International A Level Biology / Ecology

Population, community, habitat and ecosystem

Edexcel International A Level Biology (YBI11), chapter 15: Ecology, specification point 5.11. Free lesson.

Read the revision notes · International A Level Biology past papers

Population, community, habitat and ecosystem describe different parts of the living world, from one organism to interacting living and non-living parts of an area. Using each term precisely, and applying it to the place named in a question, helps you gain the marks.

How the terms fit together

The terms build outwards from one living thing. A community includes living organisms only, while an ecosystem also includes the non-living environment and the interactions between them. Organism: one individual living thing, such as one fish. Population: all organisms of one species in the same area at the same time. Community: all the populations of different species in an area. Habitat: the place where an organism lives. Ecosystem: all the organisms living in an area, together with their abiotic environment and the interactions between them.

What an ecosystem definition must include

The abiotic environment means the non-living conditions in an area. An ecosystem definition needs more than a list of living and non-living parts: it must say that they interact. Include all the organisms living in the area, meaning the community. Include the abiotic environment, such as water, light, temperature, sand or rocks. State that organisms interact with one another and with the abiotic environment.

Applying the terms to a pond

A pond makes the differences clear because its edges are easy to identify. Microorganisms, such as bacteria in the mud, count as part of the community too. One population is all the common frogs in the pond this spring. The community includes the frogs, snails, pondweed, algae and bacteria. The pond is the water snail's habitat. The abiotic conditions include temperature, light, dissolved oxygen and pH.

Answering in the context of a question

When a question asks for a definition in the context of a place, use the species and location given. A general definition may be correct, but it does not apply the idea to the question. For a population question about red squirrels on an island, write: all the red squirrels living on this island at the time of the survey. For an ecosystem question about a rocky shore, define it as all the organisms on the shore together with the abiotic environment and the interactions between them.

More lessons in Ecology

• Succession: from colonisation to climax
• Abiotic factors and population size
• Biotic factors: predation, competition and disease
• How the niche explains distribution and abundance
• Core practical 11: random sampling with quadrats
• Core practical 11: transects and abiotic measurements
• Null hypotheses and choosing a statistical test
• Spearman's rank correlation coefficient
• Student's t-test
• The chi-squared test