Ecosystem processes — A/L Biology

What must I remember about Ecosystem processes?

• The two foundational processes sustaining all ecosystems are unidirectional Energy Flow (governed by the laws of thermodynamics) and cyclical Biogeochemical Nutrient Cycling.
• Primary Productivity: Gross Primary Productivity (GPP) is the total rate of organic energy fixed by autotrophic photosynthesis per unit area per unit time; Net Primary Productivity (NPP) is the energy remaining after autotrophic cellular respiration: NPP = GPP - Ra.
• Trophic Efficiency and the 10% Ecological Rule: On average, only ~10% (range 5-20%) of the chemical energy available at one trophic level is transferred into biomass at the subsequent trophic level; ~90% is lost as metabolic heat (cellular respiration), unconsumed biomass, and feces.
• Ecological Pyramids represent trophic architecture: Pyramid of Numbers (can be upright or inverted in parasitic/tree ecosystems), Pyramid of Biomass (usually upright, inverted in marine plankton systems where phytoplankton have rapid turnover), and Pyramid of Energy (always upright, universally conforming to second law of thermodynamics).
• Biogeochemical cycles circulate chemical elements between biotic and abiotic reservoirs: Carbon cycle (photosynthetic carbon fixation, respiration, fossil fuel combustion), Nitrogen cycle (nitrogen fixation by Rhizobium/Azotobacter, nitrification by Nitrosomonas/Nitrobacter, assimilation, ammonification, denitrification by Pseudomonas), and Water cycle.

Test yourself (1 minute)

1. Why Energy Pyramids are NEVER inverted

   Energy is continually degraded and lost as entropic heat at each trophic transfer in accordance with the Second Law of Thermodynamics; energy cannot be created at higher levels.

2. Bioamplification / Biomagnification

   Fat-soluble, non-biodegradable toxins (e.g., DDT, methylmercury, microplastics) concentrate progressively at higher trophic levels because consumers eat large quantities of contaminated lower-level biomass without metabolizing or excreting the toxin.

3. Nitrogen transformation stoichiometry

   Nitrosomonas oxidizes ammonia (NH3 / NH4+) to nitrite (NO2-); Nitrobacter oxidizes nitrite (NO2-) to nitrate (NO3-), the primary form absorbed by plant root transporters.

Practise Ecosystem processes questions today

Source: NIE G.C.E. (A/L) Biology Syllabus & Resource Book, Unit 6

What should I study first?

What comes next?

Next in the syllabus

Where does Ecosystem processes fit in the syllabus?

Year 2 · Term 2