Ecosystems, Ecological Interactions & Biogeochemical Cycles
Ecosystem components (biotic and abiotic factors), trophic levels, food chains, food webs, ecological pyramids, symbiotic relationships (mutualism, commensalism, parasitism), and nutrient cycles (carbon and nitrogen cycles).
In the Kakum National Park in the Central Region, the tropical rainforest canopy food web links kapok trees, fruit bats, monkeys, and crowned eagles. Disrupting canopy trees threatens the survival of interdependent pollinators and seed dispersers.
Key Syllabus Learning Objectives
- 1Differentiate between abiotic (temperature, light, pH) and biotic ecological factors
- 2Construct and interpret food chains, food webs, and pyramids of numbers, biomass, and energy
- 3Explain the 10% energy transfer rule and why food chains rarely exceed four or five trophic levels
- 4Describe the nitrogen and carbon cycles and explain how human activities (bush burning, deforestation) disturb ecological balance
Energy Flow & Ecological Pyramids
- Food Chain: Linear feeding sequence: Guinea grass → Grasshopper → Toad → Black kite.
- Food Web: Complex network of interconnected food chains reflecting natural feeding versatility.
- Why Pyramids of Energy are always upright: Energy cannot be recycled; at every transfer step, roughly 90% is lost via cellular respiration, excretion, and movement.
- Limitation on trophic levels: By the 4th or 5th trophic level, remaining energy is insufficient to support a viable population of top predators.
In Lake Volta, microscopic phytoplankton are consumed by zooplankton, eaten by small cichlid fish (tilapia fry), which are preyed upon by electric catfish and Nile perch.
Energy flows unidirectionally and diminishes by ~90% at each successive trophic transfer; nutrients cycle continuously.
The Nitrogen Cycle & Agricultural Productivity
- Biological Fixation: Symbiotic Rhizobium in root nodules of leguminous crops (cowpeas, groundnuts, soybeans) convert N₂ into ammonium ions.
- Nitrification: Soil bacteria oxidize ammonia: Nitrosomonas converts NH₄⁺ to nitrites (NO₂⁻); Nitrobacter converts NO₂⁻ to nitrates (NO₃⁻), which plants absorb.
- Denitrification: In waterlogged, anaerobic soils, denitrifying bacteria convert nitrates back into N₂ gas, reducing soil fertility.
Farmers in the Northern Region intercrop maize with groundnuts to naturally replenish soil nitrates without expensive synthetic chemical fertilizers.
Nitrifying bacteria require aerobic oxygenated soils to produce soluble nitrates for plant root uptake.
- In food chains, arrowheads must point from the organism being eaten to the consumer that eats it (indicating direction of energy flow).
- State clearly that the pyramid of energy can NEVER be inverted, unlike pyramids of numbers which can be inverted (e.g. one huge oak tree supporting thousands of caterpillars).
- Name both nitrifying bacteria correctly: Nitrosomonas converts ammonia to nitrite; Nitrobacter converts nitrite to nitrate.
- Drawing food chain arrows backwards (pointing to food instead of consumer).
- Assuming nitrogen gas can be directly absorbed through plant stomata (plants only take up nitrates/ammonium through roots).
- Confusing commensalism with mutualism.
- Can I construct a 4-level food chain from a terrestrial grassland habitat?
- Can I explain why energy does not cycle in an ecosystem whereas carbon and nitrogen do?
- Can I outline the 4 main stages of the nitrogen cycle?
- Can I provide an example of mutualism found in Ghanaian agriculture?
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