Ecosystem are not random collections of organisms. Ecologists have identified several levels of completely in Ecosystems and recognized that different types of interactions occur at each level. The simplest level is the individual. Individuals are part of a population-a group made up of the same species. In March 2010 almost 309 million people lived in the United States. The 534 residents of Achille, Oklahoma, also represent a population. A study can look at the global population of a species or at more localized segments of it, so long as the parameters are defined in advance.