The study of how and why population size change over time is

The study of how and why population size changes over time is called "Population dynamics". It is a branch of ecology that focuses on understanding the factors that affect the size, distribution, and composition of populations of organisms, including humans. Population dynamics involves analyzing changes in population size, birth rates, death rates, immigration, and emigration to gain insights into the factors that influence population growth or decline. By studying population dynamics, ecologists and other scientists can better understand the relationships between organisms and their environment, and develop strategies to manage or conserve populations of species that are important for human well-being or ecosystem health.