Population Changes Populations of living beings are constantly changing. The number of humans, animals, insects, or plants living in a given area can vary because of two kinds of factors: biotic and abiotic. Biotic factors are living factors that can influence the size of populations, such as predators or other species competing for food. Abiotic factors are nonliving things in the surrounding environment that can cause population changes, such as weather or sunlight. Biotic and abiotic factors cause continual changes in the number of individuals that make up a population of organisms.