Which additional observation would support the conclusion that the deer population is not changing size from year to year, meaning that the net population is 0?

The presence of predators of deer which does not allow increase in the population of deer year after year.

Explanation:

In an forest ecosystem, one animal controls the population of another organisms and thus there is no increase in the population of that organism and the ecosystem is in equilibrium state. The population of deer did not increase due to the presence of predators such as lion, cheetah etc and the net population is 0.

from my research i have to the conclusion that decomposers, such as fungi, bacteria and invertebrates, play a critical role in nutrient cycling and without them the earth's ecosystem may not support life as we know it. elements such as carbon, nitrogen and phosphorus exist in limited amounts on earth, although huge amounts of carbon are being released into the atmosphere from underground sources now by humans. it is still extremely important that other nutrients be retained by the process of re-use. decomposers are organisms that are able to chemically break down dead material and make these elements available for use by other organisms. without this breakdown of dead material, dead bodies would pile up. so decomposers also play a role in clean up also. without them, life might not continue as we know it and dead material would overwhelm the earth.    

the answer to you question is   jet streams