Urbanization is the increasing percentage of people living in urban areas. It results from rural migration and population growth in cities. Functionalism views society as a complex system where all parts influence each other. Two theories are human ecology, which examines human relationships with environments, and urban ecology, which studies organisms in urban settings. Functionalism includes three theories of urban structure: concentric zone theory of socioeconomic zones radiating from the city center; demographic transition theory of population changes from high birth/death rates to low rates; and multiple nuclei theory of specialized activity centers in a city.