Size of Ecosystem
As you know an ecosystem may be as small and as simple as a cow dung pad or as complex and large as an ocean or the biosphere itself, comprising a wide variety of species. An interesting point to observe is that ecosystems occur within ecosystem. To take an example, cow dung ecosystem may be contained in a forest ecosystem which is contained in the biosphere.
In some cases, like a pond ecosystem, the boundaries are well defined. In the case of forests, grasslands and deserts there are no sharp boundaries. These ecosystems often are separated, from adjacent ecosystems by a transition zone or a diffused boundary zone called ecotone. Organisms of adjacent ecosystems intermingle in the ecotone zone; consequently they may have greater diversity of species than the neighbouring ecosystems.