Unit 1 Concept of an ecosystem
Key unit competence: Evaluate biogeochemical cycles and flow of energy among components within the ecosystem.
Key ideas
- An ecosystem is a community of organisms interacting with each other and with their abiotic (non-living) environment in a given area. Terrestrial ecosystems include forests (Nyungwe), savannah (Akagera), grasslands and deserts; aquatic ecosystems include freshwater (lakes Kivu and Muhazi, rivers, wetlands such as Rugezi) and marine (oceans, coral reefs). Aquatic zones: littoral (shallow, rooted plants), limnetic (open, lit water with plankton) and profundal (deep, dark).
- Ecological factors: abiotic — light, temperature, water, humidity, wind, soil (edaphic: pH, texture, minerals), topography; biotic — competition (intra- and interspecific), predation, parasitism, mutualism, commensalism, disease, and human activity.
- Energy flow: the Sun is the source; producers (autotrophs) fix light energy by photosynthesis; primary, secondary and tertiary consumers feed at higher trophic levels; decomposers (bacteria, fungi) and detritivores (earthworms) recycle dead matter. A food chain shows one feeding pathway; a food web shows interconnected chains. Energy flows one way and is lost at each level as heat (respiration), in faeces and urine, and in uneaten parts, so only about 10% passes to the next level; food chains rarely exceed 4–5 links.
- Productivity: gross primary productivity (GPP) is the total energy fixed by producers; net primary productivity (NPP) = GPP − energy lost in plant respiration (R); NPP is available to consumers. Efficiency of transfer = energy at higher level ÷ energy at lower level × 100%.
- Ecological pyramids: numbers (may be inverted, e.g. one tree with many insects, or parasites), biomass (dry mass per unit area; may be inverted in aquatic systems where phytoplankton reproduce fast), energy (energy per unit area per year; never inverted, the most accurate). Limitations: numbers ignore size; biomass is a snapshot and ignores turnover; energy data are hard to collect; organisms may feed at more than one level.
- Bioaccumulation is the build-up of a persistent substance in an organism faster than it is broken down or excreted. Biomagnification is the increase in concentration of such a substance at each higher trophic level. Examples: DDT (thin eggshells in birds of prey), mercury (fish, Minamata disease), lead, PCBs, some pesticides. Such chemicals are fat-soluble, not broken down, and stored in fatty tissue. Prevention: ban persistent pesticides, integrated pest management, treat industrial waste, monitor fish.
- Water cycle: evaporation, transpiration, condensation, precipitation, run-off, infiltration. Carbon cycle: CO₂ fixed by photosynthesis; returned by respiration, decomposition and combustion of fossil fuels and wood; stored in fossil fuels, carbonate rocks and oceans. Phosphorus cycle: has no atmospheric phase; weathering of rocks releases phosphate, taken up by plants, passed along food chains, returned by decomposition; lost to sediments.
- Nitrogen cycle: nitrogen fixation (N₂ → NH₃) by free-living bacteria (Azotobacter), symbiotic Rhizobium in legume root nodules, lightning and the Haber process; ammonification by decomposers (proteins → NH₄⁺); nitrification by Nitrosomonas (NH₄⁺ → NO₂⁻) and Nitrobacter (NO₂⁻ → NO₃⁻) in aerated soil; denitrification (NO₃⁻ → N₂) by anaerobic bacteria such as Pseudomonas in waterlogged soil. Plants absorb nitrates to make amino acids and nucleotides.
- Humans alter cycles: burning fossil fuels and deforestation raise atmospheric CO₂; fertilisers add nitrates and phosphates that cause eutrophication (algal bloom, decomposers use O₂, fish die). The greenhouse effect: CO₂, methane, water vapour and nitrous oxide absorb infrared radiation re-emitted by Earth, keeping it warm; extra gases cause global warming and climate change.
- Human impacts: large-scale monoculture (loss of biodiversity, pests spread, soil exhaustion), intensive livestock (methane, waste, antibiotic use, overgrazing), deforestation, habitat destruction, pollution of air (SO₂ → acid rain), water (sewage, fertilisers, oil) and land (plastics, mining). Sustainable agriculture: crop rotation, agroforestry, terracing, mulching, organic manure, integrated pest management, conservation of wetlands.
Common misconceptions
- "Energy is recycled in an ecosystem." No: nutrients are recycled; energy flows one way and is lost as heat.
- "A pyramid of energy can be inverted." No: energy always decreases at each level, so it is always upright.
- "Nitrifying bacteria fix nitrogen gas." No: nitrifying bacteria oxidise ammonium to nitrate; nitrogen-fixing bacteria convert N₂ to ammonia.
- "The greenhouse effect is harmful in itself." No: it keeps Earth habitable; the problem is the enhanced greenhouse effect from extra gases.
Worked examples
Example 1. Grass fixes 20 000 kJ m⁻² y⁻¹. Grasshoppers receive 2 000 kJ m⁻² y⁻¹ and frogs 180 kJ m⁻² y⁻¹. Calculate the transfer efficiencies.
Solution
Grass → grasshopper: 2 000 ÷ 20 000 × 100 = 10%. Grasshopper → frog: 180 ÷ 2 000 × 100 = 9%. Energy is lost as heat in respiration, in faeces and in parts not eaten.
Example 2. DDT in water is 0.00005 ppm, in zooplankton 0.04 ppm and in fish-eating birds 25 ppm. By what factor has DDT increased from water to birds? Explain.
Solution
25 ÷ 0.00005 = 500 000 times. DDT is fat-soluble and not broken down; each consumer eats many organisms from the level below and stores the DDT, so it is biomagnified.
Practice questions
Distinguish between biotic and abiotic factors with two examples each.
Show answer
Biotic: living factors, e.g. predation, competition, disease. Abiotic: non-living, e.g. temperature, light, rainfall, soil pH.
Common mistake: Calling soil organisms abiotic.
Explain why energy transfer between trophic levels is inefficient.
Show answer
Energy lost as heat in respiration; not all parts eaten (bones, roots); not all digested (faeces); excretion (urine); energy used for movement.
Common mistake: Saying energy is destroyed.
Compare pyramids of numbers, biomass and energy.
Show answer
Numbers: counts individuals, can be inverted, ignores size. Biomass: dry mass, can be inverted in water, snapshot. Energy: energy flow over time, never inverted, most accurate but hard to measure.
Common mistake: Saying all pyramids are always upright.
Explain biomagnification using DDT as an example.
Show answer
DDT sprayed enters water and plankton; fat-soluble and persistent; stored in tissues; each consumer eats many prey; concentration increases at each level; top predators get toxic doses, e.g. thin eggshells.
Common mistake: Saying DDT is broken down by birds.
Describe the nitrogen cycle.
Show answer
Fixation of N₂ to ammonia by Rhizobium, Azotobacter and lightning; plants absorb nitrates to make proteins; animals eat plants; decomposers ammonify dead matter; Nitrosomonas and Nitrobacter nitrify ammonium to nitrate; denitrifying bacteria return N₂.
Common mistake: Saying plants absorb N₂ gas directly.
Describe the carbon cycle.
Show answer
CO₂ fixed by photosynthesis into organic compounds; passed along food chains; returned by respiration of plants, animals and decomposers; combustion of fossil fuels; stored in fossil fuels, oceans and carbonate rocks.
Common mistake: Leaving out decomposers.
Explain the enhanced greenhouse effect and two consequences.
Show answer
Extra CO₂, methane and nitrous oxide from human activities trap more infrared radiation; global temperature rises; consequences: melting ice and rising sea level, droughts and floods, changes in crop yields, spread of disease vectors.
Common mistake: Confusing it with ozone depletion.
Suggest four sustainable agricultural practices and explain one.
Show answer
Crop rotation with legumes; agroforestry; terracing; mulching; organic manure; IPM. E.g. terracing reduces run-off and soil erosion on hillsides.
Common mistake: Suggesting more pesticides.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The bacteria in legume root nodules are: a. freshwater ecosystem 2. Denitrifying bacteria convert nitrate to: b. methane 3. A gas that adds to the greenhouse effect is: c. nitrogen gas 4. Lake Kivu is an example of a: d. Rhizobium Show answer
1 → d (Rhizobium) · 2 → c (nitrogen gas) · 3 → b (methane) · 4 → a (freshwater ecosystem)
- They fix nitrogen.
- In waterlogged soil.
- From cattle and rice fields.
- Inland water.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
inverted at the base · respiration and combustion · 8% · 12.5%- Carbon returns to the air by .
- Energy in herbivores is 1 500 kJ and in carnivores 120 kJ. The efficiency is .
- A pyramid of numbers for one tree with many caterpillars is .
Show answer
1: respiration and combustion · 2: 8% · 3: inverted at the base
- CO₂ is released.
- 120 ÷ 1 500 × 100.
- One large producer.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Earthworms that feed on dead leaves are: a. detritivores 2. Growing one crop over a large area is: b. energy 3. The pyramid that is never inverted is the pyramid of: c. monoculture 4. About how much energy passes to the next trophic level? d. 10% Show answer
1 → a (detritivores) · 2 → c (monoculture) · 3 → b (energy) · 4 → d (10%)
- They speed up decay.
- Reduces biodiversity.
- Energy always decreases.
- Most is lost as heat.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
12 000 kJ m⁻² y⁻¹ · eutrophication · faster flight · thin eggshells- Algal blooms caused by excess fertilisers are part of .
- DDT harmed birds of prey by causing .
- A plant has GPP 30 000 kJ m⁻² y⁻¹ and loses 18 000 in respiration. Its NPP is .
Show answer
1: eutrophication · 2: thin eggshells · 3: 12 000 kJ m⁻² y⁻¹
- Oxygen falls and fish die.
- Eggs broke easily.
- GPP − R.