What is one biotic factor that affects the size of a population in an ecosystem?

The number and type of predators existing in that ecosystem

Explanation:

The predators are defined as those organisms that are dependent on other organisms for food. These are the species of higher trophic level, that is at the top of the food chain. Some of the examples of predators include lions, bears, and foxes.

The presence of these predators directly affects the size of a population. The increasing number of this organism results in a decreasing number of prey. As they will hunt more of the organisms of the lower levels, the overall population of species will thereby fall down. Both these predators and preys undergo evolution during their time of existence.

Thus, these predators, their size and type, acts as a biotic factor that directly affects the total population size in an ecosystem.