An ecosystem (or ecological system) is a collection of communities of organisms and the environment in which they live. Ecosystems can vary greatly in size. Some examples of small ecosystems are tidal pools, a home garden, or the stomach of an individual cow. Larger ecosystems might encompass lakes, agricultural fields, or stands of forests. Landscape-scale ecosystems encompass larger regions, and may include different terrestrial (land) and aquatic (water) communities. Ultimately, all of Earth’s life and its physical environment could be considered to represent an entire ecosystem, known as the biosphere. Ecologists often invent boundaries for ecosystems, depending on the particular needs of their work. For example, depending on the specific interests of an ecologist, an ecosystem might be defined as the shoreline vegetation around a lake, or the entire lake itself, or the lake plus all the land around it. Because all of these units consist of organisms and their environment, they can properly be considered to be ecosystems.
All ecosystems have a few basic characteristics in common. They use energy (usually provided by sunlight) to build complex chemical compounds out of simple materials. At the level of plants, for example, carbon dioxide and water vapor are combined with the energy of sunlight to produce complex carbohydrates, such as starches (this process is known as photosynthesis). As plants (producers) are consumed by other organisms, more complex substances are manufactured in their bodies, and energy is passed upward through the food web. The flow of energy in an ecosystem occurs in only one direction: it is always consumed by higher levels of organisms in a food web. As a result, each level of a food web contains less energy than the levels below it. By contrast, nutrients can flow in any direction in an ecosystem. When plants and animals die, the compounds of which they are formed are decomposed by microorganisms (decomposers), returned to the environment, and are recycled for use again by other organisms. One of the greatest challenges facing humans and their civilization is to develop an understanding of the fundamentals of ecosystem organization, how they function and how they are structured. This knowledge is absolutely necessary if humans are to design systems that allow for the continued use of the products and services of ecosystems. Humans are sustained by ecosystems, and no alternative to this relationship exists.
An agrochemical is any substance used to help manage an agricultural ecosystem, or the community of organisms in a farming area. Agrochemicals include: (1) fertilizers, (2) liming and acidifying agents, (3) soil conditioners, (4) pesticides, and (5) chemicals used in animal husbandry, such as antibiotics and hormones. The use of agrochemicals has been critical to the raising crops for food. However, some of these chemicals cause substantial environmental and ecological damage, greatly reducing their benefits. Fertilizers are substances that are added to farmlands to encourage plant growth and to increase crop yields. Fertilizers may be chemically manufactured (synthetic) or be made from organic (living) material such as recycled waste, animal manure, or compost (decaying vegetation). Most fertilizers contain varying amounts of nitrogen, phosphorus, and potassium, which are inorganic (nonliving) nutrients that plants need to grow. Globally, about 152 million tons (138 million metric tons) of fertilizers are used each year. In the United States, the yearly total is about 21 million tons (19 million metric tons).
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Crops planted in soil that is either too acidic or too alkaline (basic) cannot obtain the proper nutrients they need to grow from that soil. Acidic soils, an especially common problem in agriculture, can be caused by various factors, including acid rain (precipitation that contains weak sulfuric and nitric acids) and the use of certain types of fertilizers. Acidic soils are commonly neutralized by adding compounds that contain calcium, usually in the form of powdered limestone or crushed oyster or mussel shells. Soil conditioners are materials that are added to soil, usually to increase its ability to hold water and oxygen. Materials used as soil conditioners include peat, livestock manure, sewage sludge, and even shredded newspapers. Compost is probably the best soil conditioner because it keeps soil from becoming too acidic or too alkaline and it supplies the soil with organic nutrients.
Pesticides are used to eliminate the presence of pests, any living thing that causes injury or disease to crops. Although many kinds of pesticides are used in agriculture, they can be categorized into simple groups according to the pest they are targeting. Herbicides are used to kill weeds, any non-desired plant that interferes with the growth of crops. Fungicides are used to protect agricultural plants from fungal diseases. Insecticides are used to kill insects that eat crops or stored grains. Other pesticides target snails, slugs, mites, rodents, and birds. Very large quantities of pesticides are used in modern agriculture. Globally, about 4.4 to 6.6 billion pounds (2 to 3 billion kilograms) of pesticides are used each year, at a total cost of about billion. The United States alone accounts for about one-third of all pesticide usage. Various agrochemicals are given to livestock. Antibiotics are administered, either by injection or combined with feed, to control infectious diseases and parasites that often arise when animals are raised under extremely crowded conditions. Hormones are routinely administered to increase the growth and productivity of animals, such as the bovine growth hormone given to cows.
While agrochemicals increase plant and animal crop production, they can also damage the environment. Excessive use of fertilizers has led to the contamination of groundwater with nitrate, a chemical compound that in large concentrations is poisonous to humans and animals. In addition, the runoff of fertilizers into streams, lakes, and other surface waters can increase the growth of algae, leading to the death of fish and other aquatic animals. Pesticides that are sprayed on entire fields using equipment mounted on tractors, airplanes, or helicopters often drift away from the targeted field, settling on nearby plants and animals. Some older pesticides, like the powerful insecticide DDT (dichlorodiphenyltrichloroethane), remain active in the environment for many years, contaminating virtually all wildlife, well water, food, and even humans with whom it comes in contact. Although many of these pesticides have been banned, some newer pesticides still cause severe damage. In North America, for example, it is believed that millions of wild birds are killed each year from exposure to the agricultural insecticide carbofuran.
These and other environmental effects have prompted researchers to search for nonchemical methods of enhancing soil fertility and dealing with pests. These alternatives, however, are still quite expensive at the beginning of the twenty-first century and are not yet in widespread use. In late 2000, the United Nations Environment Program organized a meeting to draft a global treaty to restrict the production and use of twelve persistent organic pollutants (POPs), especially those used as pesticides. The twelve toxic chemicals cited, which environmentalists have called the “dirty dozen,” include eight pesticides: aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, and toxaphene. Since it is still widely used in Africa to control malaria, DDT was given a special exemption. It can be used in those countries until replacement chemicals or strategies can be put into place. One hundred and twenty-two nations (including the United States) agreed to the treaty, but before it can take effect, at least fifty of those nations must also ratify it.
Dr. Badruddin Khan teaches Chemistry in the University of Kashmir, Srinagar, India.
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