Population ecology is the branch of ecology that studies the structure and dynamics of populations. Physiology studies individual characteristics and individual processes. These are use as a basis for prediction of processes at the population level.
Community ecology studies the structure and dynamics of animal and plant communities.
Population ecology provides modeling tools that can be used for predicting community structure and dynamics. Population genetics studies gene frequencies and microevolution in populations. Selective advantages depend on the success of organisms in their survival, reproduction and competition. And these processes are studied in population ecology. Evolutionary ecology is one of the major topics in population biology.