Food chain and food web: Materials Energy Within The Ecosystem – The feeding relationships among organisms at different trophic levels form a chain, the food chain. A food chain may be defined as a series of organisms that transfer food between the trophic levels of an ecosystem. All food chains begin with producers, which are usually plants in land ecosystems. The food chain continues to herbivores at the next trophic level, followed by one or more levels of carnivores, the carnivores are consumed by decomposers. An example of a food chain is shown in (fig. 19.9)
Fig. 19.9 A simple food chain
No ecosystem is simple enough to be represented by a single food chain or a few simple food chains. Most consumers feed on more than one type of food, and some consumers feed at more than one traphic level. A food web is a network of food chains representing the feeding relationships among the organisms in an ecosystems.
A food web includes all the food chains in an ecosystem. An example of a simple food web is shown in fig 19.10. It is important to note that changes in the population of an organism can affect many other populations.
Fig. 19.10 A simplified food web for an aquatic ecosystem. In nature feeding relationship are not so simple.
Energy flow
The passage of energy in a one way direction through the ecosystem is known as energy flow. In an ecosystem energy flow. Occurs in food chains, in which energy from food passes from one organism to the next in a sequence.
Energy from the sun enters an ecosystem when producers us e the energy (sunlight) to make organic matter through photosynthesis. Consumers take in this energy when they eat producers or other consumers. In this way energy moves on from one organism to the next in a sequence.
Energy from the sun enters an ecosystem when producers use the energy (sunlight) to make organic matter through photosynthesis. Consumers take in this energy when they eat producers or other consumers. In this way energy moves on from one trophic level to the next.
At each trophic level some energy is lost as heat due to the activities of the organisms. Energy is used to keep the body warm and produce motion. The energy converted into heat form cannot be used in the next trophic level and dissipates into the atmosphere. Only energy used to make biomass (i.e… organic matter) remains available (fig. 1911).
Fig. 19. 11 About 90percent of the energy in one trophic level is lost before reaching the next trophic level
19.4.3 Ecological pyramids: Cologists represent the relative amounts of energy in an ecosystem in an ecological pyramimd. Figure 19.12 shows the relative amount of energy in different trophic levels I an ecosystem. The pyramid is divided into sections, each section resenting one trophic level. An ecological pyramid can show energy, biomass, or the number of organisms in a food web.
fig. 19.12 ecological pyramids. how much of the energy that appears in one trophic level will appear in the next?
Pyramid of numbers
A pyramid of numbers shows the number of organisms at each trophic level in a given ecosystem. In pyramids of numbers, each successive trophic level is occupied by fewer organisms. Thus, the number of herbivores (such as zebras and wild beasts) is greater than the number of carnivores (such as lions)(fig. 19.13)
fig. 19.13 pyramid of numbers
Pyramid of biomass
A pyramid of biomass illustrates the total biomass at each successive trophic level. Biomass is the total amount of living matter at a trophic level. The pyramids of biomass show a progressive reduction of biomass in the successive trophic levels (fig. 19.14).
Fig. 19.14 pyramid of biomass
Pyramid of energy
A pyramid of energy indicates the energy content in the biomass of a strophic level. These pyramids show that less energy reaches each successive tropic level from the level beneath it because some of the energy at the lower level is used by the organisms to perform work, while some of it is lost (fig. 19.15)
Fig. 19.15 pyramid of energy
Flow Of Materials And Energy Within The Ecosystem – 19.4.4 Cycling of Materials in Nature.
The chemicals found in living organisms are derived originally from the biotic components of ecosystems such as soil, water, air. These materials are cycled through the ecosystems and are eventually returned to the environment when waste products and dead bodies of organisms are decomposed by bacteria and fungi. They are available for reuse by organisms.
Thus, unlike energy, matter moves through the ecosystem in cycles. The amount of matter that enters of leaves each cycle is relatively small. This fact is illustrated by carbon, water, and nitrogen cycles.
The carbon cycle
Carbon is an essential constituent of organisms. Carbon from carbon dioxide is utilized by plants in photosynthesis. This carbon becomes a part of the food made by them. The green plants are eaten by the herbivores, which in turn are eaten by the carnivores. Thus carbon is transferred from the plants to the animals. During respiration by plants and animals, CO2 is released into the atmosphere. Carbon dioxide is also released when dead organisms are decomposed by the decomposers. From the atmosphere it is again returned to the plants and cycle continues (Fig. 19.16)
Fig. 19.16The carbon cycle. Naturally the amount of carbon dioxide used in photosynthesis and released in respiration and decomposition is balanced. However, greater amount of carbon dioxide has been added to the atmosphere by human activities. Do you know how? What may be its consequences?
Materials And Energy – The nitrogen cycle
Nitrogen is important for organism because it is an essential part of proteins, nucleic acids and chlorophyll. Although earth’s atmosphere is about 78% nitrogen gas (N2) but organisms are unable to use this molecular nitrogen unless it is converted to certain compounds (such as nitrates)
The nitrogen cycle, in which nitrogen cycles between the abiotic environment and organisms have four steps. The first step in the nitrogen cycle, which is called biological nitrogen fixation, involves the conversion of gaseous nitrogen (N2) to ammonia. Biological fixation is carried out by the activity of nitrogen fixing bacteria in the soil and aquatic environment. Some nitrogen fixation to nitrates also occurs by combustion, volcanic action, lightening discharges and industrial means.
The second step of the nitrogen cycle is nitrification. It involves the conversion of ammonia (NH3) to nitrates. Nitrification is also accomplished by bacteria.
In the third step, called assimilation, plants through the roots absorb ammonia or nitrates and incorporate the nitrogen into proteins, nucleic acids and chlorophyll. When animals consume plant tissues, plant nitrogen compounds are converted to animal’s nitrogen compounds forming the bodies of animals.
The fourth and final step of the nitrogen cycle is gentrification during this stage nitrogen containing wastes and dead bodies of organisms are decomposed to ammonia and nitrates. The produced escapes to the atmosphere (Fig. 19.17)
Fig. 19.17The nitrogen cycle. All nitrogen originally comes from the atmosphere
The Water Cycle
Water continuously circulates from the oceans to the atmosphere, to the land and back to the ocean. Water evaporates from the surface and from soil, streams, rivers, and lakes in the form or water vapours.
In addition, transpiration by plants adds a considerable amount of water vapour to the atmosphere. The water vapour in the atmosphere condenses and forms clouds. It eventually comes down to the land and oceans in the form of precipitation (rain, dew, snow, or hail). Water may evaporate from land and re-enter the atmosphere directly. Alternately, it may flow in rivers and streams as run off water, reaching the ocean. Some water percolates downward in the soil to become ground water. Ground water remains in the ground for hundreds to many thousands of years, but eventually it supplies water to the soil, to streams and rivers, to plants, and to the ocean (Fig 19.18).
Fig. 19.18 The water cycle. Most of the earth’s surface water is stored in the oceans. Only about 2 percent present in the fresh lakes, ponds, rivers and streams, and in the form of precipitation (rain and snow) is available for our everyday and industrial use.