The natural world around us is comprised of a large variety of different ecosystems. An ecosystem consists of the non-living (abiotic) environment of a certain area and the community of organisms that inhabits that environment. The organisms of an ecosystem interact both with each other and with their non-living environment.

The biosphere is a term that means all the parts of our planet where life can be found.

Usually, the term "ecosystem" is used to mean a certain, well-defined part of the environment, such as a pond or a lake. The populations of an ecosystem are affected by the different environmental factors of that ecosystem. For ekosysteemillä tarkoitetaan tiettyä rajattua osaa luonnosta, kuten lampea. For example, the fish population of a lake is affected by such environmental factors as the water's oxygen and nutrient concentrations and the lake's acidity. 

Ecosystems are often connected to each other. For example, brooks and rivers often run into ponds or lakes. Likewise, these ponds and lakes are often connected by rivers to the sea. Water ecosystems also often have connections with their surrounding land ecosystems, such as forests. 

Some of our ecosystems are large, like the Baltic Sea. Larger ecosystems can also be divided into smaller parts: for example, the Baltic Sea can be divided into different archipelago ecosystems that are different to the ecosystems found in the open sea. The organisms of these ecosystems have adapted to the different conditions found in these ecosystems. For example, dark lakes are the home for organisms that survive with only a small amount of sunlight, whereas the organisms that inhabit sea ecosystems have adapted to the water's high salt concentration.