The biosphere, which encompasses all living organisms on Earth and their interactions with the environment, is not uniform in thickness. In general, the biosphere extends from about 3,000 meters (approximately 9,800 feet) above sea level, where birds and insects can be found, down to around 11,000 meters (approximately 36,000 feet) below sea level, which includes the deepest trenches of the ocean where certain microorganisms exist.
However, the most biologically active part of the biosphere is often found within a much narrower range. This primarily includes the surface layers of the soil, the atmosphere, and bodies of water, where the majority of life thrives. For instance, trees and vegetation grow in soil that is typically only a few meters deep, and marine life flourishes in the upper layers of the oceans, known as the photic zone, which can extend down to about 200 meters (approximately 656 feet).
In summary, while the biosphere can be defined in an expansive sense, its practical operational thickness where life is most concentrated varies but is most effective from several meters in soil to a few hundred meters in the oceans and extending several kilometers into the atmosphere. This complex interplay of ecosystems is essential for sustaining life as we know it on our planet.