Scope, Components and Ecosystems
What this domain tests
This domain introduces environmental science as an interdisciplinary field linking ecology, chemistry, physics, economics and law. The WBCHSE HS paper tests definitions, ecosystem structure and function, energy flow, trophic levels, ecological pyramids, productivity, nutrient cycles, succession and human impacts.
Core rules and concepts
- An ecosystem has abiotic components (light, water, temperature, soil, nutrients) and biotic components (producers, consumers, decomposers).
- Energy flows one way and is lost as heat; matter cycles. The 10% ecological efficiency rule is a teaching approximation, not a law.
- Gross primary productivity (GPP) is total photosynthesis; net primary productivity (NPP) = GPP − respiration. Only NPP is available to consumers.
- Ecological pyramids may be upright, inverted or spindle-shaped; pyramids of numbers and biomass can invert, but an energy pyramid is always upright.
- Succession changes species composition and often increases biomass and soil development until a stable or climax community forms.
Worked example
A pond's phytoplankton fix 10,000 kcal m⁻² yr⁻¹. Using 10% transfer, herbivores receive 1,000 kcal, small carnivores 100 kcal and top carnivores 10 kcal. If top carnivores require 20 kcal m⁻² yr⁻¹, the producer level must be at least 20,000 kcal under the same assumption. Reason: energy required ÷ 0.1 three times = 20 ÷ 0.001 = 20,000 kcal. Real food webs have multiple pathways, so this is a minimum estimate.
Traps that cost marks
Do not confuse GPP with NPP, or biomass with energy. Decomposers are not a trophic level in the grazing chain. Inverted pyramids are valid for numbers and biomass. Human subsidies can make energy pyramids appear inverted locally, but the sun remains the ultimate source.