Which component is NOT part of the ecosphere as defined by environmental studies?

The interstellar medium is the correct choice as it is not considered part of the ecosphere in environmental studies. The ecosphere, often referred to as the Earth's ecological system, encompasses the interactions of living organisms with each other and their physical environment, which includes the atmosphere, biosphere, and geosphere.

The atmosphere is the layer of gases surrounding the planet, crucial for providing oxygen, weather patterns, and climate regulation. The biosphere encompasses all living organisms and ecosystems, while the geosphere refers to the Earth's solid components, including rocks and soil. These three components are integral to understanding ecological interactions.

In contrast, the interstellar medium consists of the matter that exists in the space between stars in a galaxy, primarily composed of gas and dust. It does not interact with Earth's ecosystems in the same way that the atmosphere, biosphere, and geosphere do, which are vital for sustaining life and facilitating ecological processes on Earth. Thus, it is the component that falls outside the scope of ecological studies when defining the ecosphere.