We are intimately familiar with ecosystems. In effect, every centimeter of the planet is part of an ecosystem. When talking about ecosystems, the matter of scale or size is important. Dividing ecosystems in this way allows us to examine them on a global scale and think in broad terms about the challenges of managing them sustainably. However, the divisions between ecosystems are less important than the linkages between them.
Grasslands give way to savannas that segue into forests.
Fresh water becomes brackish as it approaches a coastal area.