Why is it reasonable to describe a large ecosystem like a biome as being in equilibrium, but not a very small ecosystem like a rotting log?

An ecosystem is an interdependent system comprised of living, nonliving, and dead components. Nonliving elements, such as light from the sun and carbon, are essential for plants ability to synthesize carbohydrate energy. As organisms die, their nutrients are utilized by other members of the ecosystem community. Fungus, in particular, is a decomposer that aids in breaking down dead matter.
A log is part of a dead tree. It can be home to organisms such as moss, fungi, mold, and bacteria. However, it does not contain the nonliving elements that are essential to create a full cycle of energy exchange between living, nonliving, and dead materials. Without sunlight, moss is unable to photosynthesize the energy it needs to thrive. Without oxygen, bacteria cannot metabolize the carbohydrates found in the chlorophyll of a dead plant. Although a log can be a member of an ecosystem community, it does not contain all of the components necessary to be its own ecosystem.
 
https://en.wikipedia.org/wiki/Ecosystem