ECOLOGY AND DEPENDENCE We have already noted that organisms in any area affect one another. But they also affect the environment, just as the environment affects them. Obvious as this may seem to us today, the extent of such interactions has only recently been fully appreciated. This recognition is the basis of the branch of biology called ecology.
The Ecosystem Approach to Understanding the Environment
All organisms, even humans, belong to some ecosystem. (In fact, most humans participate in numerous ecosystems.) Each ecosystem is composed of a living community of organisms and its nonliving surroundings. Ecosystems are "systems" in that they are self-contained units with smoothly interacting parts.
Individual ecosystems such as lakes and forests never exist in isolation. They continuously exchange both organisms and nonliving materials with nearby areas. One ecosystem interacts with the next,. so that all the organisms in the world and all the land, water, and air they inhabit really make up one giant ecosystem. The term bioshpere is used for the workdwide ecosystem.
One process that characterizes each ecosystem and relates one part to another is energy transfer.
Energy enters ecosystems as sunlight. Green plants trap light energy and make some of it available to other organisms in the form of food. In this sense, plants are the producers of an ecosystem. Herbivores are animals that feed on plants, and they, in turn, fall prey to carnivores (flesh-eating animals). When plants and animals die, their bodies become the energy source for a multitude of small de comers, decay-causing microorganisms and insects. This sequence of energy transfers in an system is often called a food chain.
The term
food web more accurately describes omplex nature of interdependence within natural ecosystems. There are always many kinds of and animals in a food web. And almost all animals feed on more than one species. To complicatete matters further, many animals are omni; that is to say, they eat both plants and animals.
Food webs are more than just patterns of energy transfer. They are key elements in the movement of matter between the living and nonliving . The chemicals that compose organisms are extracted by plants from soil, water, and air. Then lants these chemicals pass to herbivores, cars, and omnivores. And as organisms die, dead bodies decay and their chemical conponents are returned to the physical environment. - in a smoothly operating ecosystem, chemi:::cle between the living and nonliving components. Because life (bio-), earth (geo-), and chemistry are all involved, these are known as biogeochemical cycles. It is important to note that although chemicals continuously cycle within ecosystems, there is only a one-way flow of energy. Energy transfers are inherently inefficient. As each member of the food web uses energy, less is available to the next in line. Thus an ecosystem needs a constant supply of outside energy to keep operating.
Implications of Interdependence As living organisms we depend on ecosystems, especially on the biosphere as a whole. If we are to continue to exist, we must maintain an intact biosphere. Nevertheless, every society on earth is actively degrading its environment. Our growing numbers are dangerous. If world population continues to grow at its current rate-doubling every 36 years-we will be in deep trouble by the end of the century. In fact we are in trouble now, because we don't know how to stop our population explosion. Human multiplication is causing the food crisis to build like a thunderhead on a hot day. But that is only one of the problems we must solve to make long-term survival possible.
We may get by the impending food shortage only to perish because of a multitude of "little things." In some ways the biosphere can be compared to the economic health of a business or to our personal financial situation. People frequently complain that ever-increasing numbers of small expenses, such as taxes, interest, and similar obligations, are pushing them toward bankruptcy. Each expense by itself may be small, but together they add up and "nickel and dime us to death." The same principle applies to ecosystems. Today the ties that bind the biosphere together are being frayed. Pollution, small alterations of biogeochemical cycles, minor climatic modifications, erosion of soil, and the extinction of species each tend to pull the biosphere apart. Often one change makes another worse. Before we know it, these endless small changes in the biosphere may lead to a major disturbance. But environmental modifications don't just add up and compound one another. Usually one change precipitates another.
The very nature of ecosystems decrees that we can never do only one thing. Any alteration in an ecosystem initiates a chain reaction. Like a falling row of dominos, one effect brings another. One adjustment requires a second. Although minor fluctuations are normal in any ecosystem, major disturbances can potentially interrupt the pattern of movement of energy and essential materials. Because the flexibility of an ecosystem lies in its organisms, and because organisms change only slowly as their species evolve, ecosystems are vulnerable to fundamental change. Even the most vigorous ecosystem can succumb.
