12.4 Structure and Dynamics of Ecosystems | Introduction to Life Science | University of Tokyo


12.4Structure and Dynamics of Ecosystems


The Food Web

In the preceding section, a biocoenosis was considered an aggregate of all sorts of groups of individual constituent elements that are connected through webs via mutual interaction. On the contrary, ecosystem consists of a biotic community (biocoenosis) and the inorganic environment surrounding it, and should be studied from the aspects of its material environment as well as energy flow. The various species that represent the constituent elements of ecosystems are allocated to various trophic levels.
Trophic levels are divided into producers (mainly plants), which synthesize organic material from inorganic material through photosynthesis by receiving light of the sun, herbivores (primary consumers), carnivores (secondary consumers), which eat herbivores, and higher-order consumers. Generally, predators prey on various kinds of living organisms, and their bait also covers multiple trophic levels. Because of this, the relationship between prey and the material environment is not one straight line, but consists of multiple webs. This is called the food web.
Furthermore, the detritus eaters (organisms that break down fallen leaves in the soil, chironomids that eat organic material in river beds, gad fly larvae, etc.), which fractionize the corpses and bodily waste of organisms, and bacteria and fungi, which have the role to further breakdown those materials into inorganic material that the producer can use again, are all called decomposers. Decomposers play an important role in the cycling of materials through ecosystems.

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The Flow of Energy through Ecosystems

The source of energy of ecosystems is the energy of the sunlight that falls on the surface of the ground. Light energy is transformed through photosynthesis into chemical energy and stored in organic material. All organisms of an ecosystem live by using the chemical energy contained in such organic material. Different from substances, chemical energy is not circulated within the ecosystem: after the partial energy is being used for vital activities such as metabolism and locomotion at each stage of the transferring process toward higher trophic levels through the food web, this energy eventually changes into heat and diffuses outside the ecosystem (Table 12-1).
It should be noted in this case that across each trophic level, only 10–15% of this energy is taken in by the upper trophic level. Therefore, when all trophic levels sum up the amount of energy used per unit time as a measure, a pyramid structure called ecological pyramid can be obtained. With an ecological efficacy of 10–15% and the passage of energy through several trophic levels, the chemical energy initially fixated by plants decreases substantially (1/10,000 on the fourth trophic level in case of an ecological efficacy of 10%). Therefore, on land, at the most only up to about 5 trophic levels are found, and the number of trophic levels is limited.

Table 12-1. The Energy Flow of Ecosystems that Becomes Less at Each Trophic Level

*1 Only 1% of the amount of light energy that falls on leaves becomes organic material and is fixated for conversion.
*2 Net production at trophic levels higher than plants is determined as follows.
Net production = (Net production at the trophic level one step below) - (Amount of dead plants not eaten by animals on the upper trophic level) - (Amount eaten but not assimilated) - (Respiration rate)
Drafted based on "Fundamentals of Ecology,3rd Ed." (Odum, E.P.), Saunders, p64, 1971

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Cycling of Materials through Ecosystems

Biocenoses within ecosystems incorporate, use, and also discharge various materials, but these materials are cycled within the ecosystem by the food web. The source of carbon—one of the main materials that constitute organisms—is carbon dioxide (CO2). Producers (plants) take in CO2 and synthesize sucrose and starch through photosynthesis. When producers are eaten by herbivores and carnivores, carbon is in turn transferred to a higher-order trophic level. The carbon again returns to the atmosphere and water (carbon cycle, Fig. 12-11) through respiration and degradation of corpses into CO2. In recent years, the CO2 concentration of the atmosphere is increasing through large-scale combustion of fossil fuels such as coal and oil by human beings, which has become an issue of concern today. This is because when CO2 concentration of the atmosphere rises, the heat which is supposed to be emitted from the earth to outside the earth's atmosphere induces a greenhouse effect by which this heat remains in the atmosphere and causes an increase in its temperature (global warming).

Fig. 12-11. Schematic Diagram of the Earth Carbon Cycle

Ecosystems are greatly divided into terrestrial and marine ecosystems. In terrestrial ecosystems, CO2 that has been exhausted through respiration is accumulated in the CO2 pool of the atmosphere. In marine ecosystems, CO2 is accumulated in the CO2 pool of the ocean. Both CO2 pools are formed and destroyed periodically. The thickness of the arrows roughly indicates the amount of CO2 that is transferred.

In ecosystems, the speed with which producers fixate CO2 as an organic material is referred to as the primary production rate of an ecosystem (the unit is kcal (or J)/area/time). It consists of the total production rate and the net production rate. The total production rate represents the speed with which energy is fixated by the producers, whereas the net production rate is what remains in the production rate after the respiration rate is subtracted from the total production rate, and is used for new growth, the storage of material, and seed production. Table 12-2 shows the primary production rate of various ecosystems. Although in oceans, the primary production rate per unit area is small due to its enormous area, the primary total production of all oceans is large, and on land, tropical forests have a high primary production rate and output.

Table 12-2. Estimated Values of the Primary (Annual) Total Production Volume of Major Ecosystems

Nitrogen is contained in proteins and nucleic acids, which constitute ecologic material. However, most organisms cannot use nitrogen gas (N2) directly, and it is only fixated marginally by nitrogen-fixing bacteria and leguminous bacteria. Nitrogen is incorporated into plants as ammonium, nitrite, and nitrate salts. Through nitrogen assimilation, amino acids are synthesized, from which proteins are produced. Furthermore, nitrogen transfers via the food chain to a higher-order trophic level, or forms a part of corpses or excretion products, where it breaks down and turns into salts (nitrogen cycle, Fig. 12-12), which are again used by the producer. This indicates the importance of the work of soil microorganisms. In urban areas, the ground is further covered with asphalt and concrete. It is imperative, however, that free soil remains.

Fig. 12-12. Nitrogen Cycle of Ecosystems

In the process of the food chain, ammonium salts are generated. Following this, sulfate is accumulated via sulfurous acid and sulfuric acid-forming bacteria contained in soil and water. During absorption by plants, it is used when it turns into ammonium salt. In another cycle, atmospheric nitrogen is used and turned into sulfate by nitrogen-fixing bacteria. This process involves photochemical reactions (which also cause photochemical smog), but on the contrary there are also processes through which nitrogen is returned to atmospheric nitrogen from sulfate by denitrifying bacteria. This shows the complexity of the nitrogen cycles of ecosystems.

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