Ecologists usually study living systems at nested levels that get larger as you move up the hierarchy. In many textbooks the core order is organism (one individual), population (members of the same species in an area), community (interacting populations of different species), and ecosystem (the community plus nonliving factors such as air, water, and soil).
Broader treatments continue to biome and the biosphere—all life on Earth and the physical environment that supports it. OpenStax Biology 2e frames the four primary research levels as organism, population, community, and ecosystem.