A transitional zone between two biological communities, such as a forest and a grassland, is called:

An ecotone is a zone of transition where two different ecosystems or biological communities meet and blend into each other. Examples include the border between a forest and a grassland, between a river and its banks, between land and sea in a mangrove swamp, and an estuary, where freshwater from a river meets salty sea water.

An ecotone can be narrow, such as the edge of a forest, or wide, such as the zone between a desert and grassland. It has some typical features:

It contains species from both neighbouring communities, along with some species that are found only in the ecotone.

Because of this, an ecotone often has a greater number of species and a higher population density than either of the neighbouring communities. This tendency is called the edge effect, and the species that live mainly in the ecotone are called edge species.

Conditions in an ecotone change gradually across it, and it may be more sensitive to environmental changes.

Mangroves, estuaries and wetlands are important ecotones. They serve as breeding grounds for fish, protect coasts from storms and filter pollutants. The Sundarbans in India and Bangladesh is one of the largest mangrove ecotones in the world.

Other key ecological terms:

Niche: the role or "profession" of a species in its ecosystem, including what it eats, where it lives and how it interacts with others. The term was popularised by Joseph Grinnell and Charles Elton.

Habitat: the place where an organism lives, its "address".

Ecotype: a population of a species that is genetically adapted to local conditions.

Biome: a large regional ecosystem with a typical climate and vegetation, such as a tropical rainforest, desert or tundra.

Why not the other options?

• ✗An ecotype is a locally adapted population of a species.
• ✗A niche is the functional role of a species.
• ✗A biome is a large regional community defined by climate.