In ecosystems, the concept that aligns with the idea of a generally fixed amount is matter.
Matter cycles through ecosystems (for example, in the forms of carbon, nitrogen, and water), and while the energy flow is linear (entering, being transformed, and exiting the ecosystem), matter is recycled and reused. The Earth does have a generally fixed amount of matter because atoms cannot be created or destroyed; they just change forms and move through various biogeochemical cycles.
In contrast, energy flows through ecosystems, typically entering as sunlight, being transformed into chemical energy via photosynthesis, and exiting as heat, which cannot be recycled in the same way matter can.