The smallest group among the options provided is "population."
Here’s a brief overview of the hierarchy from smallest to largest:
- Population: A group of individuals of the same species living in a specific area.
- Community: All the populations of different species that live and interact in a particular area.
- Ecosystem: A community along with its physical environment, including both biotic and abiotic factors.
- Biome: A larger regional area characterized by specific climate conditions and types of ecosystems.
Thus, "population" is the smallest group of those listed.