Organisation of an Ecosystem

Every food chain starts with a producer — almost always a green plant or alga that uses photosynthesis to convert light energy into the chemical energy stored in glucose. From there, energy moves up through consumers.

Producer — green plant or alga (occasionally photosynthetic bacterium). On UK land: grass, oak trees, bluebells, brambles. In UK seas: phytoplankton and seaweeds.

Primary consumer — an animal that eats producers directly. Usually a herbivore. UK examples: rabbit, deer, wood mouse, slug, aphid.

Secondary consumer — an animal that eats a primary consumer. Usually a carnivore. UK examples: fox eating rabbit; tawny owl eating wood mouse; ladybird eating aphid.

Tertiary consumer — an animal that eats a secondary consumer. UK example: peregrine falcon eating a sparrowhawk that ate a small bird that ate caterpillars.

Apex predator — top of its food chain, no natural predators (apart from humans). UK apex predators include peregrine falcon, killer whale, golden eagle. Many former apex predators (wolf, bear, lynx) have been wiped out in the UK historically.

Why energy doesn't flow forever upward. Energy is lost at each level — to respiration, heat, undigested faeces, and movement. By the time you reach tertiary consumers, only a tiny fraction of the original light energy is left, which is why food chains rarely have more than four or five links.

Food chains show ONE feeding pathway from producer to apex consumer:

Oak leaves → Caterpillar → Blue tit → Sparrowhawk

The arrows always point from the food TO the feeder — they show the direction of energy flow (and biomass) along the chain.

Food webs are more realistic. In a real community, most species eat (and are eaten by) several others. A food web links many food chains together. Pull out any one chain from a web and you have a food chain.

Reading a food web on the exam:

• Identify producers (no arrows pointing IN to them — they make their own food).
• Identify apex predators (no arrows pointing OUT — nothing eats them).
• Count arrows pointing INTO an animal to find what it eats.
• Count arrows pointing OUT of an animal to find what eats it.

Why food webs matter for stability. If a species in a simple food chain disappears, the chain collapses. In a complex food web, predators usually have ALTERNATIVE prey and prey have alternative predators, so the community is more resilient — high biodiversity supports stability.

In stable communities the numbers of predators and prey do not stay perfectly constant — instead they oscillate in a regular pattern that is out of phase with each other.

The cycle:

1. Prey numbers rise (lots of food, few predators).
2. Predator numbers rise later (lots of food = lots of prey to eat → more predators survive and reproduce).
3. Prey numbers fall (too many predators eating them).
4. Predator numbers fall later (less food, predators starve or fail to breed).
5. Cycle repeats.

The classic textbook case is the lynx and snowshoe hare in Canadian forests, where Hudson's Bay Company fur records show this cycle running over more than a century. UK examples include owl and field vole populations on rough grassland.

Key features for the exam:

• Predator and prey curves are out of phase — predator peaks come AFTER prey peaks.
• Predator peak is usually SMALLER than prey peak (you always need many prey to support a few predators — a pyramid of biomass).
• Cycles persist as long as the community is stable; they break down if other factors (disease, habitat loss) intervene.

Why this happens. Predators don't immediately follow prey — there's a time lag for predator populations to respond to changes in prey availability through breeding success.

Energy keeps flowing INTO ecosystems from the Sun. Materials, however, are limited — Earth has a fixed amount of water, carbon, nitrogen and mineral ions. So ecosystems must recycle the materials they use, again and again.

The big idea is that every element a plant takes from the soil or air eventually returns when the plant — and the animals that eat it — die and are broken down. Without recycling, soils would be stripped bare, CO₂ in the atmosphere would run out (or pile up), and life would grind to a halt.

Key recycled materials on the AQA spec:

• Water (water cycle).
• Carbon (carbon cycle).
• AQA does not require the nitrogen cycle for GCSE Biology 8461, though nitrate ions from soil are referenced.

The driver of all recycling is decomposition by bacteria and fungi, plus the chemical and physical processes (evaporation, photosynthesis, respiration) that move atoms between living and non-living stores.

The water cycle moves water continuously between the oceans, atmosphere, land and living things. Almost all of it is driven by solar energy.

Stages of the water cycle:

1. Evaporation. Heat from the Sun causes liquid water in oceans, rivers and lakes to change into water vapour (a gas).
2. Transpiration. Plants lose water vapour from the underside of their leaves through stomata — significant on a global scale.
3. Condensation. Water vapour rises into the atmosphere, cools, and condenses back into tiny liquid droplets — forming clouds.
4. Precipitation. Droplets coalesce until they are heavy enough to fall as rain, snow, hail or sleet.
5. Runoff and groundwater. Water flows back to rivers, lakes and oceans, or soaks into the soil to be taken up by plant roots — and the cycle begins again.

Why the water cycle matters for organisms:

• It supplies fresh, drinkable water to terrestrial habitats.
• It is the main way water gets transported from oceans to inland UK regions.
• The cycle redistributes thermal energy around the planet (a separate physics story).

The carbon cycle moves carbon between the atmosphere (as CO₂), living things (as organic molecules), the soil (as decomposed material) and fossil fuels.

The four AQA processes you must name:

1. Photosynthesis (CO₂ → organic carbon). Plants and algae take CO₂ from the air and use light energy to make glucose. Carbon is now stored in plant tissues.

2. Respiration (organic carbon → CO₂). All living things (plants, animals, microbes) respire, breaking down glucose for energy and releasing CO₂ back to the atmosphere.

3. Combustion (organic carbon → CO₂). Burning fuels — wood, coal, oil, natural gas — releases CO₂ into the air. Fossil fuels contain carbon that was fixed by photosynthesis millions of years ago.

4. Decomposition (organic carbon → CO₂). Decomposers (bacteria and fungi) feed on dead organisms and waste. They respire as they do so, returning CO₂ to the air and mineral ions to the soil.

Carbon stores (where carbon sits):

• Atmosphere — CO₂ (small but very important store).
• Living things — biomass (plants are the biggest living store).
• Fossil fuels — vast amounts of carbon locked away for millions of years.
• Oceans — dissolved CO₂ and shells.
• Sedimentary rocks — limestone formed from ancient shells.

Why this matters now. Burning fossil fuels releases carbon that was locked away for hundreds of millions of years, adding extra CO₂ to the atmosphere faster than photosynthesis can remove it. This is the main driver of climate change (covered more under 4.7.2.4).

Microorganisms — chiefly bacteria and fungi — drive the recycling of materials. Without them, nutrients would lock up in dead bodies and waste, and ecosystems would slowly run out of usable material.

What decomposers do:

• Secrete digestive enzymes onto dead organic matter (extracellular digestion).
• Absorb the small soluble products (sugars, amino acids, mineral ions).
• Respire aerobically, releasing CO₂ and water back to the atmosphere.
• Release nitrate, phosphate and other ions back into the soil for plants to absorb.

Why this matters in ecosystems:

• Plant roots only absorb water and dissolved mineral ions. Without decomposers freeing those ions from dead leaves, plants would starve.
• The carbon cycle relies on decomposers' respiration to return CO₂ to the air.
• UK ecosystems like deciduous woodland depend on the rapid leaf-litter breakdown by fungi every autumn.

Where the AQA Required Practical 8 fits. Field investigations (transects, quadrats, sweep nets) let you sample populations and link them back to abiotic factors and recycling — for example, comparing the rate of leaf litter decomposition in different soil pH conditions.

Conditions decomposers love — covered in detail in 4.7.2.3 — are warm, moist and well-oxygenated, with the right pH. UK gardeners build compost heaps to provide exactly these conditions.

Decomposers are bacteria and fungi that feed on dead organisms and waste. They:

• Secrete digestive enzymes onto the dead material (extracellular digestion).
• Absorb the small soluble products.
• Respire aerobically, releasing CO₂ and water.
• Release mineral ions (nitrate, phosphate, etc.) back into the soil for plants.

The rate of decomposition depends on how active the decomposers are. Three abiotic factors control that activity:

1. Temperature — warmer (within limits) = faster enzyme action and faster microbial growth.
2. Water (moisture) — needed for enzyme action and for microbes to grow.
3. Oxygen — needed for aerobic respiration. Most decomposers are aerobic.

Why this is exam-friendly. You can predict decomposition rate from the conditions, and you can design experiments to test it. Real-world applications (compost heaps, food storage, biogas) link the theory to everyday life.

Temperature. As temperature rises, enzymes work faster (more kinetic energy → more successful collisions with substrates) and microbes reproduce faster. But above the optimum (around 40–50 °C for most decomposer enzymes), the enzymes start to denature — their active sites change shape and the rate plunges. So decomposition rate has a typical bell-shaped curve against temperature.

Water. Microbes need water to:

• Carry out enzyme-substrate reactions (most enzymes work in solution).
• Maintain cell volume (no water = no living cell).
• Transport nutrients into the cell.

Dry conditions stop decomposition almost completely — which is why dried foods (jerky, raisins, biscuits) keep for months without going off. Egyptian mummies, peat-bog bodies and dried herbs all survive thousands of years because the absence of water (or the wrong oxygen conditions) prevents decay.

Oxygen. Most decomposers respire aerobically, so they need oxygen for ATP production. In well-oxygenated soil or compost, decomposition is rapid. Without oxygen, only anaerobic decomposers can work — these are slower and produce different end products including methane (a useful fuel, but also a greenhouse gas) and organic acids (which create the 'rotten food' smell).

Compost heaps (speeding decay up).

• Gardeners pile up grass cuttings, vegetable peelings and leaves.
• The pile is kept moist (water for microbes).
• It's turned regularly (lets oxygen in for aerobic decomposers).
• It heats up because microbial respiration releases heat — internal temperatures of 40–60 °C accelerate decay.
• After weeks to months, you get nutrient-rich compost for the garden.

Food preservation (slowing decay down). Remove what decomposers need.

• Freezing — too cold for enzyme action and microbial growth.
• Drying / dehydrating — removes water; jerky, raisins, dried pasta last for years.
• Vacuum-packing — removes oxygen; slows aerobic decomposers.
• Salting / pickling — too salty / too acidic; microbes can't grow.
• Tinning — heat sterilises, then seal keeps air out.
• Refrigeration — slows enzyme action and microbial growth (doesn't stop it).

Anaerobic digesters / biogas generators.

• Without oxygen, decomposers produce methane (CH₄) and CO₂.
• Methane is a useful fuel (used for heating, cooking and electricity).
• Farms use animal slurry; sewage works use treated waste; landfill sites capture methane.
• UK biogas plants are growing as a renewable energy source.

Note on global warming. Methane is also a powerful greenhouse gas. Cattle and rice paddies are major natural sources — climate change links back to anaerobic decomposition.

AQA's Biology-only required-practical link expects you to be able to investigate the effect of temperature on the rate of decomposition.

Common method — pH change in milk decomposed by lipase (or with naturally occurring bacteria).

• Fresh milk is roughly neutral (pH ~6.5).
• As fats are broken down by lipase (an enzyme from decomposers or added directly), fatty acids are released and the pH falls.
• Measure pH at intervals (with universal indicator or a pH probe) at different temperatures (e.g. 10, 20, 30, 40, 50 °C).
• Plot pH against time, find the time taken for pH to fall to a given value (e.g. 5.0), and calculate rate = 1 / time.

Alternative — mass loss of fruit.

• Weigh equal pieces of fresh fruit, place in identical sealed containers.
• Keep at different temperatures.
• Reweigh daily and plot mass against time.
• Faster mass loss = faster decay.

Controls:

• Same mass / volume of food in each tube.
• Same starting microbial load (same source).
• Same surface area exposed.
• Only temperature varies (independent variable).
• Repeat readings — calculate means.

Calculation example. If milk pH falls from 6.5 to 5.0 in 12 minutes at 30 °C, rate = 1.5 / 12 = 0.125 pH units per minute (or rate = 1 / 12 = 0.083 min⁻¹ if expressing as 1 / time).

Environmental change is any shift in the abiotic factors of a habitat. AQA highlights three factors:

1. Temperature.
2. Water availability (rainfall, drought, sea level).
3. Atmospheric gas composition (CO₂, methane, oxygen, pollutants).

Three causes of environmental change:

(a) Seasonal change. In the UK, temperature, day length and water all change between winter and summer. Many species cope by:

• Migration (swallows leave for Africa in autumn).
• Hibernation (hedgehogs, dormice, bats).
• Annual life cycles (annual plants die back, leaving seeds).

(b) Geographic variation. Move from sea level to mountain top and temperature drops about 6 °C per 1000 m; water and oxygen also change. Move from south to north Britain and the climate cools. Different species suit different zones.

(c) Human activity. This is the big new factor. Activities that change the environment include:

• Burning fossil fuels → extra atmospheric CO₂ → climate change.
• Deforestation → less photosynthesis, more soil erosion, local rainfall changes.
• Industrial pollution → SO₂ and NOₓ → acid rain (still a problem near some power stations).
• Agriculture → fertiliser runoff → algal blooms in rivers and the sea.
• Urbanisation → habitat destruction.

The combination of all these means species are facing changes that are faster and larger than natural cycles.

When abiotic conditions in a habitat change, organisms respond in one of three ways:

1. Move (range shifts). Species follow their preferred conditions to new areas.

• UK butterflies like the speckled wood and the comma are spreading northwards as the climate warms.
• Some bird species (e.g. little egret) have moved INTO the UK from southern Europe.
• Mountain plants in the Cairngorms are forced to higher altitudes as the climate warms — eventually they run out of mountain.

2. Adapt (evolutionary change). Over many generations natural selection can adapt species to new conditions — but only if the change is slow enough. Rapid human-caused change often outpaces evolution.

3. Die out locally or globally. Species that cannot move or adapt fast enough may go extinct. The IUCN Red List tracks species at risk worldwide.

UK climate-change case studies:

• Red squirrels — pushed northward and into isolated strongholds by warming and by interspecific competition with greys. Range continues to shrink.
• Atlantic cod in UK waters — moving northward as the North Sea warms; UK cod fisheries are smaller than 30 years ago.
• Adders — UK populations declining in part due to milder winters disrupting hibernation patterns.
• Bumblebees — some species (e.g. the great yellow bumblebee) only survive in cooler, wetter Scottish Highlands now.

Why this matters. Changing distributions affect interdependence: pollinators, pests, predators and prey move at different rates, so existing communities are pulled apart and new ones form.

How we know environments are changing.

• Long-term temperature records (e.g. Central England Temperature series, going back to 1659).
• Phenology data — recording when species flower or migrate. UK records show many spring events are happening earlier (oak leafing, frog spawning, swallow arrival).
• Distribution surveys — UK Butterfly Monitoring Scheme, BTO bird surveys, Marine Conservation Society fish records all show range shifts.
• CO₂ measurements — Mauna Loa (Hawaii) and other stations show atmospheric CO₂ has risen from ~280 ppm pre-industrial to over 420 ppm now.

What can be done.

• Reduce greenhouse gas emissions (renewable energy, energy efficiency, low-carbon transport).
• Protect and restore habitats — Sites of Special Scientific Interest (SSSIs), National Parks, rewilding projects (Knepp Estate in Sussex is a UK example).
• Wildlife corridors — let species move between protected areas as climate shifts.
• Captive breeding and reintroduction — UK successes include red kites, pine martens, beavers.
• International agreements — Paris Agreement, biodiversity conventions.

Why GCSE students study this. Examiners ask you to interpret data (a graph of CO₂ over time, a map of species range shift) and explain it using ecology. Make sure you can connect a change in an abiotic factor to a specific effect on a named species.