Which of the following is not a biome?

To understand the answer, we need to look at the hierarchy of biological organization in ecology:

1. Biome: A large geographical area characterized by a specific climate and distinct types of plant and animal groups. Examples include Deserts, Grasslands, Tundra, and Tropical Rainforests.

2. Ecosystem: This is a smaller unit than a biome. It refers to the interaction of living organisms (biotic) with their physical environment (abiotic) in a specific area. A biome is actually made up of many different ecosystems.

In short, "Ecosystem" is a general term for the interaction itself, while Desert, Grassland, and Tundra are specific types of biomes.