Ecosystems and Environmental Components
This section covers Ecosystems and Environmental Components. The exam tests whether you can separate ecosystem structure from function, trace energy and matter through food webs, and explain why energy pyramids behave differently from pyramids of number or biomass.
Core rules
- An ecosystem is a biotic community plus its abiotic environment interacting in a defined area. Biotic parts are producers, consumers (primary to tertiary) and decomposers. Abiotic parts include light, temperature, water, soil and nutrients.
- Energy flows one way from the sun to producers to consumers; matter cycles. The 10% law says only about 10% of energy at one trophic level passes to the next, so food chains rarely exceed four or five links.
- Pyramids of energy are always upright. Pyramids of number can be inverted, as when one banyan tree supports many insects. Biomass can be inverted in some aquatic systems where phytoplankton are temporarily lighter than zooplankton.
- Gaseous cycles such as carbon, nitrogen and oxygen have atmospheric reservoirs. Sedimentary cycles such as phosphorus and sulphur have rock and soil reservoirs and are slower.
- Gross primary productivity minus respiration equals net primary productivity, the energy actually available to consumers.
Worked example
A grassland contains 10,000 kcal in grass. With 10% transfer, grasshoppers receive 1,000 kcal, frogs 100 kcal, snakes 10 kcal and hawks 1 kcal. If a hawk needs 10 kcal per day, this chain cannot support even one hawk for a day. The hawk must feed lower in the web or range over a much larger area. The reasoning is that about 90% is lost at each step, mainly as heat through respiration.
Traps
- Do not treat decomposers as just another level in the grazing chain; they form the detritus pathway.
- An inverted number pyramid is not a mistake; it reflects body size and aggregation.
- More producers do not automatically mean more top predators, because energy loss and habitat size also limit them.
Odisha examples include Chilika lagoon, where freshwater and seawater mix, and Bhitarkanika mangroves, which are producers and storm buffers.