Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Energy do ecosystems absorb on the earth.
What do ecosystems absorb on the earth?

Amount of salt are aquatic biomes often classified.
How are aquatic biomes often classified?

Our global ecological footprint depends on resource use per person and population density population factor.

Our global ecological footprint depends on resource use per person and what population factor?