

Unit Three Ecosystems: What They Are