The first principle of ecology is that every living being in continual relationship with everything that constitutes their environment. They say there is a ecosystem once it is sustainable interaction between organisms and environments.
The ecosystem is analytically differentiated into two sets that interact:
* The biotic community, composed of all living beings
* Within the ecosystem, species have these dependencies, including food. They share with each other and the environment they affect, of the energy and matter. The dead organic matter is one element.
The ecosystem concept is theoretical: it is multiscalar (multi-scale), that is to say, it may apply to portions of varying sizes of the biosphere, a pond, a meadow, or a tree death. A smaller unit is called a microcosm. It may, for example, include species that have colonized a submerged rock. A mésoécosystème could be a forest, and a macro ecosystem a region and its watershed.
The main issues facing an ecologist at the study ecosystems are:
* how was able to achieve colonization of an arid land?
* how continued this trend?
* the current state is stable?
* What are the relationships between the various elements of the system?
Ecosystems are often classified by reference to the biotopes concerned.
* continental ecosystems (or terrestrial), such as forest ecosystems (forests), grassland ecosystems (grasslands, steppes, savannas), agro-ecosystems (agricultural systems);
* inland water ecosystems, benthic ecosystems (lakes, ponds) or lotic ecosystems (rivers);
* oceanic ecosystems (seas, oceans).
Another classification can be done with reference to biological communities (eg, one speaks of forest ecosystem, ecosystem or human).
