What best defines an ecosystem?

• A. A system formed by the interaction of a community of organisms with their physical environment
• B. A random collection of organisms in a habitat
• C. A single species living in isolation
• D. An unconnected environment with no organisms

Answer: (A)

An ecosystem is a system formed by the interaction of a community of organisms with their physical environment. It includes all the living things in an area plus the nonliving factors like air, water, soil, and climate, and the ways they influence each other. This interaction drives energy flow and nutrient cycling, supporting the organisms within the area. That’s why this definition is best: it captures both the living community and the surrounding environment as an interconnected system. The other descriptions miss key parts—one talks about a mere collection of organisms without interactions, another about a single species in isolation, and another about an environment with no organisms—so they don’t describe the integrated system that characterizes an ecosystem.