An ecotone is a transition area between two adjacent ecological communities (ecosystems). It may appear on the ground as a gradual blending of the two communities across a broad area, or it may manifest itself as a sharp boundary line.

Changes in the physical environment may produce a sharp boundary, as in the example of the interface between areas of forest and cleared land (Krummholz). Elsewhere, a more gradually blended interface area will be found, where species from each community will be found together as well as unique local species. Mountain ranges often create such ecotones, due to the wide variety of climatic conditions experienced on their slopes. They may also provide a boundary between species due to the obstructive nature of their terrain; Mont Ventoux in France is a good example, marking the boundary between the flora and fauna of northern and southern France. Most wetlands are ecotones.

Ecotones are particularly significant for mobile animals, as they can exploit more than one set of habitats within a short distance. This can produce an edge effect along the boundary line, with the area displaying a greater than usual diversity of species.

The phenomenon of increased variety of plants as well as animals at the community junction is called the Edge effect and is essentially due to a locally broader range of suitable environmental conditions or ecological niches.

The word was coined from a combination of eco(logy) plus -tone, from the Greek tonos or tension – in other words, a place where ecologies are in tension.