Point 7 of 7 — Ecological Balance in Earth Systems, Biogeochemical Cycles and Food WebsQuestion bank: 0 questions (0 with pictures)
07Worked examples
A field of wheat fixes 20,000 kJ per square metre per year. Estimate the energy available to rats and then to owls. — Rats ≈ 2,000 kJ, owls ≈200 kJ; the owl therefore needs a large territory for its food.
Why is a pyramid of energy never inverted? — Because each transfer loses at least 90% as respiration and heat; a level can never store more energy than entered it.
Name two bacteria and the process each performs in the nitrogen cycle. — Rhizobiumfixes N₂ in legume nodules; Nitrosomonasoxidises ammonia to nitrite (nitrification); Pseudomonasreduces nitrate to N₂ (denitrification).
A lake receives phosphate-rich detergent effluent and later the fish die. Explain the steps. — Phosphate is the limiting nutrient → algal bloom → algae shade and die → bacteria decompose the biomass → BOD rises and DO falls → fish and invertebrates suffocate.
An island lake has one species of fish and a new predatory fish is introduced. Predict and justify. — The native fish, with no evolved defence and no refuges, is likely driven to extinction; the web simplifies and the ecosystem loses resilience — this is the standard invasive-species argument.
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