Cambridge O Level Biology · Syllabus 5090 · Relationships of Organisms with One Another and with the Environment
Ecosystem
What is Ecosystem?
A unit containing the community of organisms in an area and the non-living parts of their environment, all interacting with one another. The living part is the community, made up of every population of every species present; the non-living part includes factors such as light, temperature, water, dissolved oxygen and mineral ions in the soil. Because the organisms and their surroundings affect each other continuously, an ecosystem is defined by the interaction, not merely by the boundary drawn around it.
This definition is part of the Relationships of Organisms with One Another and with the Environment chapter in Cambridge O Level Biology.
Ecosystem in context
Ecosystems run on one input and two very different accounting systems. Light energy from the Sun enters through photosynthesis, passes along food chains by feeding, and is transferred to the environment as thermal energy at every step — so energy flows through an ecosystem and is never recycled. The atoms carried with it are different: carbon and nitrogen move from air to producer to consumer to decomposer and back to the air, so matter cycles and is used again. Everything else in this topic — short food chains, ecological pyramids, the carbon and nitrogen cycles, eutrophication, conservation — is a consequence of those two sentences.
Common mistakes with Ecosystem
- “Energy is recycled by decomposers.” Why it is wrong: decomposers respire like any organism, so the energy they release is transferred to the environment as thermal energy. Nothing returns it to the producers. Exam-safe: decomposers recycle matter, releasing inorganic nutrients. Energy flows through the ecosystem and is not recycled.
Examiner tips on Ecosystem
- Four conclusions that lose marks. “Ban all fishing” — ignores that conservation manages use rather than ending it. “Education will solve it” — education changes understanding, not the removal rate, on its own. “Quotas always work” — they work only when set correctly and enforced. “Replanting restores the ecosystem” — it restores tree cover; biodiversity is a separate measurement. Each of these states an outcome without the mechanism or the evidence.
Questions students ask about Ecosystem
Does replanting a forest undo deforestation?
Partly. Replanting restores tree cover, so soil is bound again, rain is intercepted, photosynthesis resumes and a future timber supply is secured. But a plantation of one fast-growing species provides far fewer habitats and food sources than a diverse natural forest, so much of the lost biodiversity does not return. Monitoring biodiversity, not just tree cover, is what settles whether the ecosystem has recovered.