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.