An ecosystem consists of all the organisms in an area together with the abiotic conditions of that area, including climate, soil or water conditions and sunlight exposure. These biotic and abiotic components are interconnected through nutrient cycles and energy flows. In most, but not all, types of ecosystems, energy enters in the form of sunlight, allowing photosynthesis, which creates plant tissue. Animals, by feeding on plants and on each another, play an important role in the movement of matter and energy through the system. Dead organic matter is broken down by decomposers, including bacteria and fungi, releasing carbon back into the atmosphere in the form of carbon dioxide, which can be readily absorbed by plants, and converting other nutrients back into forms that can be reused by the various organisms.
Each ecosystem, including seemingly very similar ones, is unique. Ecosystems can exist in any size, and smaller ecosystems can exist within larger ecosystems. Examples of the smallest ecosystems include the inside of an animal's intestine, the surface of a human's skin, a puddle, and a decaying tree trunk. Examples of an intermediate-sized ecosystem are a lake, a forest and a small island. Examples of a very large ecosystem include an entire rainforest, a desert, and a large island. Changes can occur within ecosystems due to changes in the abiotic conditions, such as a change in climate, and due to changes in biotic conditions, such as the introduction of a new species or the extinction of an existing one.