Population biology in English is studying how many organisms there are and how the populations interact with each other. I'm fairly sure that the French term Biologie des populations is used this way as well.

According to Wikipedia's definition: Population biology is a study of biological populations of organisms, especially in terms of biodiversity, evolution, and environmental biology. The term population biology is often used interchangeably with population ecology, although the term with biology is more frequently used when studying diseases, viruses, and microbes, and the term with ecology is used more frequently when studying plants and animals.