Population ecology is the study of these and other questions about what factors affect population and how and why a population changes over time. Population ecology has its deepest historic roots, and its richest development, in the study of population growth, regulation, and dynamics, or demography. Human population growth serves as an important model for population ecologists, and is one of the most important environmental issues of the twenty-first century. But all populations, from disease organisms to wild-harvested fish stocks and forest trees to the species in a successional series to laboratory fruit files and paramecia, have been the subject of basic and applied population biology.