What is the basic unit of ecological hierarchy known as?

Individual

The basic unit of ecological hierarchy is individual because a community starts building from an individual which grows onto form an entire ecosystem.

It cannot be Ecosystem as that is one of the largest unit in hierarchy. Many biological communities together form an ecosystem and many individuals in a given space form a population.

The ecological hierarchy goes like this:

individual---> population---> community ----> ecosystem ----> biome ---> biosphere.