Final answer:
A population at the zero population growth isocline is stable in size, indicating that birth rates and death rates are equal and the population is at equilibrium.
Step-by-step explanation:
A population at the zero population growth isocline is stable in size. This concept represents a point where the population size does not change because the birth rate equals the death rate. It is not increasing exponentially, nor is it undergoing logistic growth, because logistic growth implies an initially rapid increase that slows down and reaches stability when environmental carrying capacity limits the population size. A zero population growth isocline indicates that the population has already reached a level where growth is neither positive nor negative, reflecting a stable condition.