Ecosystem: a complex system
ecosystem is a complex system make of beings that live in a specific habitat. Animals and plants are biotic components of ecosystem, wherease subsoil, air and water, light, temperature, climate, rain etc. they are abiotic component. Biotic and abiotic components found between them a connects whole that charaterize same ecosystem and take it to atemporary balance.
According to their function in the ecosystem, biotic components(beings), we can divided in :
•	producers (plants,seaweed and some bacterium): they are "autotrofi" beings that produce thierselves nourishment to live and grow, useing simply inorganic molecules like water, carbon dioxide(CO2) and nitrates.
•	consumers: they are "eterotrofi" beings, because they can't produce own nourishment and so they eat producer ( for example herbivous consumer like cows amh shepps, that eat garden's grass) or others consumer( carnivorous consumer like lions or same human)
•	decomposers (mushrooms and bacteria): they feed on decomposing tissues of dead organisms.
Each ecosystem contains a defined amount of organic matter that includes all its plant and animal organisms, the term of biomass, it identifies the weight of the substance, calculated on the dry and per unit of area occupied by the ecosystem itself.

Energy transfer in the ecosystem
Relations between the different components of an ecosystem are so narrow that if one of them is damaged, the entire ecosystem is perturbed. The main relations are those formed by flows of energy and nutrient flows. The ecosystem is an open system than the energy, the energy in and out continuously from the system. The energy enters the ecosystem primarily by the sun crosses the biotic community and its food chain, and comes out as heat, organic matter and organisms produced. More specifically, the bodies producers are able to capture and use the sun's energy to transform some inorganic compounds into organic compounds through the process of photosynthesis. These compounds are used by the producers themselves in two ways: to live and grow (in this case the organic compounds are to form new structural elements of plants like leaves and new branches). Herbivores, eating plants, they take and metabolize organic matter, that is transformed into other organic materials suitable for use in all vital functions (breathing, movement, body heat and others) and as a structural material for their growth. Similarly, behave carnivorous animals, feeding on other animals. The process continues from one category of organisms to another. The transfer of energy through the food chain, however, is accompanied by losses in the form of heat dissipated in the environment to the second law of thermodynamics. This means that in the diet of carnivores will be available less energy than is available to herbivores, which in turn receive from their food less energy than is used by plants in the metabolism of the food itself, (the organic matter formed through the process of photosynthesis). The transfer percentage of energy from one tropic level to the next is called "eco-efficiency” or effectiveness of the food chain.

Tropic chain
There are two types of food chains: the chain of grazing and the chain of debris. The first part of green plants, through herbivores grazing, then the top-level carnivores that eat herbivores, then to the second level carnivores that eat other carnivores. The second part instead of dead organic matter, passes through the micro-organisms from these animals detritivores, and ending their predators, that is carnivores. Food chains are densely interconnected, and that is why we talk about food web (or food). In natural ecosystems, organisms that obtain food from the sun with the same number of passes in the same trophic level. Then, the green plants (producers) occupy the first tropic level, organisms that feed on plants occupy the second level (primary consumers), carnivores belong to the third level to fourth level predators and carnivores (secondary consumers and tertiary). The source and quality of energy available indicate, for all levels, the type and number of organisms, and their development processes.

Nutrients
Unlike energy, which enters and exits from the ecosystem along a linear path, the field follows a circular route going from non-living compartment to living organisms and back to the abiotic compartment. These paths are defined biogeochemical cycles. Carbon, hydrogen, nitrogen, phosphorus and calcium are needed by living organisms in large quantities and this is called macronutrients. Other elements such as iron, magnesium, manganese, zinc, etc... Are required in lesser quantities and are defined for this micronutrient. This division is purely academic; as the organizations grow and reproduce they need all these substances in varying degrees depending on the time of their physiological development. Substances that can be defined as essential will vary from species to species. Both the numerical development of a population, both the individual growth of the organism depend on that element or compound that, given the needs of the organism, is found in lesser quantities in the environment: the limiting factor.

Primary productivity 
The primary productivity of an ecosystem is defined as the rate at which solar energy is converted by photosynthesis into organic matter. 
Then: 
• Gross primary productivity (PPL), the total rate of photosynthesis.
• Net primary productivity (PPN), the rate of storage of organic matter produced, net of the one used by the plant to live.
• Net productivity of the community (PNC) is the rate of storage of organic matter not used by herbivores and carnivores.
• Productivity secondary (PS) is the rate of storage of organic matter for energy purposes by consumers. 
A high rate of primary productivity in ecosystems is achieved when physical factors are favorable. The presence of secondary forms of energy can help to raise the rate of primary productivity. One example is that of estuaries, one of the most productive ecosystems in the world. In estuaries, fresh water mixes with the seawater. The plants here are extensive photosynthetic mat. Stems and roots trap large amounts of food particles, and completed their life cycle, providing additional share decompose organic matter to the ecosystem. The secondary energy is represented here by the action of tides, which helps on the one hand, the rapid movement of nutrients and, helps remove waste products, so the organisms that live here (larvae of sea bass, sea bream , mullet and clams) did not spend energy to get food or to eliminate waste and can grow more quickly.

Alteration of the food chain 
Technological innovations applied to agriculture to combat the pest organisms that damage crops has led to the use of pesticides in large quantities and for a long time. These substances are toxic and their accumulation has changed the balance of the biosphere because they have harmful effects on many organisms including man. DDT is a substance which, introduced into the environment, has caused damage to ecosystems, creating a phenomenon of widespread pesticide contamination on a global scale. Studies have found that the amount of DDT present in organisms have confirmed its presence in fish from around the world, in the Inuit, in animals living in Polar Regions and in breast milk. This passage through the various levels of DDT in the food chain is possible because the molecule remains unchanged, it does not degrade easily. It follows that at each jump of level, the concentration of DDT in organisms increases by about 10 times. That is, if the mass of bodies became the tenth part, the concentration of the pesticide becomes ten times higher. 
The damage done to the bodies by DDT are alarming: when the molecule reaches the sea, it slows down the photosynthetic activity of algae, birds' eggs are fragile and break easily during the hatching because they are poor in calcium, decreases the number of individuals population, humans relate to damage organs and systems
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The ecosystem is important for humans
Terrestrial and aquatic ecosystems are "factories" natural complex and perfectly organized that produce all that is necessary to enable life on Earth and to cover basic human needs: food, fiber, water. Some of these functions of ecosystems are essential for humans, such as air and water purification, climate control, the cycle of nutrients, soil fertility. In addition, some ecosystems (beaches, forests, lakes, high mountains, isolated valleys) are the ideal places for recreation and tourism. In addition, it also recognized the importance in economic terms: 50% of the inhabitants of the world finds employment in agriculture, forestry and fisheries. This percentage rises to 70% when considering only the sub Saharan countries, Asia and the Pacific. 25% of the countries of the world has an economy that still depends almost entirely from the areas mentioned above. Only agriculture produces 1.3 trillion dollars in food and fiber every year.

The biome 
A forest, a lake, a river, a meadow, a beach, the sea, even the green spaces of the city are all ecosystems. In short, every inch of our planet is, or is part of an ecosystem. Ecosystems can have very different sizes. Ecosystems are considered to be the temperate forest, which occupies most of North America, Europe and northern Asia, and the cavity filled with water and life of a beech forest that is part of the same. 
The Earth itself can be considered one large ecosystem. The division into smaller ecosystems and defined is necessary in the case of targeted studies, but in fact the limits of an ecosystem normally blend into those of another, and most of the bodies may belong to different ecosystems at different times. For example, fresh water becomes brackish near the coast, and in this way the ecosystem of the sea and fresh water are linked by flows of energy and food. The boundaries of an ecosystem can also vary over time due to various factors that alter the balance, including the extinction of a species, human intervention, the introduction of exotic species in an ecosystem and others. Under ideal conditions, areas that have uniform physical and chemical characteristics should have ecosystems that are well defined and easily recognizable. In nature, however, is not possible to find conditions so uniform. Especially in the case of terrestrial ecosystems, it is easier to identify groups of ecosystems. In particular, neighbors that share ecosystems and biogeochemical cycles have similar abiotic components are called "biomes". Terrestrial ecosystems can thus be grouped in several biomes. 

Earth's ecosystems
Based on the type of vegetation that characterizes them most, ecosystems are recognizable, and are divided into: 
• Deserts
• savannah
• steppe
• temperate forests
• tropical
• boreal forest (taiga) 
• tundra
• Mediterranean
Aquatic ecosystems are divided in hand: 
• freshwater ecosystems: lakes and ponds, rivers and streams, swamps and marshes
• marine ecosystems: coral reefs, oceans, continental shelves, areas of slope of nutrients, estuaries. 
On Earth, however, there are not only natural ecosystems, but also those created artificially by humans at the same time that development has led him to organize his social life and way of living and produce according to specific characteristics.
The artificial ecosystems are divided into: 
• Urban-industrial ecosystems (metropolis) 
• ecosystems rural (small towns) 
• agro ecosystems (cultivated field).

A changing world: the ecological succession
The story of an ecosystem, from birth to maturity, is called ecological succession. The ecological succession, in practice, is a continuous sequence of changes in biotic and abiotic components of an area, so we come to a stable ecosystem characterized by a balance between its various components, that is, no precedence over other decreed the extinction. The sequence of communities that gradually replace the ecosystem is known as "serious" and are called "serial stages" the different stages of transition. In some cases, the people themselves alter the environment in which they live and cause their own extinction for the benefit of other species of organisms. Examples of this evolutionary process can easily be found in nature, where the formation of any new environment initially determines the spread of organisms known as "pioneers," meaning they are able to grow despite the difficult conditions in the area. The vital activity of these early organisms changes the environment, creating new conditions that are favorable for other organisms more demanding. These are developed often causing the elimination of the entities pioneers.