amazon.co.ukA-Level Biology revision on ecology and ecosystems: energy flow and productivity (GPP/NPP), nutrient cycles, population dynamics, succession and conservation, with required practicals (AQA RP12, OCR PAG12) and multi-board exam practice.
An ecosystem is a community plus its abiotic environment, with energy flowing through it and nutrients cycling within it. Trophic levels: producers (autotrophs) β primary consumers (herbivores) β secondary consumers β tertiary consumers, with decomposers (saprotrophs) recycling nutrients at every level by extracellular digestion - secreting enzymes onto dead matter and absorbing the products. Food webs show interlinked food chains; removing one species ripples through the web (keystone species questions). Energy flow starts with gross primary productivity (GPP) - the total chemical energy fixed by photosynthesis per area per time. Producers use some of that in respiration (R): NPP = GPP β R; NPP is what is available to primary consumers and for producer growth/reproduction. Similar equations apply to consumers: energy ingested = energy lost in faeces + energy assimilated (respired or made into new biomass).
Why only ~10% transfers between trophic levels (the guaranteed four-marker): energy is lost as heat from respiration (most energy), faeces (undigested material, egestion), urine (excretion of nitrogenous waste) and uneaten parts (bones, roots); only the energy in consumed, digested, assimilated biomass becomes new biomass for the next level; the second law of thermodynamics means entropy increases so transfers are never 100% efficient. Consequences: food chains rarely exceed 4-5 trophic levels (insufficient energy left), pyramids of energy are always upright (energy always decreases), and human diets based on plants feed more people per hectare (less energy wasted between levels). Pyramids of number and biomass can invert (one tree supports many aphids; phytoplankton biomass drops momentarily before a zooplankton bloom) - know when each pyramid type is appropriate and their limitations. Calculations: percentage efficiency = (energy transferred Γ· energy available) Γ 100%; NPP calculations from GPP and R are standard.
Nutrients cycle between abiotic reservoirs (atmosphere, soil, ocean) and living biomass; energy flows one-way but nutrients are recycled. Carbon cycle: photosynthesis fixes atmospheric COβ into organic molecules; respiration returns COβ (plants, animals, decomposers); combustion of fossil fuels/wood releases COβ; decomposition by saprotrophs returns carbon to soil and COβ; ocean uptake dissolves COβ (and carbonate shells); methane (CHβ) from anaerobic decomposition and livestock also contributes. Anthropogenic enhancement: fossil fuel combustion and deforestation raise atmospheric COβ, driving climate change (greenhouse effect) - link to conservation questions.
Nitrogen cycle - know each stage with its bacteria: (1) nitrogen fixation: Rhizobium (mutualistic, in root nodules of legumes) and Azotobacter (free-living in soil) use nitrogenase to reduce atmospheric Nβ to ammonia (NHβ/NHββΊ); Rhizobium gains carbohydrates from the plant; (2) nitrification: Nitrosomonas oxidises ammonia to nitrite (NOββ»), Nitrobacter oxidises nitrite to nitrate (NOββ») - nitrates are the form plants assimilate; (3) assimilation: plants absorb nitrate by active transport and build amino acids/nucleotides, which pass through food chains; (4) ammonification: saprotrophs decompose dead matter and waste, releasing ammonia; (5) denitrification: Pseudomonas (anaerobic, waterlogged soils) reduces nitrate back to Nβ, depleting soil nitrogen. Lightning fixes small amounts of nitrogen directly. Exam questions test whether you link each process to its bacteria and conditions (aerobic nitrification vs anaerobic denitrification) and to agriculture (crop rotation with legumes, fertiliser use).
Carbon sinks and budgets: the oceans are the largest carbon sink (dissolved COβ, carbonate sediments, phytoplankton), soils store more carbon than vegetation, and forests fix carbon rapidly in growth - deforestation releases stored carbon and removes the sink, so it contributes doubly to rising atmospheric COβ. Agricultural links: crop rotation with legumes exploits Rhizobium to add nitrogen without synthetic fertiliser; ploughing increases aeration, boosting nitrification but also decomposition and COβ release; waterlogged paddy fields favour denitrification and methanogenesis (CHβ), so rice and livestock farming increase methane - quote these when asked to link nutrient cycles to farming or climate-change questions.
A population is all the individuals of one species in a habitat at a time; the community is all species; the habitat is the place; the niche is the role (two species cannot share the same niche - competitive exclusion). Population growth curve: lag phase (few individuals, slow growth while adapting); log/exponential phase (unlimited resources, maximum growth rate); stationary phase (limiting factors bite - birth rate = death rate); death/decline phase (resources exhausted, waste accumulates). The carrying capacity (K) is the maximum sustainable population size, set by limiting factors: abiotic (temperature, light, water, space) and biotic (food, predation, competition, disease). Limiting factors questions: identify from data which factor limits a population, and predict the effect of adding/removing it.
Predator-prey cycles (Lotka-Volterra): predator and prey populations oscillate out of phase - prey rise, predator numbers follow after a lag (time to reproduce); rising predation reduces prey, which then reduces predators; fewer predators let prey rise again. Explain the lag and the phase difference - the standard interpretation question. Sampling methods (AQA RP12; OCR PAG12): quadrats (random placement with random number coordinates to avoid bias; measure abundance as frequency, % cover or ACFOR scale) and belt transects (along an environmental gradient - distribution questions); mark-release-recapture for mobile animals: capture, mark (non-harmful), release, recapture - N = (nβ Γ nβ) Γ· n_marked; assumptions: population closed (no migration), random mixing, marks do not affect survival/catchability, constant population. Know the assumptions - evaluation questions ask what happens if they fail (overestimates if marked animals less catchable, etc.).
r/K selection explains population strategies: r-selected species (many small offspring, little parental care, rapid reproduction - mice, insects) exploit unstable environments and recover fast; K-selected species (few offspring, heavy parental investment - elephants, whales) sit near carrying capacity in stable environments and recover slowly - conservation of K-selected species is harder because populations recover slowly from decline. Sampling evaluation: quadrat estimates are only as good as their sample size - larger quadrats and more samples reduce error; random placement avoids bias toward convenient areas; along a gradient, belt transects with quadrats at regular intervals (or interrupted) show distribution change; mark-release-recapture overestimates if marked animals are easier or harder to catch again, and fails if the population changes between samples - state the assumption being violated in evaluation questions.
Succession is the progressive, directional change in a community over time, driven by each stage modifying the environment for the next. Primary succession starts on bare substrate with no soil (lava, dunes, retreating glaciers): (1) pioneer species colonise - lichens/mosses adapted to harsh conditions (tolerate drought, low nutrients, fix nitrogen); (2) pioneers die and decompose, building soil, retaining water and nutrients; (3) intermediate communities - grasses, then shrubs, then fast-growing trees (each outcompeting the previous by growing taller, shading them out); (4) climax community - the stable end community in equilibrium with the climate (e.g. oak/beech woodland in the UK); biodiversity and biomass rise through succession and stabilise at the climax. Secondary succession starts where soil already exists (after fire, clearing) - faster, skipping pioneer stages. Deflected succession: human activity (grazing, mowing, burning, trampling) halts succession, maintaining a stable intermediate (plagioclimax, e.g. grazed grassland, heather moorland); if the activity stops, succession resumes. Conservation applications: managing succession maintains early-stage habitats for species that need them (e.g. grazing to keep chalk grassland, cutting to maintain heath); understand why climax woodland is not always the conservation goal - species diversity peaks at intermediate stages. Exam questions: sequence the stages, explain why each stage enables the next (soil depth, water retention, shelter), and define/apply deflected succession to a scenario.
Eutrophication (the guaranteed sequence question): (1) excess nitrates/phosphates from fertiliser runoff or sewage enter water; (2) algal bloom at the surface blocks light; (3) submerged plants die from lack of light; (4) saprobic bacteria decompose the dead biomass aerobically, depleting dissolved Oβ; (5) aerobic organisms (fish, invertebrates) die from oxygen shortage; (6) anaerobic bacteria flourish, releasing toxic products (HβS) - dead zones with very low biodiversity. Explain each step as a chain of cause and effect; the BOD (biochemical oxygen demand) rises as Oβ falls - occasionally examinable. Climate change: enhanced greenhouse effect from COβ (combustion, deforestation) and CHβ (livestock, anaerobic decomposition, rice paddies); impacts - species range shifts, phenology mismatches (earlier flowering), coral bleaching, ocean acidification (COβ + HβO β carbonic acid, lowering pH and dissolving carbonate shells).
Conservation maintains biodiversity (species, genetic, ecosystem diversity) by managing ecosystems sustainably: in situ methods (nature reserves, habitat restoration, managing succession, legal protection) are preferred - conserve whole ecosystems and the evolutionary context; ex situ methods (zoos, seed banks, captive breeding, gene banks) back up in situ for critically endangered species. Sustainable management: using resources without depleting them - e.g. fishing quotas (maintain breeding stock), coppicing (regrowth without succession to woodland), rotational forestry (replanting). Evaluation questions want both sides: economic development vs conservation (tourism income vs habitat loss); conflict between local needs and global biodiversity; the value of keystone species and ecosystem services (pollination, water purification, carbon storage). Biodiversity measurement: species richness (number of species) vs species evenness (relative abundance) - Simpson's diversity index D = 1 β Ξ£(n/N)Β² (higher D = more diverse); OCR and Edexcel expect the calculation and interpretation, and the comparison of two habitats (which is more diverse and why).