Population biology is a study of biological populations of organisms, especially in terms of biodiversity, evolution, and environmental biology.
Malthus can almost be considered an early population biologist, even though his training was in economics and the term population biology had not been coined. Although his work An Essay on the Principle of Population only dealt with humans for the most part, it gave Charles Darwin some inspiration for his seminal work The Origin of Species.
Charles Darwin, from his autobiography. (1876)

See also

Carrying capacity
Paul R. Ehrlich
Population dynamics
Lotka-Volterra equation