An individual is one organism; a population is a group of the same species in one area; a community contains interacting populations of different species; and an ecosystem includes the community plus its abiotic environment and their interactions. This ecological hierarchy is core content for both SL and HL in IB ESS Subtopic 2.1.
The Ecological Hierarchy
| Level | IB ESS meaning | Serengeti example |
|---|---|---|
| Individual | One organism that is a member of a species | One lion |
| Population | Organisms of the same species living in the same area at the same time and capable of interbreeding | All lions in the Serengeti |
| Community | A collection of interacting populations within an ecosystem | Lions, wildebeest, zebras, grasses and decomposers |
| Ecosystem | A community interacting with its physical environment as a system | The Serengeti community interacting with soil, water, sunlight and climate |
Moving from population to community introduces interspecific interactions, including predation, competition, herbivory, parasitism and mutualism. Moving from community to ecosystem adds abiotic factors, such as temperature, sunlight, water, pH and soil.
An ecosystem is an open system: energy and matter enter and leave. Solar energy enters the Serengeti, while heat leaves; water and migrating organisms cross its boundaries. Tourism, livestock farming and conservation management can alter populations and interactions, linking ecological and societal factors.
A common misconception is that a community includes non-living factors. It does not: non-living components are included only at the ecosystem level.
Exam Technique
For a distinguish question, define both terms and make the difference explicit. For example: “A population contains one species, whereas a community contains interacting populations of multiple species.” Avoid writing only “a population is a group of animals” because populations may include plants, fungi or microorganisms, and must consist of the same species.