Here's why:

* Biosphere: The biosphere encompasses all life on Earth and the parts of the planet where life exists, including the atmosphere, hydrosphere, and geosphere. It's a global system that includes all ecosystems.

* Ecosystem: An ecosystem is a community of living organisms (biotic) in conjunction with the nonliving components of their environment (abiotic) interacting as a system. It's a smaller, localized unit within the biosphere.

So, while ecosystems are important components of the biosphere, the biosphere is the overarching term for the entire living realm on Earth.