A person who studies the interrelationships of organisms is typically referred to as an ecologist. An ecologist examines how organisms interact with each other and their environments.
To clarify the other terms:
- Entomologist: A scientist who studies insects specifically.
- Economist: A professional who studies economic systems and issues, which is unrelated to biological interrelationships.
- Environmentalist: Someone who is concerned with or advocates for the protection of the environment, but they may not necessarily study the scientific interrelationships of organisms.
So, the best term for someone studying interrelationships of organisms is ecologist.