An ecosystem is a unit composed of all living things that inhabit specific physical media and the interaction of this medium with those agencies and of these together. Act within an ecosystem biotic (animals, plants, microorganisms) and abiotic (without life components such as minerals and water). Two or more living things interact in positive relationships (when all involved will benefit, as the lichen symbiosis in), or negative, when at least one is harmed, like a deer predation by a lion. If the relationship between two individuals there is no benefit or harm is said that it is neutral. The terrestrial ecosystems can be classified as grasslands, savannas, forests and deserts, freshwater such as rivers, lakes and marshes, marine and coral reefs, coastal water and open sea.
Among the most significant impacts arising from human action are, the soil pollution, accumulation of gases that cause greenhouse gases in the atmosphere and altering the environment, and cultivated plants and animal species introduced in places different from source.
In ecosystems have changed in the short term, generally stable and predictable (weather stations, tidal monthly cycle), medium-term variations are related to population cycles and long-term transformations, which act for centuries, are and respond to unpredictable geological events and the evolution of species.