A region of transition between two biological communities. An ecotone between two habitats is often richer in species than either.1
Note this means a region of space, not a progression over time.
Air currents, light, heat, and sound waves are shaped by habitat in transitional areas and in the unique areas on each side of the transition. This creates diverse opportunities for food and shelter, which many species capitalize on.
The edge of a forest against an open field. Mammals and reptiles pass by the edge because, for one, there is shade variability. Owls and hawks use trees along the edge to perch and do fly-bys.
A beach that transitions from sand to tide pool to coral reef to big open water. Water flows and sediments are noticeably different across this gradient. The area between the coral reef edge and big open water is where sharks are most active.
Why are ecotones especially biologically rich? The ground and surface features on either side of a transition region are more uniform; the middle is a mix-up. When you have continuity and then a disruption of that continuity, evolutionary and ecological processes take advantage of novelties.
Dictionary v2.3.0, Apple Inc.