Which of the following factors can directly decrease population size?

Emigration directly decreases population size because it involves individuals leaving a population or area. When members of a species emigrate, they remove themselves from the population, resulting in a lower number of individuals remaining in that area. This movement can occur for various reasons, such as seeking better resources, habitat, or overcoming competition.

In contrast, immigration would increase population size as individuals move into a new area. High natality, which refers to a high birth rate, increases the population size by adding more individuals. Exponential growth describes a situation where population increases rapidly in a favorable environment, which also does not contribute to a decrease in size. Thus, emigration is the only factor among the options that directly leads to a reduction in population size.