Ecosystems, Energy flow in an ecosystem.



   The examples above illustrate the dependence of animals upon plants and each other for their sources of food; they also indicate that plant communities are influenced by the activities of animals. The soil, on which plant life depends, is affected by the plants growing on it, the small animals, plants and bacteria living in it and the activities of the larger animals moving about on its surface. The plant roots modify the crumb structure of the soil the larger soil organisms such as termites and beetle larvae make tunnels which affect the drainage; bacteria and fungi decompose the dead remains of plants and animals and so make their chemicals available for plant nutrition.

The study of the interaction of plants and animals with each other and their environment is called ecology. A self-supporting system of plants and animals in, for example, a pond, an ocean or a forest is called an ecosystemn and the interdependent groups of plants and animals are referred to as communities

In a stable ecosystem on the land, the producers (i.e. the green plants) will use carbon dioxide from the air and water and salts from the soil for their growth. Animals will eat the plants and each other. The dead remains of both will return to the soil and provide food for the decomposers, i.e. the fungi and bacteria, which will thus replace the organic matter needed for a good soil structure and the mineral salts needed by the plants. Thus, the materials in a stable ecosystem are not lost but recycled. The recycling of two elements, nitrogen and carbon, will now be described in more detail. In this case, the ecosystem is the entire Earth or, rather, that part of it which contains living organisms, the biosphere.

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The carbon cycle

The carbon cycle describes, in essence, the processes which Increase or decrease the carbon dioxide in the environment.

Removal of carbon dioxide from the atmosphere. Green plants, by their photosynthesis remove carbon dioxide from the atmosphere or from the water in which they grow. The carbon of the carbon dioxide is incorporated at first into carbohydrates such as sugar or starch and eventually into the cellulose of cell walls, and the proteins, pigments and other organic compounds which comprise living organisms. When the plants are eaten by animals the organic plant matter is digested, absorbed and built into compounds making the animals

Addition of carbon dioxide to the atmosphere.

(a) Respiration. Plants and animals obtain energy by oxid- izing carbohydrates in their cells to carbon dioxide and water. These products are excreted and the carbon dioxide returns once again to the environment.

(b) Decay. The organic matter of dead animals and plants is used by bacteria and fungi as a source of energy. The micro- organisms decompose the plant and animal material, convert- ing the carbon compounds to carbon dioxide.

(c) Combustion. In the process of burning carbon-containing fuels such as wood, coal, petroleum and natural gas, the carbon is Oxidized to carbon dioxide. The hydrocarbon fuels originate from communities of plants such as prehistoric forests or deposits of marine algae which have only partly decomposed over the millions of years since they were buried.

Thus, an atom of carbon which today is in a molecule of Carbon dioxide in the air, may tomorrow be in a molecule of cellulose in the cell wall of a blade of grass. When the grass is eaten by a cow, the carbon atom may become one of many in a protein molecule in the cow's muscle. When the protein molecule is used for respiration the carbon atom will enter the air once again as carbon dioxide. The same kind of cycling applies to nearly all the elements of the Earth. No new matter is created but it is repeatedly rearranged. A great proportion of the atoms of which you are composed will, at one time, have been an integral part of many other organisms.

Today, man's activities affect these cycles. For example, the nitrogen present in his excretory products is not usually recycled to the land producing his food; the carbon fuels are being burned in ever-increasing quantities, depleting their sources and adding more carbon dioxide to the atmosphere.

The nitrogen cycle 

When a plant or animal dies its tissues decompose, largely as a result of the action of enzymes and bacteria. One of the important products of this decomposition is ammonia, which is washed into the soil where it forms ammonium compounds.

Nitrifying bacteria. In the soil are many other bacteria and certain of these, the nitrite bacteria, oxidize the ammonium compounds to nitrites. Others, the nitrate bacteria, further oxidize nitrites to nitrates, and these can be taken up in solution by plants. The faeces of animals contain organic matter that is similarly broken down, while their urine is rich in nitrogenouss waste products such as ammonia that can be Oxidized to nitrates by soil bacteria.

   The bacteria derive energy from these oxidative processes in much the same way as plants and animals derive energy from respiration by oxidizing carbohydrates to form carbon dioxide and water.

  Nitrogen-fixing bacteria. Although green plants cannot utilize the nitrogen in the atmosphere, there are bacteria in the soil which absorb and combine it with other elements, soo making nitrogen compounds. This is called the fixation of nitrogen. Such nitrogen-fixing bacteria, as well as existing free in the soil, are also found in special root swellings, or nodules (see Plate 8), in plants of the pea family such as Crotalaria and ground-nut.

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Denitrifying bacteria. There are also bacteria in the soil that obtain energy by breaking down compounds of nitrogen tob gaseous nitrogen which consequently escapes to the atmos- phere. There are Similar cycles for other minerals, e.g. phosphates.

The process of bacterial decay releases these minerals slowly intoo the soil at a rate which, in a stable ecosystem, matches the rate at which they are taken up by plants.

Energy flow in an ecosystem

With the exception of atomic energy and tidal power, all the energy released on Earth is derived from sunlight. The energy released by animals is derived ultimately from plants that they or their prey eat and the plants depend on sunlight for making their food. The energy in organic fuels also comes ultimately from sunlight trapped by plants. Coal is formed from fossilized forests and petroleum comes from the cells of ancient marine plants.

To try and estimate just how much life the Earth can support it is necessary to examine how efficiently the sun's energy is utilized. The amount of energy from the sun reaching the

Earth's surface in one year ranges from 2 million to 8 million kilojoules per square metre (2-8 X10° Jm-2 y1) depending on the latitude. When this energy falls on to grassland, about 20 per cent is reflected by the vegetation, 39 per cent is used in evaporating water from the leaves (transpiration), 40 per cent warms up the plants, the soil and the air, leaving only about 1 per cent to be used in photosynthesis for making new organic matter in the leaves of the plants. This figure of 1 per cent will vary with the type of vegetation being considered and with climatic factors such as availability of water and the soil temperature. Sugar cane grown in deal conditions can convert 3 per cent of the solar energy into photosynthetic products and sugar beet at the height of its growth has nearly a 9 per cent efficiency. Tropical forests and swamps are far more productive than grassland but it is difficult, at the moment, to effectively harvest and utilize their products.

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In order to allow crop plants to approach their maximum efficiency they must be provided with sufficient water and mineral salts. This can be achieved by irrigation and the application of fertilizer. In some cases the small amount of carbon dioxide in the air may limit the rate of photosynthesis but little can be done about this except in an artificially enclosed ecosystem such as a green-house.

Having considered the energy conversion from sunlight to plant products the next step is to study the efficienccy of transmission of energy from plant products to first orderConsumers. On land, first order consumers eat only a small proportion of the available vegetation. In a deciduous forest only about 2 per cent is eaten; in grazing land, 40 per cent of the grass may be eaten by cows. In open water, however, where the producers are microscopic plants (phytoplankton) and swal- lowed whole by the first order consumers in the zooplankton, 90 per cent or more may be eaten. In the land communities, the parts of the vegetation not eaten by the first order consumers will eventually die and be used as a source of energy by the decomposers.

A cow is a first order consumer; of the grass it eatS Over 60 per cent passes through its alimentary canal without being digested. Another 30 per cent is used in the cow's respiration to provide energy for its movement and other life processes. Less than 10 per cent of the plant material is converted into new animal tissue to contribute to growth. This figure will vary with the diet and the age of the animal. In a fully growwn animal all the digested food will be used for energy and replacement and none will contribute to growth. Economically it is desirable to harvest the first order consumers before their rate of growth starts to fall of.

The transfer of energy from first to second order consumers i probably more efficient since a greater proportion of the an food is digested and absorbed than is the case with piai material. The transfer of energy at each stage in a food cna may be represented by classifying the organisms in a com munity as producers, first, second and third order consumers and shoWing their relative masses in a pyramid such as shown but on a more accurate scale. In the width ol the horizontal bands is proportional to the masses (dry weign) of the organisms in a shallow pond.

In human communities, the use of plant products to feed animals which provide meat, eggs and dairy products is very wasteful because only 10per cent of the plant material is Converted to animal products. Although a proportion is undigested whether it is a man or a chicken which eats wheat, it is far more economical for the man to eat bread made from the wheat than to feed the wheat to hens and eat the eggs and chicken meat because it avoids using any part of the enerey in the wheat to keep the chicken alive and healthy. Energy losses can be red uced by keeping hens indoors in small cages where they lose little heat to the atmosphere and cannot use much energy in movement. The same principles can be applied in intensive methods of rearing calves but many people feel these methods are less than humane and the saving of energy is less than if the plant products were eaten directly by man.

It is estimated that about 70g protein is needed each day by an adult. Since animal protein contains more essential amino acids than plant protein, about half this amount should come from animal sources. However, animal protein eaten in excess of about 40g per day is wasteful of food resources.

In Europe, much of the animal protein harvested from a dwindling fish population is used not to feed man but to provide fish meal for animal feed stuffs.

Consideration of the energy flow in a modern agricultural system reveals other sources of inefficiency. To produce 1 tonne of nitrogenous fertilizer takes energy equivalent to burning 5 tonnes of coal. Calculations show that if the energy needed to produce the fertilizer is added to the energy used to produce a tractor and to power it, the energy derived from the food so produced is less than that expended in producing it.


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