Ecology: real-world examples
Named cases you can use in an answer, each under the syllabus point it illustrates and linked to the lesson it comes from.
2 Ecology
2.1 Individuals, populations, communities, and ecosystems
2.1.2An individual organism is a member of a species.
A zebra and a horse can interbreed to produce a zebroid, but the zebroid is sterile and cannot itself reproduce, so zebra and horse are not the same species under the fertile-offspring definition.
Demonstrates the biological species concept by showing that producing offspring is insufficient: fertility of the offspring is the decisive criterion for species membership.
2.1.3Classification of organisms allows for efficient identification and prediction of characteristics.
Carl Linnaeus (Swedish botanist, b. 1707) devised binomial nomenclature and the taxa kingdom–phylum–class–order–family–genus–species in Systema Naturae, cataloguing roughly 7,700 plants and 4,400 animals using morphology-based homologous traits.
Shows how a standardised hierarchical classification system enables efficient identification and prediction of shared characteristics across the ~2 million known species.
Uppsala University, Sweden IndividualsThe domestic cat is classified Animalia > Chordata > Mammalia > Carnivora > Felidae > Felis > catus, with each rank defined by predictive traits (vertebral column, lactation and three ear bones, retractable claws), giving the binomial Felis catus.
Demonstrates how following an organism down the taxonomic hierarchy allows prediction of characteristics and reveals it is more closely related to a cow than a chicken.
2.1.4Taxonomists use a variety of tools to identify an organism.
Modern taxonomists use genetic testing (e.g. RNA polymerase similarity showing Archaea are more closely related to Eukarya than to Bacteria) alongside Linnaeus's morphology-based methods, and this molecular evidence largely agrees with classifications based on structure and form.
Shows the variety of tools taxonomists deploy, contrasting traditional morphological keys with genetic analysis that resolved the three-domain system.
2.1.5A population is a group of organisms of the same species living in the same area at the same time, and which are capable of interbreeding.
Humpback whales are divided into 14 distinct population segments worldwide, of which four are classified as endangered, illustrating spatially separated groups of the same species.
Demonstrates that a species can comprise multiple distinct populations defined by area and interbreeding.
2.1.8A niche describes the particular set of abiotic and biotic conditions and resources upon which an organism or a population depends.
In European deciduous forests the Great Spotted Woodpecker (Dendrocopos major) deep-excavates wood-boring beetle larvae from main trunks (2–10 cm depth, foraging upward), while the Eurasian Nuthatch (Sitta europaea) gleans the surface (0–2 cm) foraging headfirst downward and caches seeds, partitioning the same trees to reduce interspecific competition.
Shows how two species occupy distinct niches through different foraging depths, techniques and diets despite sharing habitat.
2.1.9Populations interact in ecosystems by herbivory, predation, parasitism, mutualism, disease and competition, with ecological, behavioural and evolutionary consequences.
In British deciduous forests, introduced grey squirrels outcompete native red squirrels, leading to competitive exclusion and local extinction of red squirrels, with niche partitioning only where coexistence occurs.
Shows interspecific competition producing competitive exclusion and evolutionary/behavioural consequences like niche differentiation.
United Kingdom PopulationsWhite-nose syndrome, a fungal disease (*Pseudogymnoascus destructans*), has caused population crashes of over 90% mortality in North American bat populations, driving some colonies to local extinction and disrupting ecosystem services such as insect control.
Demonstrates disease as a population interaction with dramatic density-dependent mortality and cascading ecological consequences.
North America Populations
2.1.10Carrying capacity is the maximum size of a population determined by competition for limited resources.
On St. Matthew Island the reindeer population overshot carrying capacity because a 2–3 year time lag masked lichen depletion; overgrazing reduced future carrying capacity so slowly-regenerating lichen could never again support even a small herd.
Shows how competition for a single limiting resource (lichen) sets carrying capacity and how overshoot permanently degrades it.
St. Matthew Island, Alaska, USA Population Size
2.1.11Population size is regulated by density-dependent factors and negative feedback mechanisms.
On St. Matthew Island the 1963–64 crash was driven by density-dependent starvation: at peak density (18.1 reindeer per km²) the depleted lichen could not feed the herd, causing mass die-off evidenced by bone marrow depletion and declining body weights.
Demonstrates a density-dependent factor (food scarcity intensifying with density) driving negative feedback on population size.
St. Matthew Island, Alaska, USA Population Size
2.1.12Population growth can either be exponential or limited by carrying capacity.
In 1944 the US Coast Guard introduced 29 reindeer to St. Matthew Island (332 km²) in the Bering Sea, Alaska; with abundant lichen and no predators the herd irrupted to about 6,000 by 1963, overgrazed the lichen mat from 100 mm to under 25 mm, then crashed to just 42 animals after the severe winter of 1963–64 and went extinct by the 1980s.
Demonstrates a J-curve exponential growth followed by overshoot and crash rather than an S-curve stabilising at carrying capacity.
St. Matthew Island, Alaska, USA Population Size
2.1.15Population abundance can be estimated using random sampling, systematic sampling or transect sampling.
A transect laid at 1 m intervals away from a field gate showed daisy density per m² rising with distance from the gate, attributed to reduced trampling (a biotic factor) further from the entrance.
Demonstrates transect sampling revealing how abundance changes along an environmental gradient caused by a specific pressure.
2.1.16Random quadrat sampling can be used to estimate population size for non-mobile organisms.
Dr Acton measured a field as ~620 m², used a random number generator to place a 1 m² quadrat at 10 coordinates, counted Bellis perennis (daisies) to find a mean of 7 per m², then multiplied by 620 to estimate a total population of 4,340 daisies.
Shows random quadrat sampling used to estimate the population size of a non-mobile organism by scaling mean density up to total area.
2.1.17Capture–mark–release–recapture and the Lincoln index can be used to estimate population size for mobile organisms.
In an ant colony study, researchers vacuumed and counted 300 surface foragers, marked them with fluorescent printer's ink diluted in ether, then 24 hours later recaptured 300 ants of which ~150 (50%) were marked, giving a Lincoln index estimate of (300×300)/150 ≈ 600 foragers.
Demonstrates capture-mark-release-recapture and the Lincoln index applied to a real mobile-organism population, showing both the marking method and the (M×N)/R calculation.
2.1.21Sustainability is a natural property of ecosystems.
StudyA 2024 PLOS Climate study of California kelp (1910–2016) found central California, where ~3,000 southern sea otters survived, expanded its kelp canopy and withstood the same disease, heatwave and El Niño that collapsed northern California, with otter density the strongest predictor of kelp change.
Demonstrates that intact keystone predation and functional redundancy confer the resilience that sustains an ecosystem through disturbance.
Central California coast, USA Communities and ecosystems
2.1.22Human activity can lead to tipping points in ecosystem stability.
Northern California's kelp forests lost over 95% of canopy across ~350 km of coast (2013–2025) after sea star wasting disease killed sunflower sea stars and the 2014–2016 'blob' marine heatwave and El Niño raised sea temperatures 2–3°C, letting purple urchins explode with no otters (extirpated in the 1800s fur trade) to check them.
Shows how loss of a keystone predator plus climatic stress pushed a resilient system past a tipping point into a near-irreversible urchin-barren state.
Mendocino County, California, USA Communities and ecosystems
2.1.23Keystone species have a role in the sustainability of ecosystems.
Sea otters are a keystone species in California's bull kelp forests: a single otter eats ~25% of its body weight (up to 15 lb) daily in sea urchins, and this top-down control of urchin grazers allows kelp forests supporting over 1,000 species to persist rather than collapse into urchin barrens.
Demonstrates how a species of low abundance exerts disproportionately large control over ecosystem structure and biodiversity via a trophic cascade.
Monterey Bay, California, USA Communities and ecosystems
2.1.24The planetary boundaries model indicates that changes to biosphere integrity have passed a critical threshold.
StatisticThe planetary boundaries model shows biosphere integrity has crossed a critical threshold, with current extinction rates running at 100–1000 times the natural background rate.
Quantifies the degree to which the biosphere-integrity boundary has been transgressed in the planetary boundaries framework.
2.1.25To avoid critical tipping points, loss of biosphere integrity needs to be reversed.
The River Otter Beaver Trial in Devon, England found that reintroducing beavers raised plant species richness by 28% and bird species by 26%, and tripled fish populations in some areas, while Scottish beaver populations now support over 30 specialist wetland plant species.
Shows how restoring a keystone ecosystem engineer reverses biodiversity loss and rebuilds biosphere integrity across multiple niches.
River Otter, Devon, England Communities and ecosystems
2.1.26There are advantages of using a method of classification that illustrates evolutionary relationships in a clade.HL
Cladistic analysis places crocodiles closer to birds than to lizards or snakes, uniting them as archosaurs sharing four-chambered hearts, nest-building and vocalisation, and reveals the traditional class 'Reptilia' to be paraphyletic because it excludes birds.
Shows how clade-based classification uncovers true evolutionary relationships and corrects misleading paraphyletic groupings.
2.1.27There are difficulties in classifying organisms into the traditional hierarchy of taxa.HL
The giant panda's 'thumb' looks like a human thumb but is actually a modified wrist (sesamoid) bone that evolved independently, and polar and brown bears interbreed to produce fertile 'pizzly'/'grolar' hybrids, showing gene flow across supposedly distinct species.
Illustrates how analogous traits and interbreeding make discrete traditional taxonomic boundaries difficult to define.
2.1.28The niche of a species can be defined as fundamental or realized.HL
Joseph Connell's 1961 study on Scottish rocky shores showed that Chthamalus stellatus has a fundamental niche spanning the upper and middle intertidal zones, but is competitively excluded from the middle zone by Semibalanus balanoides, restricting its realised niche to the upper zone; when Connell experimentally removed Semibalanus, Chthamalus colonised the middle zone.
Demonstrates experimentally that a species' realised niche is narrower than its fundamental niche due to competitive exclusion.
2.1.30Knowledge of species’ classifications, niche requirements and life cycles help us to understand the extent of human impacts upon them.HL
In the Netherlands, winter moth (Operophtera brumata) caterpillars evolved to hatch as oak buds burst, and great tits (Parus major) time breeding to peak caterpillar abundance; warming springs advance bud burst while great tit cues (day length) stay fixed, creating a phenological mismatch that reduces chick survival.
Shows how knowledge of synchronised life cycles reveals climate-change-driven phenological mismatch cascading through trophic levels.
Pied flycatchers (Ficedula hypoleuca) leave West African wintering grounds on a photoperiod cue that has not changed, yet spring caterpillar peaks in European breeding woodlands have shifted earlier with warming, so birds arrive too late for peak food, causing population declines across parts of Europe.
Demonstrates that species locked into fixed photoperiod cues cannot adjust migration timing to human-caused warming, exposing them to mismatch impacts.
2.2 Energy and biomass in ecosystems
2.2.1Ecosystems are sustained by supplies of energy and matter.
Coral reef ecosystems are sustained by dual inputs: solar energy captured by photosynthesising zooxanthellae living inside coral polyps, and matter delivered by ocean currents (dissolved nitrates, phosphates and minerals for skeleton formation), supporting biodiversity from plankton up to apex reef sharks. Blocking the light or halting the nutrient flow degrades the whole system.
Shows how continuous, separate supplies of both energy and matter are jointly required to sustain a real ecosystem.
Great Barrier Reef, Australia EnergyOn Bear Island (Bjørnøya), Norway, at 74°N, Charles Elton conducted one of the first systematic food-web studies in 1921, mapping an Arctic community that links terrestrial, freshwater and marine subsystems, for example phytoplankton → zooplankton → Arctic cod → ringed seal → polar bear.
Shows how an ecosystem is sustained by energy and matter flowing through interconnected trophic relationships across sub-systems.
Bear Island (Bjørnøya), Norway Biomass and food websThe California kelp forest is sustained by giant kelp (Macrocystis pyrifera) with a net primary productivity of ~5,000 g dry mass m⁻² yr⁻¹, one of the highest on Earth, which feeds urchins, otters and detritus-based food chains alike.
Shows how a supply of energy captured by producers underpins an entire ecosystem and all its consumer pathways.
California kelp forest, USA Energy flows
2.2.6Cellular respiration releases energy from glucose by converting it into a chemical form that can easily be used in carrying out active processes within living cells.
A sprinting cheetah's muscle cells rapidly break down glucose via cellular respiration, converting stored chemical energy into ATP to power the active process of muscle contraction during the chase.
Shows respiration converting glucose into an immediately usable chemical form (ATP) to drive an active life process.
2.2.7Some of the chemical energy released during cellular respiration is transformed into heat.
In cellular respiration only about 38–40% of glucose's chemical energy is captured as usable ATP while the remaining ~60–62% is transformed into heat, which is why an exercising human gets hot and loses roughly 2,000–2,500 kcal of heat energy daily that must be replaced by eating.
Quantifies the fraction of respiratory energy converted irreversibly to heat and links it to daily food requirements.
Southern sea otters (Enhydra lutris nereis) lack blubber and have a field metabolic rate of ~687 kJ kg⁻¹ day⁻¹ (2–3× that of similar-sized land mammals), consuming 25% of their body mass daily so that most assimilated energy is lost as heat maintaining body temperature in cold water, giving an ecological efficiency of only ~5% from urchins to otters.
Demonstrates how chemical energy from respiration is transformed into heat, and why endotherms in cold environments show especially low ecological efficiency.
California kelp forest, USA Energy flowsStudyThe stick-insect and cabbage-white caterpillar (Pieris brassicae) experiments show consumers lose far more assimilated biomass to respiration than producers, because mobile animals expend energy on locomotion, foraging and (in homeotherms) maintaining body temperature.
Demonstrates that chemical energy released in respiration is transformed into heat, explaining high respiratory losses in consumers.
2.2.8The second law of thermodynamics states that energy transformations in ecosystems are inefficient.
An oak woodland receives 7,000,000 kJ/m²/yr of solar radiation but fixes only 42,000 kJ as GPP; of the 16,800 kJ NPP, just 1,680 kJ (10%) passes to herbivores and 168 kJ (10%) to primary carnivores, with ~89% of NPP passing to decomposers and most energy lost as heat.
Quantifies the inefficiency of energy transformations, giving ~10% trophic transfer efficiency and <1% photosynthetic efficiency.
In a North Atlantic marine ecosystem the pyramid of biomass is inverted, with zooplankton (21.0 g m⁻²) exceeding phytoplankton (4.0 g m⁻²), because fast-reproducing phytoplankton are grazed almost as fast as they grow, yet the corresponding pyramid of energy remains upright.
Counter-example showing biomass pyramids can invert while energy pyramids cannot, reinforcing the second law's constraint on energy flow.
North Atlantic Ocean Biomass and food websStudyIn a stick-insect feeding experiment, ten insects fed privet leaves assimilated 2.4 g (gross productivity) but gained only 0.3 g biomass, meaning 2.1 g (about 87.5%) was lost to cellular respiration.
Quantifies the inefficiency of energy transformation predicted by the second law, with most assimilated energy lost rather than stored as biomass.
In an African grassland food chain, grass captures ~10,000 kJ of solar energy, a zebra eating the grass obtains ~1,000 kJ (10%), and a lion eating the zebra obtains only ~100 kJ (10% again), with the rest lost as respiratory heat. This explains why vast grassland areas support only a few lions and why food chains rarely exceed four or five levels.
Demonstrates how repeated ~90% energy losses per transfer limit food-chain length and predator abundance.
In the California giant kelp (Macrocystis pyrifera) forest food chain, energy flow falls from 50,000 kJ m⁻² yr⁻¹ in kelp to 4,000 kJ m⁻² yr⁻¹ in purple sea urchins (Strongylocentrotus purpuratus) to just 200 kJ m⁻² yr⁻¹ in southern sea otters (Enhydra lutris nereis), giving an overall efficiency of only 0.4% from producer to secondary consumer.
Quantifies the second law's inefficiency of energy transfer with real trophic-level energy values, showing losses well below and above the 10% approximation.
California kelp forest, USA Energy flows
2.2.18Pollutants that are non-biodegradable, such as polychlorinated biphenyl (PCB), dichlorodiphenyltrichloroethane (DDT) and mercury, cause changes to ecosystems through the processes of bioaccumulation and biomagnification.
The Minamata Bay disaster (Japan, discovered 1956): the Chisso Corporation discharged mercury-laden wastewater which bacteria converted to methylmercury; it biomagnified from 0.000003 ppm in seawater to ~200 ppm in fish-eating birds (a ~67-million-fold increase), causing over 1,500 deaths and thousands of cases of severe neurological damage in people who ate contaminated fish.
Demonstrates how a non-biodegradable, lipophilic pollutant (mercury) bioaccumulates in fatty tissue and biomagnifies up the food chain to devastate top predators including humans.
Minamata, Japan Pollutants, Bioaccumulation and Biomagnification
2.2.19Non-biodegradable pollutants are absorbed within microplastics, which increases their transmission in the food chain.
The Great Pacific Garbage Patch, the largest of five oceanic gyres concentrating plastic debris, contains areas where microplastics outnumber plankton in surface samples; hydrophobic PCBs and DDT bind to their large surface areas, so filter feeders ingest plastic-borne pollutants at the base of the food chain.
Demonstrates how microplastics act as vectors concentrating non-biodegradable pollutants and transmitting them into food chains while disrupting energy transfer at low trophic levels.
Great Pacific Garbage Patch Pollutants, Bioaccumulation and Biomagnification
2.2.20Human activities, such as burning fossil fuels, deforestation, urbanization and agriculture, have impacts on flows of energy and transfers of matter in ecosystems.
Between 1988 and 2008, deforestation in the Brazilian Amazon ran at 10,000–30,000 km² per year (65–70% driven by cattle ranching), releasing carbon from a biome storing roughly 150–200 billion tonnes of carbon in its biomass.
Shows how deforestation removes producers, disrupts food webs and releases stored carbon, altering energy flows and matter transfer while creating a climate feedback loop.
Amazon Rainforest, Brazil Pollutants, Bioaccumulation and BiomagnificationIowa (USA) has converted over 85% of its land to corn and soybean monoculture using ~30 million kg of nitrogen fertiliser annually, whose runoff feeds a Gulf of Mexico dead zone covering up to 22,000 km² each summer.
Demonstrates how agriculture simplifies food webs, removes matter via harvest, and links terrestrial nutrient loading to eutrophication and disrupted energy flow in a distant aquatic ecosystem.
2.2.23Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements.HL
At deep-ocean hydrothermal vents (2000–4000 m depth), first discovered in 1977, chemoautotrophic bacteria oxidise inorganic hydrogen sulphide to fix carbon, powering an entire ecosystem including giant tube worms (Riftia pachyptila) with no reliance on sunlight.
Shows primary productivity driven by an external chemical energy source rather than light, using inorganic carbon and chemosynthesis.
2.2.26Maximum sustainable yields (MSYs) are the net primary or net secondary productivity of a system.HL
The Grand Banks cod fishery off Newfoundland, Canada collapsed in 1992 after annual catches (e.g. 0.8 million tonnes in 1970) repeatedly exceeded the maximum sustainable yield, crashing estimated cod biomass from 1.6 million tonnes in 1960 to under 0.1 million tonnes, triggering a moratorium that still restricts the fishery over 30 years later.
Demonstrates that harvesting Atlantic cod (Gadus morhua) above the net secondary productivity (MSY) depletes the stock and causes population collapse.
Grand Banks, Newfoundland, Canada Productivity, Sustainable Yields and Thermodynamics
2.2.27Sustainable yields are higher for lower trophic levels.HL
In a simplified agricultural food chain, crop plants receiving 10,000 kJ pass 1,000 kJ to cattle and only 100 kJ to humans, so eating crops directly gives people ten times more energy than eating beef.
Quantifies why sustainable yields are higher at lower trophic levels, making plant-based diets more sustainable than meat-based ones.
2.2.28Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next level.HL
In the kelp forest case study, ecological efficiency from producers to secondary consumers was only ~0.4%, and sea otters showed unusually low efficiency (~5%) because their high metabolic rate diverts much assimilated energy to heat.
Shows that real ecological efficiencies deviate widely from the 10% rule, with high-metabolism endotherms passing on very little energy.
2.3 Biogeochemical cycles
2.3.1Biogeochemical cycles ensure chemical elements continue to be available to living organisms.
In the global carbon cycle photosynthesis removes about 120 GtC/year from the 800 GtC atmospheric store while respiration and decomposition return about 60 GtC/year, keeping carbon continuously available to organisms with an atmospheric residence time of only 3-5 years.
Quantifies the biogeochemical fluxes that recycle carbon and keep it available to living organisms.
2.3.7Fossil fuels are stores of carbon with unlimited residence times. They were formed when ecosystems acted as carbon sinks in past eras and become carbon sources when burned.
The global fossil fuel pool holds about 10,000 GtC accumulated over hundreds of millions of years, yet humans are releasing roughly 9 GtC per year, converting stores with effectively unlimited residence times into a rapid carbon source.
Quantifies how ancient carbon sinks with near-infinite residence times become sources when burned, releasing millions of years of stored carbon in decades.
2.3.8Agricultural systems can act as carbon stores, sources and sinks, depending on the techniques used.
Draining and repeatedly ploughing a wetland for arable land exposes its organic matter to oxygen, accelerating decomposition and oxidation so the soil switches from a carbon store to a source, whereas no-till farming, cover crops and crop rotation keep soils acting as stores or sinks.
Demonstrates how agricultural technique choice determines whether a soil system is a carbon store, sink or source.
2.3.9Carbon dioxide is absorbed into the oceans by dissolving and is released as a gas when it comes out of a solution.
Ocean-atmosphere exchange moves roughly 90 GtC in each direction annually, but the oceans currently take up a net ~2.5 GtC per year, acting as a carbon sink even though this is far less than the 9+ GtC/year humans emit.
Quantifies bidirectional dissolving and out-gassing of CO2 and shows the ocean's net uptake is limited relative to emissions.
2.3.10Increases in concentrations of dissolved carbon dioxide cause ocean acidification, harming marine animals.
Ocean surface pH has fallen from about 8.2 in pre-industrial times to roughly 8.1 today; because pH is logarithmic this represents a ~26% increase in acidity, reducing calcification in molluscs and corals such as the Great Barrier Reef, which is experiencing bleaching and reduced calcification rates.
Quantifies how rising dissolved CO2 lowers ocean pH and demonstrates the concrete harm to calcium-carbonate-forming marine animals.
Great Barrier Reef, Australia A Tour of the Carbon Cycle
2.3.11Measures are required to alleviate the effects of human activities on the carbon cycle.
IPCC-style carbon budget figures show anthropogenic emissions of 9.4 GtC/year from fossil fuels plus 1.6 GtC/year from land-use change, against natural sinks of only 3.4 GtC/year (land) and 2.5 GtC/year (ocean), leaving a net ~4 GtC/year accumulating in the atmosphere.
Quantifies why sources exceed sinks and justifies mitigation measures targeting emission reduction over sink enhancement.
2.3.14In past geological eras, organic matter from partially decomposed plants became fossilized in coal, and partially decomposed marine organisms became fossilized in oil and natural gas held in porous rocks.HL
Coal formation peaked during the Carboniferous period (~360–300 million years ago), when vast swamp forests of ferns, horsetails and early trees near the equator died in waterlogged, anaerobic conditions and were buried and compressed over tens of millions of years.
Demonstrates that coal derives from partially decomposed plant matter fossilized under specific geological-era conditions, as required by the syllabus point.
Oil and natural gas formed mainly during the Jurassic (201–145 mya) and Cretaceous (145–66 mya) periods, when warm climates, high CO₂ and extensive shallow seas produced enormous plankton productivity; dead marine algae and zooplankton sank, were buried, and were transformed by heat and pressure into oil and gas held in porous rocks.
Shows that oil and gas originate from marine organisms fossilized in porous rock during specific eras of high productivity, illustrating the syllabus point in detail.
2.3.16Methane has a residence time of about 10 years in the atmosphere and is eventually oxidized to carbon dioxide.HL
Methane has an atmospheric residence time of only about 10 years before being oxidized to CO₂, yet it has roughly 80× the warming potential of CO₂ over a 20-year period, meaning cuts to methane emissions can deliver rapid climate benefits.
Quantifies methane's short residence time and its oxidation fate, showing why potency and residence time must be considered together.
2.3.19Denitrification only happens in anaerobic conditions, such as soils that are waterlogged.HL
The Flow Country of Caithness and Sutherland, northern Scotland, is Europe's largest blanket bog (~4,000 km²) whose waterlogged, acidic, anaerobic peat soils cause denitrification and leaching to strip available nitrogen, so insectivorous sundews (Drosera rotundifolia) trap and digest insects to obtain nitrogen instead.
Demonstrates how anaerobic waterlogged soils drive denitrification/leaching and create nitrogen-poor conditions favouring insectivorous plants.
Flow Country, Scotland The Nitrogen Cycle: From Bacteria to Planetary Boundaries
2.3.20Plants cannot fix nitrogen so atmospheric dinitrogen is unavailable to them unless they form mutualistic associations with nitrogen-fixing bacteria.HL
Brazil, the world's largest soybean producer, has farmers inoculate seeds with Bradyrhizobium japonicum so the legume–bacteria mutualism fixes atmospheric N₂ into ammonia, saving the sector billions of dollars annually in synthetic fertiliser costs (Embrapa).
Shows the agricultural significance and competitive advantage of the legume–Rhizobium mutualism as a route for plants to access unavailable atmospheric nitrogen.
2.3.22Human activities such as deforestation, agriculture, aquaculture and urbanization change the nitrogen cycle.HL
Synthetic nitrogen fertiliser on intensive maize in the US Corn Belt (Iowa, Illinois, Indiana) generates nitrate run-off carried down the Mississippi, feeding the Gulf of Mexico 'dead zone', a hypoxic area of roughly 15,000 km² and one of the world's largest cases of eutrophication.
Quantifies how agricultural nitrogen inputs alter the nitrogen cycle and produce large-scale downstream eutrophication.
Gulf of Mexico dead zone The Nitrogen Cycle: From Bacteria to Planetary Boundaries
2.3.23The Haber process is an industrial process that produces ammonia from nitrogen and hydrogen for use as fertilizer.HL
The Haber-Bosch process, developed by Fritz Haber in 1909 and scaled by Carl Bosch for BASF, produces ammonia from N₂ and H₂, now sustaining roughly half the world's food production while consuming about 1–2% of global energy, almost entirely from fossil fuels.
Shows both the advantage (feeding half the world) and disadvantage (fossil-fuel dependence and emissions) of industrial ammonia production for fertiliser.
2.3.24Increases in nitrates in the biosphere from human activities have led to the planetary boundary for the nitrogen cycle being crossed, making irreversible changes to Earth systems likely.HL
The Baltic Sea, bordered by nine countries, is one of the most eutrophied seas in the world because agricultural nitrogen run-off from all of them has caused severe oxygen depletion, illustrating the consequences of exceeding the proposed 62 Tg N yr⁻¹ nitrogen planetary boundary.
Demonstrates the transboundary eutrophication that results from crossing the nitrogen planetary boundary driven by fertiliser use.
2.3.25Global collaboration is needed to address the uncontrolled use of nitrogen in industrial and agricultural processes and bring the nitrogen cycle back within planetary boundaries.HL
Sri Lanka's 2021 abrupt ban on synthetic fertiliser imports, intended to force a switch to fully organic farming, collapsed rice and tea yields because there was no phased transition or alternative nitrogen source.
Counter-example showing that reducing inorganic fertiliser dependence requires careful, phased global collaboration rather than abrupt bans.
2.4 Climate and biomes
2.4.2A biome is a group of comparable ecosystems that have developed in similar climatic conditions, wherever they occur.
Tropical rainforests in the Amazon Basin, the Congo Basin and Southeast Asia share recognisable structural features (tall canopy, dense undergrowth, high diversity) despite containing entirely different species, because similar climatic conditions produce convergent structures.
Shows the defining 'wherever they occur' idea that comparable climates produce parallel ecosystem features across continents.
Congo Basin Biomes: Characteristics, Distribution and Change
2.4.5The tricellular model of atmospheric circulation explains the behaviour of atmospheric systems and the distribution of precipitation and temperature at different latitudes. It also explains how these factors influence the structure and relative productivity of different terrestrial biomes.
The Sahara (≈25°N), Arabian (≈25°N) and Australian (≈25°S) deserts all sit at roughly 30° latitude because dry, cool air descends there as the poleward limb of the Hadley cell, creating a permanent zone of high pressure and arid, low-productivity conditions.
Demonstrates how the descending arm of the Hadley cell produces high-pressure arid biomes at a predictable latitude in both hemispheres.
Sahara Desert Climate, Atmospheric Circulation and Biome DistributionWestern Europe, including the UK (≈45°–60°N), experiences frequent rainfall and variable weather because it lies in the Ferrel cell where warm subtropical air rises over cold polar air at ≈60°, cooling and condensing to produce the moderate precipitation that supports temperate forest.
Shows how the rising air and polar front of the Ferrel cell generate the mid-latitude precipitation and variable weather characteristic of temperate biomes.
United Kingdom Climate, Atmospheric Circulation and Biome Distribution
2.4.6The oceans absorb solar radiation and ocean currents distribute the resulting heat around the world.
London (51°N) averages about 5°C in January while Calgary, Canada (51°N) averages about −7°C, because the Gulf Stream and its extension the North Atlantic Drift carry warm water from the Gulf of Mexico across the Atlantic, warming the westerly winds that reach western Europe.
Quantifies how a warm ocean current redistributes tropical heat to give two locations at the same latitude very different climates.
The Humboldt Current brings cold water northward along the west coast of South America, cooling the adjacent coast and contributing to the aridity of the Atacama region.
Shows how a cold current transports polar water towards the equator and shapes the climate and biomes of nearby land masses.
Humboldt Current, west coast of South America Climate, Atmospheric Circulation and Biome Distribution
2.4.7Global warming is leading to changing climates and shifts in biomes.
Along the Dalton Highway ~232 miles north of Fairbanks, Alaska, researchers study the boreal-tundra treeline, the largest ecosystem transition on Earth; Arctic warming is occurring ~6 times faster than at the equator, but although spruce may advance north the tundra extends to the Arctic Ocean and has nowhere to retreat.
Demonstrates poleward biome shift under global warming and why the tundra is squeezed with nowhere to migrate.
Dalton Highway, Alaska, USA Biomes: Characteristics, Distribution and ChangeCanada's boreal forest, part of the largest relatively untouched forest on Earth (1.9 billion hectares globally, supporting 3.7 million people), is contracting at its southern margin as milder winters and hotter summers cause drought stress, stomatal closure and mountain pine beetle outbreaks, while expanding at its northern margin into Arctic tundra.
Shows a biome being squeezed from both margins simultaneously, illustrating that biome shifts are not smooth migrations.
2.4.8There are three general patterns of climate types that are connected to biome types.HL
London (maritime temperate) has a January average of around 5°C while Calgary (continental temperate) at the same latitude averages around -10°C, showing how proximity to the ocean, not latitude, determines the temperate subdivision and biome potential.
Demonstrates the distinction between maritime and continental subdivisions of the temperate climate type at equal latitude.
Calgary, Canada Climate Types, ENSO and Tropical Cyclones
2.4.9The biome predicted by any given temperature and rainfall pattern may not develop in an area because of secondary influences or human interventions.HL
Nairobi, Kenya, sits at just 1°S latitude but at an altitude of 1,795 m, so the environmental lapse rate (~6.5°C per 1,000 m) cools it enough that it does not develop the tropical rainforest predicted by its equatorial latitude.
Demonstrates how a secondary influence (altitude) overrides the biome predicted by latitude-based temperature and rainfall.
Nairobi, Kenya Climate Types, ENSO and Tropical CyclonesThe Atacama Desert in Chile receives under 15 mm of precipitation per year despite lying on the Pacific coast, because the rain shadow of the Andes and the cold Humboldt Current suppress moisture and rainfall.
Shows two secondary influences (rain shadow and cold ocean current) preventing the moister biome expected at a coastal location.
Atacama Desert, Chile Climate Types, ENSO and Tropical Cyclones
2.4.10The El Niño Southern Oscillation (ENSO) cycle is the fluctuation in wind and sea surface temperatures that characterizes conditions in the tropical Pacific Ocean. The two opposite and extreme states are El Niño and La Niña, with transitional and neutral states between the extremes.HL
The catastrophic 1972-73 El Nino decimated Peru's anchoveta population, killed millions of seabirds and crashed Peru's export economy, illustrating the irregular and severe extremes of the ENSO cycle.
Shows the extreme El Nino state of the ENSO cycle and the unpredictability of event intensity over decades.
2.4.11El Niño is due to a weakening or reversal of the normal east–west (Walker) circulation, which increases surface stratification and decreases upwelling of cold, nutrient-rich water near the coast of north-western South America. La Niña is due to a strengthening of the Walker circulation and reversal of other effects of El Niño.HL
During the 2023-24 El Nino, one of the five strongest on record with eastern Pacific SSTs ~2.0°C above average, Peru cancelled the June 2023 anchoveta fishing season because collapsed upwelling drove fish below 100 m, cutting fishmeal production 28% and raising global feed prices since Peru supplies ~20% of world fishmeal.
Quantifies how weakened Walker circulation suppresses upwelling off Peru and collapses marine productivity with global economic knock-on effects.
2.4.13Rises in ocean temperatures resulting from global warming are increasing the intensity and frequency of hurricanes and typhoons because warmer water and air have more energy.HL
Hurricane Milton (October 2024) intensified from tropical storm to Category 5 in ~24 hours over a Gulf of Mexico that Climate Central found 400-800 times more likely to be so warm due to climate change; World Weather Attribution concluded that without warming it would have made landfall as Category 2 rather than 3, with rainfall 20-30% higher.
Provides attribution-study evidence linking elevated ocean temperatures to rapid intensification and greater cyclone severity.
Gulf of Mexico Climate Types, ENSO and Tropical Cyclones
2.5 Zonation, succession and change in ecosystems
2.5.1Zonation refers to changes in community along an environmental gradient.
At Studland Bay, Dorset, walking inland from the shore produces clear sand dune zonation: marram grass peaks (~63% cover) around 20 m from the sea while heather only appears from 40 m and dominates (100% cover) at 60–70 m, as salt spray falls and soil depth and humus rise.
Shows community changing along a distance-from-sea environmental gradient, with species replacing one another as abiotic conditions shift.
Studland Bay, Dorset, England Zonation and Succession
2.5.2Transects can be used to measure biotic and abiotic factors along an environmental gradient in order to determine the variables that affect the distribution of species.
Students at Studland Beach laid three parallel transects at 90° to the sea, sampling 1 m² quadrats at 10 m intervals to record percentage cover of marram grass and heather alongside slope angle, revealing that the two species barely overlap and are separated along the gradient.
Demonstrates transect methodology (parallel replicates, regular quadrats, biotic and abiotic factors) used to determine variables affecting species distribution.
Studland Bay, Dorset, England Zonation and Succession
2.5.3Succession is the replacement of one community by another in an area over time due to changes in biotic and abiotic variables.
Surtsey, a ~1 km² volcanic island off southern Iceland formed by eruptions in 1963–67, shows primary succession on bare rock (lithosere): first insects arrived in 1964, mosses and lichens as pioneers, the first willow shrub in 1999, and by 2008 it had 69 vascular plant species (versus 460 on mainland Iceland), with 2–5 new species arriving each year.
Quantifies primary succession from a documented start date, showing pioneer colonisation of newly formed substratum and slow community development due to isolation.
Surtsey, Iceland Zonation and SuccessionThe Studland Bay dune psammosere progresses over ~400+ years from foredunes (sea rocket, pH 8.5) through mobile and fixed dunes to climax oak-ash woodland (>40% humus, pH 4.5), so walking the 2.5 km inland from the 16th-century oldest dunes is effectively walking forward through time.
Demonstrates succession as temporal replacement of communities, with the spatial dune sequence mirroring change over centuries.
Studland Bay, Dorset, England Zonation and Succession
2.5.4Each seral community (sere) in a succession causes changes in environmental conditions that allow the next community to replace it through competition until a stable climax community is reached.
In the Studland Bay psammosere, marram grass traps sand and adds leaf-litter organic matter, raising humus and lowering soil pH from ~8.0 to 6.5, so that red fescue and heather eventually outcompete the marram, which itself requires sand burial to thrive: each sere engineers its own replacement.
Shows a seral community modifying abiotic conditions (pH, humus) so the next community can outcompete and replace it, the core mechanism of succession.
Studland Bay, Dorset, England Zonation and Succession
2.5.6Secondary successions happen on bare soil where there has been a pre-existing community, such as a field where agriculture has ceased or a forest after an intense firestorm.
The Broadbalk Wilderness at Rothamsted Research, Hertfordshire, England, is one of the world's longest-running ecological experiments: a section of the Broadbalk wheat field abandoned in the 1880s underwent unmanaged secondary succession, progressing through annual weeds, perennial grasses, hawthorn and blackthorn scrub, and by the present day a closed-canopy mixed deciduous woodland of oak, ash and sycamore after roughly 140 years.
Demonstrates secondary succession on pre-existing soil with a seed bank following the cessation of agriculture, reaching woodland in about 100 years.
Rothamsted Research, Harpenden, Hertfordshire, England Secondary Succession, Changes During Succession and Resilience
2.5.7Energy flow, productivity, species diversity, soil depth and nutrient cycling change over time during succession.
Along a sand-dune succession transect, the strandline pioneer stage supports only 2–3 species while the dune heath supports dozens of plant species plus associated animals, and soil humus rises from near zero to over 40%, showing species diversity and soil depth both increase through the seres.
Quantifies the systematic rise in species diversity and soil organic matter across successional stages.
At Broadbalk Wilderness the deepening leaf litter and steadily rising soil organic carbon since 1881, alongside the shift from short simple food chains to a branched woodland food web, provides direct long-term evidence that soil depth, nutrient cycling and energy-flow complexity increase during succession.
Quantifies how soil depth, nutrient cycling and energy flow change over time using a documented 140-year dataset.
Rothamsted Research, Harpenden, Hertfordshire, England Secondary Succession, Changes During Succession and Resilience
2.5.8An ecosystem's capacity to tolerate disturbances and maintain equilibrium depends on its diversity and resilience.
Sustained trampling on the sand dunes at Studland Bay, Dorset, destroys marram grass and exposes bare sand, resetting succession and creating a plagioclimax; because the pioneer community has low diversity and small biomass storage it is pushed toward a tipping point, unlike a resilient climax oak woodland.
Shows how low diversity and small storage reduce resilience, and how human interference (trampling) diverts succession to a plagioclimax.
Studland Bay, Dorset, England Secondary Succession, Changes During Succession and ResilienceThe displacement of diverse North American prairie ecosystems with monoculture crops drastically reduces species diversity and the size of biomass and nutrient storages, simplifying food webs and lowering the ecosystem's resilience to disturbance.
Demonstrates how human activity reduces diversity and storage, thereby lowering resilience and moving a system closer to tipping points.
Great Plains, North America Secondary Succession, Changes During Succession and Resilience
2.5.9The type of community that develops in a succession is influenced by climatic factors, the properties of the local bedrock and soil, geomorphology, together with fire and weather-related events that can occur. There can also be top-down influences from primary consumers or higher trophic levels.HL
The Flow Country of Caithness and Sutherland, Scotland, is one of the world's largest blanket bogs (~4,000 km²) where flat topography, impermeable bedrock and high rainfall keep soils waterlogged; Sphagnum moss, cotton grass and sundews dominate instead of the forest predicted by latitude, because anaerobic conditions halt decomposition and tree establishment.
Shows how geomorphology and poor drainage arrest succession at a bog seral stage rather than forest.
Flow Country, Caithness and Sutherland, Scotland Factors Influencing Succession, Productivity and PlagioclimaxThe serpentine soils of the Troodos Mountains, Cyprus, form from ultra-basic oceanic crust that weathers to soils very high in nickel and chromium but low in calcium and potassium; instead of the Mediterranean woodland predicted by climate, only sparse endemic metallophyte plants grow, halting succession by soil chemistry.
Demonstrates how bedrock and extreme soil chemistry override climate to determine the community that develops.
Troodos Mountains, Cyprus Factors Influencing Succession, Productivity and Plagioclimax
2.5.12The concept of a climax community has been challenged, and there is uncertainty over what ecosystems would develop naturally were there no human influences.HL
Frans Vera's wood-pasture hypothesis proposes that post-glacial Europe was a shifting mosaic of woodland, scrub and grassland maintained by large herbivores such as aurochs, wild horses and European bison, rather than the closed-canopy forest of the traditional monoclimax view; early Holocene pollen and open-habitat beetle fossils are cited as support, though the evidence remains contested.
Demonstrates how the single-climax concept is challenged and uncertainty over the 'natural' human-free ecosystem.
2.5.13Human activity can divert and change the progression of succession leading to a plagioclimax.HL
The open heather moorland of the Scottish Highlands is a plagioclimax: wolves and lynx were extirpated centuries ago, and combined with sheep grazing and deer-stalking management this prevented regeneration of the native Caledonian pine forest, until rewilding by Trees for Life in Glen Affric let Scots pine, birch and rowan recover.
Shows how removing top carnivores plus human grazing diverts succession into a maintained plagioclimax that recovers when pressure is removed.
Glen Affric, Scotland Factors Influencing Succession, Productivity and Plagioclimax