- Build food chains and food webs and place organisms in trophic levels.
- Apply the 10% rule to pyramids of energy, number and biomass.
- Trace the carbon, nitrogen, water and phosphorus cycles and name the organisms and processes in each.
- Explain ecological balance, biodiversity and the effect of human disturbance.
🌳 Concept tree
Ecological Balance and the Cycles of Matter
├── Structure of an ecosystem│ ├── Bioticproducers, consumers, decomposers
│ └── Abioticlight, temperature, water, gases, salts
├── Feeding relationships
| Comparison aspect | Main idea | Keywords |
|---|
| │ ├── Grazing chain | plant → herbivore → carnivore | plantherbivorecarnivore |
| │ ├── Detritus chain | dead matter → decomposers → detritivores | deadmatterdecomposersdetritivores |
| • | │ └── Food web = the network of intersecting chains | Foodnetworkintersectingchains |
├── Pyramids│ ├── Energyalways upright, 10% transfer
│ ├── Numbermay be inverted (one tree, many insects)
│ └── Biomassinverted in the ocean (phytoplankton turn over fast)
├── Biogeochemical cycles
| Comparison aspect | Main idea | Keywords |
|---|
| │ ├── Carbon | - photosynthesis
- respiration
- decomposition
- fossil fuels
- ocean
| photosynthesisrespirationdecompositionfossil |
| │ ├── Nitrogen | - fixation (Rhizobium
- Azotobacter
- lightning)
- nitrification
| fixationRhizobiumAzotobacterlightning |
| • | - │ │ assimilation
- ammonification
- denitrification
| assimilationammonificationdenitrification |
| │ ├── Water | - evaporation
- transpiration
- condensation
- run-off
- infiltration
| evaporationtranspirationcondensationinfiltration |
| │ └── Phosphorus | - rock weathering
- no gaseous phase
- sediments
| rockweatheringgaseousphase |
└── Balance and its threats
├── Biodiversity, resistance and resilience
- └── Invasive species
- pollution
- habitat loss
- over-exploitation
| Comparison aspect | Main idea | Keywords |
|---|
| Ecosystem | a functioning unit of interacting living organisms and their non-living environment | functioningunitinteractingliving |
| Habitat | - is where an organism lives
- niche is its role — what it eats
- what eats it
- when and how it lives. Two species cannot keep the same niche indefinitely
| organismlivesnicherole |
| Producers (autotrophs) | photosynthetic or chemosynthetic organisms that build organic matter from inorganic compounds | photosyntheticchemosyntheticorganismsbuild |
| Consumers (heterotrophs) | - herbivores
- primary carnivores and secondary carnivores. Decomposers (bacteria and fungi) break down dead matter externally and absorb it
- detritivores (earthworms
- woodlice) ingest the debris
| herbivoresprimarycarnivoressecondary |
| Trophic level | - the feeding position in a chain — level 1 producers
- level 2 primary consumers
- and so on
| feedingpositionchainproducers |
| Food web | - the network formed when several food chains cross-link
- the more complex the web
- the more stable the ecosystem
| networkformedseveralchains |
| Ecological balance (homeostasis) | the ability of an ecosystem to keep its structure and function through the feedbacks between its components | abilityecosystemkeepstructure |
| Biodiversity | - variety of genes
- species and ecosystems
- it is the main guarantee of resilience
| varietygenesspeciesecosystems |
| Biogeochemical cycle | - the circular path of an element between the living (bio)
- the soil
- water and air (geo) and back again
| circularpathelementbetween |
| Comparison aspect | Main idea | Example | Keywords |
|---|
| Lindeman’s 10% rule (trophic-dynamic efficiency) | - only about 10% of the energy of one level appears as biomass in the next
- the rest is lost as respiration
- heat
- movement and unconsumed or undigested matter
| — | energylevelappearsbiomass |
| Consequence | - food chains rarely exceed four or five levels
- because there is not enough energy left to support another
| — | foodchainsrarelyexceed |
| Example | - if plants fix 10
- 000 kJ
- herbivores store about 1
- 000 kJ
- primary carnivores 100 kJ and top carnivores 10 kJ
| — | plantsherbivoresstoreprimary |
| Primary productivity | - gross (GPP) minus respiration (R) equals net (NPP) — NPP is what the ecosystem actually offers to consumers
- tropical rainforests and coral reefs are the highest
- deserts and open ocean the lowest per unit area
| — | grossminusrespirationequals |
| Nitrogen fixation | - N₂ + 8H+ + 8e− + 16 ATP → 2NH₃ + H₂
- catalysed by nitrogenase in Rhizobium in legume root nodules
- free-living Azotobacter and Clostridium
- and cyanobacteria
| Anabaena | 2NH₃catalysednitrogenaseRhizobium |
| Nitrification | - NH₃ → NO₂− (Nitrosomonas) → NO₃− (Nitrobacter)
- denitrification by Pseudomonas returns N₂ to the air under anaerobic conditions
| — | NitrosomonasNitrobacterdenitrificationPseudomonas |
| Comparison aspect | Main idea | Example | Keywords |
|---|
| A grazing food chain in a pond | - phytoplankton → zooplankton → small fish → large fish → human or osprey. A detritus chain: dead leaves → fungi and bacteria → earthworms → moles → owls. In nature the two run together as a web
- so the loss of one species rarely starves another completely
| — | phytoplanktonzooplanktonsmallfish |
| Pyramid of energy | - the energy entering each level per unit time. It is always upright
- because energy can never increase along the chain
| — | enteringlevelunittime |
| Pyramid of number | - individuals per level
- it can be inverted — one tree supports thousands of insects and a few birds — or partly inverted in a parasitic chain
| — | individualslevelinvertedtree |
| Pyramid of biomass | - the dry mass per level
- usually upright on land but inverted in the sea
- where phytoplankton have a small standing crop yet reproduce so fast that they support a larger mass of zooplankton
| — | levelusuallyuprightland |
| Bioaccumulation travels the same levels | non-biodegradable substances | - DDT or mercury increase in concentration at each step
- so the top predator carries the highest dose (biomagnification)
| biodegradablesubstancesmercuryincrease |
| Comparison aspect | Main idea | Keywords |
|---|
| In | photosynthesis fixes atmospheric CO₂ into carbohydrate — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — and the carbon passes along the food chain | photosynthesisfixesatmosphericcarbohydrate |
| Out | - respiration in all organisms returns CO₂
- decomposers release it from dead matter
- combustion of wood
- coal
- oil and gas returns ancient carbon that had been locked out of the cycle for millions of years
| respirationorganismsreturnsdecomposers |
| The ocean | - is the largest active reservoir: CO₂ dissolves and equilibrates with HCO₃− and CO₃2−
- shell-forming organisms make CaCO₃
- and when they sink the carbon becomes limestone or is returned by upwelling
| largestactivereservoirdissolves |
| • | - Because the ocean absorbs CO₂
- its pH falls — ocean acidification —
- dissolves the shells of corals
- molluscs and plankton
| Becauseoceanabsorbsfalls |
| Terrestrial sinks | - living biomass and soils hold far more carbon than the atmosphere
- so deforestation both releases carbon and removes the sink
| livingbiomasssoilshold |
| Human disturbance | burning fossil fuels and clearing forests move carbon into the air faster than photosynthesis and dissolution can take it back | burningfossilfuelsclearing |
Nitrogen — the bacterial cycle
| Comparison aspect | Main idea | Keywords |
|---|
| • | - Plants can only absorb nitrogen as NO₃− or NH₄+
- never as N₂ gas
- so fixation is indispensable
| Plantsabsorbnitrogennever |
| Sequence | N₂ —fixation→ NH₃/NH₄+ —nitrification→ NO₂− → NO₃− —assimilation→ plant and animal protein —death and excretion— NH₄+ —ammonification→ —denitrification→ N₂ | fixationnitrificationassimilationplant |
| Legumes host Rhizobium in root nodules in a mutualism | - the bacterium gets sugars
- the plant gets amino acids — the basis of crop rotation with clover
- beans and alfalfa
| bacteriumgetssugarsplant |
| Human interference | - the Haber–Bosch process fixes more nitrogen industrially than all natural fixation
- and excess fertiliser runs off into water (eutrophication) or is denitrified to N₂O
- a powerful greenhouse gas
| HaberBoschprocessfixes |
Water
- Evaporation from oceans, transpiration from plants, condensation into clouds, precipitation, then run-off, infiltration into aquifers and return to the sea — driven by solar energy and gravity, so it is a cycle that needs no chemical change of the substance.
Phosphorus — no gaseous phase
- Phosphate is released by the weathering of rocks, taken up by plants, passed through consumers, returned by decomposers, and finally precipitates in sediments on the sea floor; it returns to land only by uplift or by guano and fishing.
- Because there is no air reservoir, phosphorus is the limiting nutrient in many freshwaters, and its addition from detergents triggers algal blooms.
| Comparison aspect | Main idea | Example | Keywords |
|---|
| An ecosystem keeps its balance through negative feedbacks | - prey increase → predators increase → prey decrease → predators decrease. The more species and the more interconnections
- the more ways there are to absorb a shock (resistance) and to recover (resilience)
| — | preyincreasepredatorsdecrease |
| Keystone species | - hold a community together out of proportion to their numbers
- removing them changes the whole structure
| — | holdcommunitytogetherproportion |
| Succession | - is the orderly replacement of communities after a disturbance — lichens and mosses
- then grasses
- shrubs and finally forest on bare rock (primary succession)
- or re-growth on existing soil after a fire (secondary)
| — | orderlyreplacementcommunitiesafter |
| Threats | - habitat loss and fragmentation
- over-exploitation and over-fishing
- pollution of water air and soil
- invasive alien species that find no natural enemies
- and climate change that shifts the ranges of species faster than they can adapt
| — | habitatlossfragmentationover |
| Egypt’s measures | protected areas | - Ras Mohammed and Wadi El-Gamal
- breeding and re-introduction programmes
- the national biodiversity strategy
- fishing and hunting regulation
- afforestation belts (the “Green Wall”) against desertification
- and wetland restoration in the north
| protectedareasMohammedWadi |
- 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. — Rhizobium fixes N₂ in legume nodules; Nitrosomonas oxidises ammonia to nitrite (nitrification); Pseudomonas reduces 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.
| Comparison aspect | Main idea | Keywords |
|---|
| Decomposers are not consumers in the trophic pyramid sense | - they feed on all levels
- state that they recycle matter
- not that they “eat the producers”
| feedlevelsstaterecycle |
| Matter cycles, energy flows. | - Energy enters as light and leaves as heat and is never recycled
- only the elements return. This is the single most-tested distinction in the topic
| enterslightleavesheat |
| 10% is an average | answers must use it only to estimate and must state that the loss is respiration and heat | answersmustestimatestate |
| Nitrification is aerobic, denitrification anaerobic | waterlogged soil loses nitrogen for that reason | waterloggedsoillosesnitrogen |
| Plants absorb nitrates and ammonium, not atmospheric nitrogen | however abundant it is | howeverabundant |
| Phosphorus has no gaseous phase | do not draw an air reservoir in its cycle | drawreservoircycle |
| Inverted pyramids | - number and biomass may invert
- energy never
| numberbiomassenergynever |
| Food web ≠ food chain | the web is the network of many chains and it is what gives stability | networkmanychainswhat |
