EcosystemNEET Botany · Class 12 · NCERT Chapter 5

Ecosystem appears in almost every NEET paper, contributing 3 to 5 questions. High-yield topics are ecological pyramids (especially which pyramid can be inverted), the 10% law, GPP vs NPP, decomposition steps, and nutrient cycling. NEET 2023 had 4 questions from this chapter.

Gross Primary Productivity (GPP) is the total rate of organic matter production by autotrophs through photosynthesis. Net Primary Productivity (NPP) is the energy available to consumers after the producers have used some for their own respiration. NPP = GPP minus Respiration (R). NPP represents the actual biomass available to herbivores. Tropical rainforests have the highest NPP on land; open ocean has the lowest per unit area.

Decomposition has 5 steps: (1) Fragmentation: detritivores (earthworms, millipedes, fungi) break detritus into smaller pieces, increasing surface area. (2) Leaching: water-soluble nutrients move into the soil. (3) Catabolism: bacteria and fungi secrete enzymes to degrade detritus into simpler molecules. (4) Humification: formation of dark, colloidal humus that is resistant to further microbial breakdown. (5) Mineralisation: further degradation of humus releases inorganic nutrients into the soil. Remember: F-L-C-H-M (Fragmentation, Leaching, Catabolism, Humification, Mineralisation).

The pyramid of energy is always upright because energy always decreases from one trophic level to the next due to the 10% law. You cannot get more energy at a higher level than was present at the lower level. Pyramids of biomass and number can be inverted in specific ecosystems. Example: In a tree-insect-parasites food chain, one large tree (low number, high biomass) supports thousands of insects (high number), which support even more parasites. So the pyramid of number becomes inverted. In a marine ecosystem, phytoplankton (reproduce rapidly, low standing biomass at any moment) support zooplankton (higher standing biomass), giving an inverted pyramid of biomass.

The 10% law was proposed by Raymond Lindemann (1942). It states that only 10% of energy at one trophic level is transferred to the next trophic level. The remaining 90% is lost as heat, used in respiration, or goes to decomposers. Example: If producers have 1000 kcal, primary consumers get 100 kcal, secondary consumers get 10 kcal, tertiary consumers get 1 kcal. This is why food chains are typically limited to 4 to 5 trophic levels.

Phosphorus exists as phosphate ions in soil and rock, not in a volatile gaseous form. So its cycle is called a sedimentary cycle: phosphate is released by rock weathering, absorbed by plants, passed through food chains, released by decomposition, and eventually returns to rocks/sediments. Carbon, on the other hand, exists as CO2 in the atmosphere, and plants fix CO2 via photosynthesis while all organisms release it via respiration, making it a gaseous cycle with an atmospheric reservoir.