TEXT B: ECOSYSTEMS

An ecosystem is the most complex level of organization in nature. It consists of the biological and physical environments of an area. The biological environment is made up of all living things in the community. The nonliving or physical environment includes climate, air, soil, water, nutrients, energy and weather. All these biological and physical factors interact within an ecosystem. They compose a network of complex relationships that control population growth. Ecologists try to link together the many different physical and biological activities in an environment. They study the flow of energy and the cycling of materials through an ecosystem. They generally use powerful computers to help understand the data obtained from field research and to predict future development.

Energy flow. Ecologists categorize the elements that make up or affect an ecosystem into six main parts, based on the flow of energy and nutrients through the system: (1) the sun, (2) abiotic (nonliving or physical) substances, (3) primary producers, (4) primary consumers, (5) secondary consumers, and (6) decomposers. A simplified ecosystem is illustrated in this article.

The sun provides the energy that nearly all primary producers need to make food. Primary producers consist mainly of green plants, such as grass and trees, which make food by the process of photosynthesis. Plants also need abiotic substances, such as phosphorus and water, to grow. Primary consumers include mice, rabbits, grasshoppers, and other plant-eating animals. Foxes, skunks, and other secondary consumers —or predators—eat animals. Decomposers, such as bacteria and fungi, break down dead plants and animals into simple nutrients. The nutrients go back into the soil and are used again by plants.

The series of stages energy goes through in the form of food is called a food chain. In one simple food chain, grass is the primary producer. A primary consumer, such as a rabbit, eats the grass. The rabbit, in turn, may be eaten by a secondary consumer, such as a fox or a hawk. Decomposing bacteria break down the uneaten remains of dead grass, rabbits, foxes, and hawks, as well as animal body wastes.

Most ecosystems have a variety of producers, consumers, and decomposers, which form an overlapping network of food chains called a food web. Food webs seem especially complex in many tropical and oceanic ecosystems.

Some species eat many things, but others have very specific food requirements. Such primary consumers as koalas and pandas eat chiefly one type of plant. Koalas eat primarily eucalyptus and pandas eat primarily bamboo. If these plants died off, so would the animals.

Energy moves through an ecosystem in a series of transformations. First, primary producers change the light energy of the sun into chemical energy that is stored in plant protoplasm (cell material). Next, primary consumers eat the plants, changing the energy to a different kind of chemical energy that is stored in body cells. This energy changes again when the secondary consumer eats the primary consumer.

Most organisms have a low ecological efficiency. This means they are able to convert only a small fraction of the energy available to them into stored chemical energy. For example, green plants can change only about 0.1 to 1 percent of the solar energy that reaches them into plant protoplasm. Most of the energy captured by the plants is burned up during plant growth and escapes into the environment as heat. Similarly, herbivores (plant-eating animals) and carnivores (meat-eating animals) convert into their own body cells only about 10 to 20 percent of the energy produced by their food.

Because so much energy escapes as heat at each step of the food chain, all ecosystems develop a pyramid of energy. Plants (primary producers) form the base of this pyramid. Herbivores (primary consumers) make up the next step, and carnivores (secondary consumers) form the top. The pyramid reflects the fact that more energy passes through the plants than through the herbivores and more through the herbivores than through the carnivores. In many land ecosystems, the pyramid of energy results in a pyramid of biomass. This means that the total biomass (weight) of the plants is greater than the total weight of the herbivores, which in turn exceeds the total weight of the carnivores. In the oceans, however, the biomass of plants and animals is about the same. Small plants grow so rapidly in the oceans that they can support proportionately more animals than can the plants on land.

Ecologists have collected information on a pyramid of biomass on Isle Royale. They studied the relationship in the pyramid among plants, moose, and wolves. In one study, ecologists found that it takes 762 pounds (346 kilograms) of plant food to support 59 pounds (27 kilograms) of moose. This is the amount of moose needed to support 1 pound (0.45 kilogram) of wolf.

Cycling of materials. All living things are composed of certain chemical elements and compounds. Chief among these are water, carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur. All of these materials cycle through ecosystems again and again.

The cycling of phosphorus provides an example of this process. All organisms require phosphorus. Plants take up phosphorus compounds from the soil, and animals get phosphorus from the plants or other animals they eat. Decomposers return phosphorus to the soil after plants and animals die.

Changes in ecosystems occur daily, seasonally, and, as in the case of ecological succession, over periods of many years. Sometimes changes take place abruptly, as when a fire sweeps through a forest or a hurricane batters a seashore. But most of the day-to-day changes, especially in the nutrient cycles, are so subtle that ecosystems tend to appear stable. This apparent stability among plants and animals and their environment has been called the " balance of nature. " In the past, this concept of balanced, largely unchanging ecosystems was thought to be especially descriptive of climax communities. But these earlier views were based on short-term studies. Now that ecologists have had an opportunity to study ecosystems over longer periods, they have had to alter some of their ideas. Biologists refer to the relative stability of each population within a community as the balance of nature.

Notes: climax - клімакс (відносно стабільний стан рослинності чи живих організмів, що знаходяться у рівновазі з навколишнім середовищем)

Знайдіть в тексті абзац, в якому мова йде про “харчовий ланцюжок”. Як ви розумієте це поняття? Наведіть свій приклад “харчового ланцюжка.” Перекладіть письмово цей та попередній абзаци.

2. З’єднайте слова у словосполучення, опираючись на текст:

3. Складіть речення з поданих слів:

1. Biological, all, physical, and, factors, interact, an, ecosystem, within.

2. Moves, energy, an, ecosystem, in, a, through, of, transformations, series.

3. Organisms, have, a, low, most,efficiency, ecological.

4. Things, are, of, certain, all, living, composed, chemical, elements, and, compounds.

5. Return, to, phosphorus, the, soil, plants, and, animals, die, decomposers.

4. Підберіть визначення до наступних термінів:

5. Дайте відповіді на запитання:

1. What does ecosystem consist of?

2. What things is the biological environment made up?

3. What does the physical environment include?

4. How do ecologists classify the elements that make up an ecosystem? (What are these six main parts?)

5. What is a food web?

6. Describe a series of transformations of energy moves.

7. What forms the base of a pyramid of energy?

8. Do carnivores form the top of this pyramid?

9. What does a pyramid of biomass mean?

10. What do scientists call the “balance of nature”?

Поиск по сайту: