Ecotone

The edge of two or more ecosystems. For example, the two banks of the river, the interface between the woods and an open field, a hedgerow next to a field of tall grass, the meeting of freshwater with salt water (i.e., estuary), etc. This zone is a region of high biodiversity and attracts plants and animals to it that cannot usually be found in any of the bordering ecosystems. For this reason, alone the ecotone is a valued (albeit small) piece of real estate.

Links
  http://www.western.edu/faculty/jsowell/ecotones.html