The Earth is made up of balanced systems of plants, animals and the environment they exist in. These are ecosystems.
Dictionary.com defines an ecosystem as a "system formed by the interaction of a community of organisms with their environment."
An ecosystem is defined by the plants and animals that have adapted for survival in a particular environment. In this vein, a mountain ecosystem includes plant life and animals that are suited to live in a cooler, mountainous situation.
Ecosystems greatly vary in size from very small to very large depending on the landscape and geography of a region.
For as many types of environments as there are on Earth, there are that many ecosystems.
Because ecosystems are simply smaller units of the biomes that make up the life on Earth, it is important the balance of ecosystems be maintained to prevent the loss of species from the planet. The loss of a smaller ecosystem affects everything connected with it in a negative way.