Organizational ecology (also organizational demography and the population ecology of organizations) is a theoretical and empirical approach in the social sciences that is considered a sub-field of organizational studies. Organizational ecology utilizes insights from biology, economics, and sociology, and employs statistical analysis to try to understand the conditions under which organizations emerge, grow, and die. The ecology of organizations is divided into three levels, the community, the population, and the organization. The community level is the functionally integrated system of interacting populations. The population level is the set of organizations engaged in similar activities. The organization level focuses on the individual organizations (some research further divides organizations into individual member and sub-unit levels). What is generally referred to as organizational ecology in research is more accurately population ecology, focusing on the second level. Development Wharton School researcher William Evan called the population level the organization-set, and focused on the interrelations of individual organizations within the population as early as 1966. However, prior to the mid-1970s, the majority of organizational studies research focused on adaptive change in organizations (See also adaptive management and adaptive performance). The ecological approach moved focus to the environmental selection processes that affect organizations. In 1976, Eric Trist defined population ecology as "the study of the organizational field created by a number of organizations whose interrelations compose a system at the level of the whole field". He also advocated for organizational studies research to focus on populations and individual organizations as part of open rather than closed systems that have both bureaucratic (internal) regulation and ecological (community environment) regulation (see also Open and closed systems in social science). The first explicit formulation of a theory of population ecology, by Michael T. Hannan and the late John H. Freeman in their 1977 American Journal of Sociology piece "The population ecology of organizations" and later refined in their 1989 book Organizational Ecology, examines the environment in which organizations compete and how a process like natural selection occurs. This theory looks at the death of organizations (firm mortality), the birth of new organizations (organizational founding), as well as organizational growth and change. Organizational ecology has over the years become one of the central fields in organizational studies, and is known for its empirical, quantitative character. Ecological studies usually have a large-scale, longitudinal focus (datasets often span several decades, sometimes even centuries). The books The Demography of Corporations and Industries by