Final answer:
The smallest unit in the ecological hierarchy is an organism, which is the building block for larger ecological structures such as populations, communities, ecosystems, biomes, and the biosphere.
Step-by-step explanation:
The smallest unit in the ecological hierarchy is an organism. In the ecological hierarchy, organisms are the individual living entities that form the basic unit of biology and ecology. When multiple organisms of the same species live together, they form a population. Populations of different species living together in an area form a community. A community and its nonliving environment are known as an ecosystem, which can be part of a larger biome. Finally, the biosphere encompasses all ecosystems on Earth, representing the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, hydrosphere, and atmosphere.