Coaching Centre
Ecosystems review
A summary of the whole Ecosystems chapter, key words, and a mixed test covering the carbon and nitrogen cycles, photosynthesis, respiration, relationships between organisms, measuring and changing populations, and caring for Country.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 3A Carbon cycle | Energy flows (lost as heat); matter cycles. Carbon stores: air, living things, soil, oceans, fossil fuels, limestone. Photosynthesis removes CO₂; respiration, decomposition and combustion return it. CO₂ has risen from about 280 ppm before 1800 to about 425 ppm, enhancing the greenhouse effect and acidifying oceans. Blue carbon in mangroves and seagrass. |
| 3B Nitrogen cycle | N₂ is 78% of air but unusable by most organisms. Fixation (bacteria in legume nodules, free-living bacteria, lightning, Haber process) → ammonium; nitrification → nitrate; plants absorb nitrate; decomposers return ammonium; denitrifying bacteria return N₂. Excess fertiliser → leaching, eutrophication, N₂O. Crop rotation with legumes. |
| 3C Leaves, light and water | Chlorophyll absorbs red and blue, reflects green; carotenoids; absorption and action spectra; anthocyanins in red new growth. Stomata and guard cells: CO₂ in, water out (transpiration), cooling. Adaptations: large thin leaves and drip tips in shade; hanging, waxy, sclerophyll eucalypt leaves; rolled spinifex leaves with hidden stomata; sunken stomata. C4 plants concentrate CO₂; CAM plants open stomata at night. |
| 3D Respiration | Glucose + oxygen → carbon dioxide + water + energy, in stages ending in the mitochondria; energy stored in ATP. Anaerobic: lactic acid in muscles; alcohol + CO₂ in yeast. Net gas exchange; the compensation point (photosynthesis = respiration); shade plants have low compensation points. Limewater and hydrogencarbonate indicator test for CO₂. |
| 3E Relationships | Within species: cooperation, mating, competition. Between species: mutualism (+/+), commensalism (+/0), parasitism (+/−), predation (+/−), competition (−/−). Symbiosis = close long-term relationship. Allelopathy (penicillin). Predator–prey cycles; predator peaks lag behind prey peaks. |
| 3F Measuring populations | Population, community, density, distribution, biodiversity. Random sampling. Quadrats: mean per quadrat × area. Transects for changing conditions. Capture–recapture: N = first catch × second catch ÷ marked recaptured. Camera traps, drones, eDNA, citizen science. |
| 3G Changing populations | Births + immigration − deaths − emigration. Growth curve: slow, rapid, levels off at the carrying capacity. Biotic and abiotic limiting factors. Boom and bust: the 2021 mouse plague (carrying capacity rises after rain, overshoot, crash). Devil facial tumour disease, Murray–Darling fish kills, introduced species. |
| 3H Caring for Country | Sustainability. Brewarrina fish traps (Ngemba, Baiame's Ngunnhu), Budj Bim eel traps. Anangu habitats (puli, puti, pila) and five seasons. Buffel grass. Early dry season savanna burning cuts late wildfires, methane and nitrous oxide; carbon credits fund rangers. Protected areas, catch limits, pest control, habitat restoration, two-way science. |
Putting it together
The whole chapter runs on one idea: energy flows through an ecosystem once, but matter goes round and round. Photosynthesis captures the Sun's energy and pulls carbon out of the air; respiration releases that energy and returns the carbon. Bacteria keep nitrogen moving between air, soil and living things. The amount of matter and energy available sets how many organisms an ecosystem can support, which is why populations level off at a carrying capacity, and why predators, disease, fire, drought and introduced species can push them up or down. Sustainable management, whether through fish trap rules or national parks, means keeping those cycles and populations working so the ecosystem can keep providing for the future.
Key words
abiotic · action spectrum · aerobic respiration · allelopathy · anaerobic respiration · ATP · biodiversity · biotic · blue carbon · C4 plant · CAM plant · capture–recapture · carrying capacity · carotenoid · chlorophyll · chloroplast · commensalism · community · compensation point · Country · decomposition · denitrification · eutrophication · fossil fuel · interspecific · intraspecific · legume · limiting factor · mitochondria · mutualism · nitrification · nitrogen fixation · parasitism · photosynthesis · population · population density · predation · quadrat · sampling · stomata · sustainability · symbiosis · transect · transpiration
Test yourself
Which statement about ecosystems is correct?
Atoms cycle; energy flows through.
Which process converts nitrate back into nitrogen gas?
Denitrifying bacteria return N₂ to the air.
A plant in a sealed jar in bright light will
In bright light photosynthesis is faster than respiration.
Mistletoe growing on a eucalypt is an example of
The mistletoe takes water and minerals from the tree, harming it.
30 frogs are marked and released. Later 20 are caught and 4 are marked. The estimated population is
30 × 20 ÷ 4 = 150.
A population levels off at the carrying capacity because
Shortages raise deaths and lower births until they balance.
Spinifex has rolled leaves with its stomata in hairy grooves on the inside. This mainly helps it to
Still, moist air trapped in the grooves slows evaporation.
Name the processes in the carbon cycle.
Which cycle does each process belong to?
Put the stages of a mouse plague in order.
Water vapour escapes from leaves through the , and respiration is completed in the . Legumes have bacteria in root that fix nitrogen. A relationship where both species benefit is . Excess nutrients in water cause . Using resources so that enough remains for the future is .
A wetland next to farmland has been partly cleared and drained, and fertiliser runs into it. Explain how these changes could affect the carbon cycle, the nitrogen cycle and the populations living in the wetland, and propose a sustainable management plan.
Carbon cycle. Wetland plants and waterlogged soils store a lot of carbon. Clearing the plants reduces photosynthesis, so less CO₂ is removed from the air. Draining lets oxygen into the soil, so decomposers break down the stored organic matter faster and release it as CO₂. A carbon store becomes a carbon source.
Nitrogen cycle. Fertiliser nitrate washes into the remaining water. Algae and cyanobacteria bloom, then die; decomposers use up the dissolved oxygen as they break them down, so fish, yabbies and insect larvae suffocate. Some nitrate may be converted by bacteria into nitrous oxide, a greenhouse gas.
Populations. Losing habitat lowers the carrying capacity: frogs and waterbirds lose breeding sites and food, so their populations fall, and predators that depend on them decline too. Disturbed ground can be invaded by weeds and introduced species such as carp.
Plan. Leave a strip of native vegetation along the wetland edge to absorb nitrate before it reaches the water; apply fertiliser in smaller amounts when crops need it and not before heavy rain; restore natural water flows and replant wetland plants so it stores carbon again; control carp and weeds; and monitor recovery with quadrats, capture–recapture of frogs and eDNA sampling, working with the local Traditional Owners whose knowledge of the wetland's seasons can guide the timing of the work.