The nutritional relationship among the components of the ecosystem is realized through the food chain and food web. The food chain is a chain-like food dependence relationship between different organisms in the ecosystem, and each link in the food chain is called trophic level. Each biological population is at a certain trophic level, and a few species are at two trophic levels at the same time, such as omnivores. The food chain in the ecosystem includes two main types: living food chain and scavenging food chain. The food chain of living food starts with green plants fixing solar energy and producing organic matter, which belongs to the first trophic level, herbivores belong to the second trophic level, and various carnivores constitute the third, fourth and higher trophic levels. The saprophytic food chain starts from the biological residues, and returns to the environment in the form of inorganic substances for re-absorption by green plants through the crushing decomposition of soil animals and the decomposition and transformation of bacteria and fungi. According to the trophic level, the decomposer is in the fifth or higher trophic level. Rats feed on grain, weasels feed on mice, and eagles feed on weasels. The remains of eagles are decomposed into inorganic substances by various microorganisms, which is an example of a simple food chain. However, the food chain in nature does not exist in isolation. It is an easy-to-understand fact that almost no consumer is specifically targeting a certain plant or animal, and no plant or animal is just the food of a certain consumer. For example, mice eat all kinds of grains and seeds, and grains are the food of many birds and insects. Insects are eaten by frogs, which are the food of snakes, and snakes are finally captured by eagles as food. The straw of grain is still the food of cattle, and beef has become the food of human beings (Figure 10-7). It can be seen that food chains often cross each other to form a complex feeding network, which is called food web. Generally speaking, the more complex the food web structure of an ecosystem, the greater the stability of the system.

4. Function of ecosystem

The functions of ecosystem are mainly manifested in biological production, energy flow and material circulation, which are realized through the core part of ecosystem-biological community.

(1) Biological production of ecosystem

Biological production of ecosystem refers to the process in which biological organisms recombine energy and substances in the process of energy and substance metabolism to form new products (carbohydrates, fats, protein, etc.). ). The production process of green plants absorbing and fixing solar energy through photosynthesis and transforming inorganic substances into organic substances is called plant production or primary production; The production process in which consumers metabolize primary products and assimilate them to form heterotrophs's own substances is called animal production or secondary production.

The solar energy fixed by plants through photosynthesis in unit area and unit time is called total primary production (GPP), and the unit is j m-2a-1or g dw m-2a-1(dw is dry weight). Net primary production (NPP) is the total primary production minus the consumption of plants due to respiration (R). The relationship between them is

GPP-R

Another concept related to primary production is biomass. For plants, it refers to the total weight of plants per unit area, and the unit is km m-2. The plant biomass at a certain moment is the primary production accumulated before that moment.

It is estimated that the global net primary production (dry matter) is172.5×109tA-1,and the biomass (dry matter) is 184 1× 109t, depending on different ecosystem types. It should be pointed out that this estimate is very rough, but it still has certain reference value for understanding the general quantitative characteristics of primary production and biomass of global ecosystems.

The ratio of the energy contained in the organic matter accumulated by plant photosynthesis on the unit ground to the solar energy irradiated on the same ground is called the light energy utilization rate. The average light energy utilization rate of green plants is 0. 14%, while the light energy utilization rate of farmland ecosystem with modern farming technology is only about 1.3%. The earth's ecosystem is the organic matter produced by such low utilization rate of light energy to maintain the survival of the animal kingdom and human beings.

(2) Energy flow of ecosystem

The biological production of ecosystem begins with the fixation of solar energy by green plants, and solar energy is transformed into biochemical energy through photosynthesis of plants, which becomes the available basic energy in ecosystem. One-way flow is an important feature of energy flow among various components of the ecosystem, which shows that a large part of energy is utilized by organisms of various trophic levels and dissipated in the form of heat through respiration, but the heat energy lost to the environment cannot return to the ecosystem to participate in energy flow, because no organisms using heat energy as energy to synthesize organic matter have been found, but the proportion of energy used to form higher trophic levels is very small (Figure 10-8).

Energy transfer and transformation in the ecosystem follow the laws of thermodynamics. According to the first law of thermodynamics, the energy input into the ecosystem is always equal to the energy stored, transformed and released by biological organisms, thus keeping the total energy value of the ecosystem and its environment unchanged. According to the second law of thermodynamics, the energy of ecosystem is changing and transferring at any time. When one form of energy is transformed into another, some energy is always consumed in the form of heat energy, so the entropy of the system tends to increase. For an isolated system with thermodynamic non-equilibrium, its entropy always tends to increase spontaneously, which makes the degree of order of the system lower and lower, and finally reaches a state of disorder and chaos, that is, thermodynamic equilibrium. However, the earth's ecosystem is experiencing a development process that is contrary to the second law of thermodynamics, that is, from simple to complex, from disorder to order. According to the viewpoint of non-equilibrium thermodynamics, an open system far from equilibrium can introduce negative entropy flow from the environment to offset the increase of entropy generated inside the system and make the system transform from disorder to order. Ecosystem is an open system, and both energy exchange and material exchange are carried out between biological communities and their environment. Through the input of energy and matter, the ecosystem constantly "eats" the negative entropy flow and maintains a highly orderly state.

As mentioned earlier, every time you go through a trophic level, you will lose a lot of energy. So, what is the energy conversion efficiency of the ecosystem? Lin Deman, an American scholar, has calculated the energy conversion efficiency of lake ecosystem, and the average result is 10%, that is, about 90% of energy is lost when it flows from one trophic level to another, which is the famous "one-tenth law" (Figure 10-9). For example, if a person gains 0.5kg by drinking water products, he will eat 5kg of fish, which feeds on 50kg of zooplankton, while 50kg of zooplankton consumes about 500kg of phytoplankton. Because this "law" comes from the study of natural lakes, it is more in line with the situation of aquatic ecosystems and does not apply to terrestrial ecosystems. Generally speaking, the energy conversion efficiency of terrestrial ecosystems is lower than that of aquatic ecosystems, because only a small part of the net production on land can be transferred to the previous trophic level, and most of it can be directly transferred to decomposers.

(3) Material circulation of ecosystem

The development and change of ecosystem not only needs a certain amount of energy input, but also contains various material movements as energy carriers in essence. For example, green plants store solar energy in the form of chemical energy in synthetic organic matter through photosynthesis, and energy and material movement coexist at the same time. The movement of various elements and compounds in nature in the ecosystem is a circular flow, which is called biogeochemical cycle.