http://eschooltoday.com/ecosystems/scales-of-an-ecosystem.html

Ecosystems come in indefinite sizes. It can exist in a small area such as underneath a rock, a decaying tree-trunk, or a pond in your village, or it can exist in large forms such as an entire rain forest. Technically, the Earth can be called a huge ecosystem.

The illustration above shows an example of a small (decaying tree trunk) ecosystem

To make things simple, let us classify ecosystems into three main scales.

Micro: A small scale ecosystem such as a pond, puddle, tree trunk, under a rock etc.

Messo: A medium scale ecosystem such as a forest or a large lake.

Biome: A very large ecosystem or collection of ecosystems with similar biotic and abiotic factors such as an entire Rain forest with millions of animals and trees, with many different water bodies running through them.

Ecosystem boundaries are not marked (separated) by rigid lines. They are often separated by geographical barriers such as deserts, mountains, oceans, lakes and rivers. As these borders are never rigid, ecosystems tend to blend into each other. This is why a lake can have many small ecosystems with their own unique characteristics. Scientists call this blending “ecotone”

Ecosystems can be put into 2 groups. If the ecosystem exists in a water body, like an ocean, freshwater or puddle, it is called an aquatic ecosystem. Those that exists outside of water bodies are called terrestrial ecosystems.