Study of the relationships between organisms and their environment. Physiological ecology focuses on the relationships between individual organisms and the physical and chemical features of their environment. Behavioral ecologists study the behaviours of individual organisms as they react to their environment. Population ecology is the study of processes that affect the distribution and abundance of animal and plant populations. Community ecology studies how communities of plant and animal populations function and are organized; it frequently concentrates on particular subsets of organisms such as plant communities or insect communities. Ecosystem ecology examines large-scale ecological issues, ones that often are framed in terms of measures such as biomass, energy flow, and nutrient cycling. Applied ecology applies ecological principles to the management of populations of crops and animals. Theoretical ecologists provide simulations of particular practical problems and develop models of general ecological relevance. See alsosystems ecology.