First, the basic concept of ecosystem

(1) biological population and community

The so-called population refers to the sum of individuals of the same species with a certain space and time. The same species that make up a population must have potential compatible individuals (male and female), otherwise the population will disappear, so the population is a species-specific unit of existence, reproduction and evolution.

There is not only communication between different populations of the same species, but also obvious geographical isolation. For example, the population on the islands in the ocean is obviously geographically isolated from the mainland. However, people living in one city communicate with people in another city, even when different people cross the ocean.

Community refers to the aggregation of all biological populations in a certain spatial range. This aggregate includes the populations of plants, animals and microorganisms. The living things on the earth are interdependent. No creature can live alone, but many creatures live together to form an interdependent, interactive and mutually restrictive organism. Community is such an organic synthesis. Community has a certain structure, a certain species combination and a certain interspecific relationship, which can be repeated in different areas with similar environmental conditions. However, the community is not a random combination of any species. Species living in the same community have been preserved through long-term historical development and natural selection, and their interaction is not only conducive to their own survival and reproduction, but also conducive to maintaining the stability of the community.

(b) food chain and trophic level

Simply put, the food cycle is that one creature feeds on another creature, and the other creature feeds on the third creature ... and they form a chain relationship with food as the button. For example, in the grassland, rabbits feed on grass, foxes feed on rabbits, wolves prey on foxes, and wolves prey on fiercer tigers or lions. This is a food chain. In fact, the food chain is a very complicated relationship in essence, and there are mutual constraints between organisms, which is an important factor restricting the stability of a community.

Nutritional level refers to every link in the food chain. For example, green plants are a trophic level, rabbits or herbivores are a trophic level, and wolves or carnivores are a trophic level (figure 1 1.7). The trophic level is actually a combination of organisms that store energy. It can be divided into the first trophic level (located at the bottom, such as green plants), the second trophic level (such as herbivores), the third trophic level (carnivores), and the fourth and fifth trophic levels.

(3) Ecosystem and its composition

The term ecosystem was put forward by British plant ecologist Tansley in 1935. The current definition refers to the sum of all living and non-living things in a certain time and space (table 1 1.2). Ecosystem is a complex complex. Living things and living or non-living things are interrelated, influenced and interdependent through energy flow, material circulation and information transmission, forming a complex with self-organization and self-regulation functions.

Ecosystems can range from the global biosphere to a pond. Regardless of the size, the ecosystem should include four components: producers, consumers, decomposers and inorganic environment.

Table 1 1.2 Ecosystem structure

Inorganic environment is abiotic substance and energy, including water, gas, soil and sunlight, and sunlight is the main source of living things. Producer is the most active factor in the ecosystem, which includes all green plants and some bacteria. They convert inorganic substances such as carbon dioxide and water into organic substances through photosynthesis, store solar energy, and make the ecosystem gain energy. Consumers are the negative factors of the ecosystem. They can't produce organic matter and energy for the ecosystem, but rely directly or indirectly on producers for survival. Consumers include various animals. Decomposer is also a positive factor in the ecosystem, which gradually decomposes some complex organic matter in the ecosystem into simple inorganic matter, which is convenient for producers to absorb. Including bacteria, fungi, soil protozoa and so on.

(d) Energy flow and material circulation in ecosystems

The energy of the ecosystem is constantly flowing. Energy flow can not only make all parts of the ecosystem gain energy and maintain stability, but also establish relationships among various organisms in the ecosystem. The energy flow in the ecosystem is completed through the food chain (Figure 1 1.7). For example, green plants convert solar energy into energy in the ecosystem, while herbivores transfer the energy in green plants to the second trophic level, and carnivores transfer the energy to the third trophic level. In this way, the energy in the ecosystem flows down step by step.

The energy flow in the ecosystem follows the law of conservation of energy, which will neither produce nor die. Because the ecosystem is an open system, the energy flow is a one-way irreversible process, which always flows from the energy-rich low trophic level to the high trophic level. Another principle of energy flow in the ecosystem is the "one tenth" rule (Figure 1 1.8), that is, in the process of energy flow, the energy efficiency of energy transfer from the previous trophic level to the latter trophic level is generally around 10%, and other energy is exported to the ecosystem through heat dissipation, respiration and excretion.

The material circulation in the ecosystem refers to the non-stop movement of various substances and elements from environment to biology and from biology to environment. Various chemical elements necessary for life, such as oxygen, carbon, nitrogen and phosphorus, circulate among producers, consumers and decomposers through the food chain. Take carbon as an example. In the form of CO2, it is stored by green plants or algae through photosynthesis. Some herbivores transfer this part of carbon to their own bodies, and part of it is excreted into the atmosphere through respiration in the form of CO2, and then circulated through photosynthesis. Part of it is excreted as waste, decomposed by decomposers and absorbed by plants, and the rest is transferred to carnivores through the food chain.

(5) Ecosystem classification

There are many schemes for the classification of ecosystems. According to the biological composition of the ecosystem, it can be divided into plant ecosystem, animal ecosystem, microbial ecosystem and human ecosystem. According to the abiotic components and characteristics of ecosystems, they can be divided into terrestrial ecosystems and aquatic ecosystems. According to human activities and their influence, it can be divided into natural ecosystem, semi-natural ecosystem and artificial composite ecosystem. The types of ecosystems can be summarized as follows:

Second, forest ecosystem.

Forest ecosystem is one of the most important ecosystems on the earth, which not only plays a regulatory role in the global environment, but also is the most important biomass storage place in the biosphere. The main contributions of forest ecosystem to the earth's environment are:

(1) Cyclic changes of carbon fixation, oxygen production and carbon dioxide are the basis of the existence of the earth's biosphere. The breathing process of animals and various combustion processes carried out by humans consume a lot of oxygen and release a considerable amount of carbon dioxide. Without the function of green plants to absorb carbon dioxide and produce oxygen, the nature of the earth's atmosphere will be fundamentally changed, and living things will be difficult to survive. According to the research, 1hm2 broad-leaved forest absorbs 1t CO2 and releases 0.73t oxygen every day. Forests around the world can convert about 55 billion tons of carbon dioxide into wood every year, while releasing more than 40 billion tons of oxygen.

(2) The important hub of material and energy exchange forest is not only an important reservoir of carbon dioxide, but also the exchange capacity of inorganic substances such as water and nitrogen through the forest is huge. Forests absorb about 25 billion tons of water and a corresponding amount of carbon dioxide every year to synthesize organic matter. The amount of water transpiration through the forest is larger, with about 480,000 tons of water transpiration into the air every year, which needs to consume 1.005× 1.022J of heat. The fixed solar energy of forest ecosystem photosynthesis accounts for about half of the whole biosphere.

(3) Water conservation and loose soil in water-soil forest land, coupled with the well-developed root system in the soil, often have a large number of litter on it, all of which have water conservation ability, and can keep the water falling on the forest land from losing. According to the measurement, the soil on the slope land without forest can only absorb 56% of the water reduction, while the forest belt with the width of 10m can absorb 84%. If the forest width reaches 80m, surface runoff can be completely converted into underground runoff, and 50,000 mu of forest is equivalent to a reservoir with a storage capacity of 100× 104m3. Because the breaking of forest branches and leaves weakens the kinetic energy of raindrops and the fixation of tree roots, the scouring effect of rainwater on soil layer is greatly reduced. According to the measurement, the soil erosion of forest land is less than that of bare land 100 times. Therefore, after forest destruction, it will destroy the ecological balance and cause a series of serious consequences.

(4) Clean woodland can absorb harmful gases, purify particles and dust in the air, and make the atmosphere clean, which is more suitable for human survival. According to research, elm leaves can retain 3.39g of dust per square meter, and mountain forests can absorb about 4.5t of dust per mu every year. Many trees can absorb some gases, Robinia pseudoacacia absorbs 42kg of chlorine per hectare, ginkgo, citrus and walnut trees absorb carbon dioxide, tea trees and camellia sinensis absorb hydrogen fluoride, and poplar and mulberry leaves absorb lead dust.

Third, ecological balance.

In any normal ecosystem, energy flow and material circulation are always going on. In a certain time and space, there is a dynamic stability among producers, consumers and decomposers, which is called ecological balance. This balance includes the stability of ecosystem structure, function and energy input and output.

It is very important to maintain ecological balance, because ecosystems are of great significance to human survival and development. This significance is mainly manifested in its function. For example, forest ecosystem is one of the most important ecosystems on the earth, which has the functions of carbon fixation and oxygen generation, water conservation, soil erosion prevention, air purification and material exchange. If there were no forests on the earth, the landscape of the earth would be unimaginable.

There are many factors that destroy the ecological balance, both natural and man-made. Natural factors, such as earthquakes, volcanic eruptions, forest fires, droughts, floods, etc. , may destroy a component or trophic level of the ecosystem, resulting in the imbalance of the ecosystem. Human factors are mainly manifested in unreasonable development and utilization of natural resources or man-made destruction. Such as large-scale deforestation, excessive reclamation of grasslands, urbanization development, water pollution and so on. , will destroy the ecological balance. For example, the catastrophic floods in the Yangtze River basin from 65438 to 0998 were closely related to the destruction of the forest ecosystem in the upper reaches of the Yangtze River basin.

The stability (or balance) of ecosystem is related to the structure of ecosystem, the way of energy flow and material circulation. Generally speaking, it is easy to maintain stability in an ecosystem with diverse components, complex energy flow and material circulation paths; On the contrary, ecosystems with simple components and structures are usually fragile. Therefore, forest ecosystem is more stable than grassland ecosystem, and grassland ecosystem is more stable than tundra ecosystem. We often say that maintaining biodiversity is for the stability of the ecosystem. However, the regulation ability of an ecosystem is limited. Beyond this limit, the adjustment ability of the ecosystem itself will no longer play a role, and the ecological balance will be destroyed.

Four. Biological disaster

Biological disasters mainly include insect disaster, forest fire, red tide, rat disaster, plague and so on. Forest fire is one of the most serious disasters threatening forest ecosystem at present. Due to abnormal climate, forest fires occurred frequently all over the world in 1980s and 1990s, such as forest fires in Lake Baikal in Russia, forest fires in Mongolia and forest fires in Indonesia. A forest fire broke out in Daxing 'anling 1987, with the burned area of 1 14000 hm2 (i.e. hectares), and the direct economic loss was more than 500 million yuan. 1997 the forest fire in Indonesia burned for three months, and smoke filled the sky over Jakarta. Pedestrians in the street wore masks, which destroyed more than 300,000 hectares of forest and killed 300 people.

Insect disasters occur every year, affecting all parts of the world. The annual loss of forestry caused by pests and forest fires in China is 2 billion yuan, and the agricultural loss is 654.38+0 billion yuan. Insect disasters mainly include locust disaster, cotton bollworm disaster, rice insect disaster and pine caterpillar disaster. Rodents are mainly food loss and disease spread. 1347 ~ 1350 The Black Death, which was prevalent in Europe in the 20th century, was a plague, which claimed the lives of13 Europeans (more than 20 million people). In 1980s, the average annual grain loss caused by rodents in China reached 720 million kilograms. In addition, there are a series of biological disasters such as termites eating wooden houses and furniture, and mosquitoes spreading diseases.