Biology
Ecosystem
3,734 words, 10 sections
The ecosystem is the major unit of ecology: a self-sustained area in which living organisms interact with their non-living surroundings. This chapter holds the machinery the rest of Biology leans on, namely trophic levels, the one-way flow of energy, the nutrient cycles, and the way one community replaces another through time. Its status needs stating plainly, because it is unusual. The PMDC syllabus table names no learning outcome for this chapter, and its measured examination weight is zero: no question in the paper bank carries a topic tag for it, so there are 0 distinct questions across 0 official papers, no years examined and no boards recorded. What exists is 211 active questions on this chapter in the practice bank. The material therefore belongs with the official ecology and environment content carried inside the Biology portion of the paper, worth 81 of the 180 marks, rather than with a separately measured chapter. A student who has it can place any organism at a trophic level, follow nitrogen from air to soil and back, and explain why every energy pyramid in it must be upright.
Two words carry the definition. The eco part refers to the environment and the system part to a collection of related parts functioning as a unit, which is why an ecosystem is self-sustained rather than a list of species on the same ground. It has two interacting components: the biotic (plants, animals, fungi, micro-organisms) and the abiotic (atmosphere, climate, soil, water). Above the individual the chapter builds a ladder. Individuals of one species in one place at one time form a population; interacting populations of different species form a community; a major ecosystem over a broad geographical region, with a uniform vegetation form such as grass or coniferous trees, is a biome; all the biomes together make the planetary ecosystem. The whole is the biosphere, a thin life-containing blanket extending about to above the surface and the same distance down into the oceans.
8 more sections in this chapter
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