9.2.1 Energy transfer between trophic levels

Energy is lost between trophic levels

1. A trophic level is a feeding position in a food chain, with producers at trophic level $1$.
2. Only energy stored in an organism's new biomass can be transferred to the next trophic level when that biomass is eaten.
3. Some organisms or body parts are not eaten, so the chemical energy in that material is not transferred to the consumer.
4. Some eaten material cannot be digested and is egested in faeces, so its chemical energy is not absorbed into the consumer's body.
5. Some absorbed substances are excreted, including nitrogen-containing waste in urine, so this material cannot become food for the next trophic level.
6. Respiration transfers chemical energy from food for movement, active transport, growth, repair and temperature control, and much of this energy eventually spreads to the surroundings as heat.

Less energy supports fewer organisms

1. Each higher trophic level receives less usable chemical energy, so it can build less new biomass.
2. Less biomass provides food for fewer individuals, which is why populations are usually smaller at higher trophic levels.
3. Top predators need many prey organisms across a large feeding area because so little of the original energy reaches them.
4. After several transfers, too little energy remains to support another stable consumer population, which limits most food chains to a small number of trophic levels.

Biomass pyramids narrow upwards

1. A pyramid of biomass uses horizontal bars whose widths are proportional to the total dry mass of organisms at each trophic level.
2. The producer bar is placed at the bottom, followed by primary, secondary and higher consumers in feeding order.
3. Biomass normally decreases at each transfer because only part of the material at one level becomes new biomass at the next level.
4. The bars therefore become narrower towards the top, showing why less living material and usually fewer organisms can be supported.

Pyramid diagrams must match the data

1. Draw separate horizontal bars rather than a triangle, because each bar must show the biomass of one trophic level.
2. Label the producer at the bottom and arrange every consumer above it in the order shown by the food chain.
3. Use the numerical values to set the relative widths instead of drawing equally spaced steps.

Transfer rates are not fixed

1. The often-used $10\%$ transfer figure is a rough model, not a rule that applies to every ecosystem or every trophic level.
2. Transfer depends on factors such as how much of each organism is eaten, how much is digestible and how much energy the organisms use in respiration.
3. Use values supplied in a question when explaining a particular food chain instead of assuming that every transfer is $10\%$.

9.2.2 Efficiency of energy transfers

Efficiency is the percentage transferred

1. Energy transfer efficiency compares the energy passed to the next trophic level with the energy available at the trophic level below.
2. Use the same energy unit in the numerator and denominator so the units cancel and the final answer is a percentage.
3. The general equation is $efficiency=\frac{amount\; transferred}{amount\; available}\times 100$.
4. For energy data, write it as $efficiency\; of\; energy\; transfer=\frac{energy\; at\; the\; next\; trophic\; level}{energy\; at\; the\; previous\; trophic\; level}\times 100$.

The denominator is the previous level

1. The amount transferred belongs on top of the fraction and the amount available before the transfer belongs underneath.
2. Dividing in the opposite order can produce a value above $100\%$, which is not possible for trophic transfer efficiency.
3. If the figures use different units, convert them before calculating, such as changing both values to $\mathrm{kJ}$ or both to $\mathrm{J}$.
4. A correct final value can receive full calculation marks, but showing the substitution makes an arithmetic slip easier to identify.

Biomass uses the same calculation

1. Biomass is usually measured as dry mass because water content changes and does not represent stored organic material fairly.
2. For biomass data, use $biomass\; transfer\; efficiency=\frac{biomass\; at\; the\; next\; trophic\; level}{biomass\; at\; the\; previous\; trophic\; level}\times 100$.
3. The biomass values may be totals or values per unit area, but both values in one calculation must use the same basis and units.

Percentages can find a missing value

1. Convert a percentage to a decimal before multiplying when the transferred amount is unknown.
2. For an efficiency of $8\%$, the decimal multiplier is $\frac{8}{100}=0.08$.
3. The transferred amount is found from $amount\; transferred=amount\; available\times \frac{efficiency}{100}$.

Use the data given

1. Read the labels carefully to identify the two adjacent trophic levels involved in the transfer.
2. Keep full calculator values during working and round only the final answer to the precision requested.
3. A transfer percentage is not the same as a percentage decrease, so use the numerator named in the efficiency equation.

Check the method and units

1. A sensible answer is smaller than $100\%$ because some energy and biomass fail to reach the next trophic level.
2. An energy answer needs an energy unit such as $\mathrm{kJ}$, whereas an efficiency answer needs $\%$.

9.2.3 Human interactions and biodiversity

Biodiversity changes when populations change

1. Biodiversity is reduced when human activity causes native populations to fall, removes species or simplifies food webs.
2. An interaction can have both benefits and costs, so its effect depends on how it changes habitats, competition, predation, disease and pollution.
3. A strong biological explanation follows a chain from the human action to a changed condition, then to a population change and finally to biodiversity.

Fish farming can help or harm

1. Fish farms can produce a predictable food supply and may reduce pressure on wild fish populations if fewer wild fish are caught for people to eat.
2. Controlled feeding, protection from predators and selective breeding can increase survival and yield.
3. High stocking density lets pathogens and parasites spread rapidly, and infections can pass to wild fish near open cages.
4. Uneaten feed and faeces add organic matter and mineral ions to the water, which can reduce oxygen concentration and change the species able to survive.
5. Medicines and chemicals used in farms can affect non-target organisms, while escaped farmed fish may compete or breed with wild populations.
6. Some farmed fish are fed fishmeal made from wild-caught fish, so farming can still reduce wild populations lower in the food web.

Introduced species alter competition

1. A non-indigenous species may be introduced deliberately for food, ornament or biological control, or arrive accidentally in cargo, ballast water or on equipment.
2. An introduced species can add a useful food source or control a pest, but it may become invasive if it spreads rapidly and causes ecological harm.
3. Without its usual predators or diseases, its population may grow quickly and compete with native species for food, light, space, oxygen or breeding sites.
4. It may eat native organisms, carry new pathogens or alter habitats, causing native populations to decline and disrupting food webs.
5. If native species disappear locally, species richness and biodiversity decrease.

Eutrophication removes oxygen

1. Nitrate or phosphate ions from fertiliser runoff, animal waste or sewage enter a pond, lake or slow-moving river.
2. The extra mineral ions remove a growth limitation, so algae reproduce rapidly and form an algal bloom near the surface.
3. The dense algal growth blocks light, so submerged plants photosynthesise less and may die.
4. Dead algae and plants are broken down by decomposer microorganisms.
5. The decomposers respire aerobically and use dissolved oxygen faster than it is replaced.
6. The oxygen concentration falls, so fish and other oxygen-dependent animals may suffocate, move away or die.
7. The loss of sensitive species and dominance of a few tolerant species reduce biodiversity.

Balance benefits against biodiversity costs

1. A valid judgement identifies the benefit of the human activity and weighs it against the size, likelihood and reversibility of its ecological effects.
2. Management can reduce damage through lower fish stocking densities, waste treatment, secure cages, quarantine and controls on releasing non-indigenous species.
3. The conclusion should be based on the evidence in the question because the same interaction can have different effects in different ecosystems.

9.2.4 Benefits of maintaining biodiversity

Biodiversity keeps ecosystems functioning

1. High biodiversity gives a food web more feeding links, so the loss of one population is less likely to collapse the whole ecosystem.
2. Different species provide pollination, decomposition, nutrient cycling, soil formation, water purification and control of pest populations.
3. Genetic variation within wild populations provides alleles that may help species survive disease or environmental change.
4. Wild species are possible sources of foods, medicines and genes for breeding disease-resistant or climate-tolerant crops and livestock.
5. Habitats and wildlife also support recreation, tourism, scientific study and cultural value.

Animal conservation prevents extinction

1. Protecting and restoring habitats gives animals food, shelter, breeding sites and space to maintain viable populations.
2. Legal controls can restrict hunting, collection and trade, while protected areas reduce disturbance and habitat destruction.
3. Captive-breeding programmes pair individuals to retain genetic variation, then reintroduce offspring when threats in the wild have been reduced.
4. Monitoring population size and genetic diversity shows whether conservation action is working and whether inbreeding is becoming a risk.
5. Preventing extinction keeps the conserved species in its food web as a predator, prey, competitor, pollinator or seed disperser.

Reforestation rebuilds habitats

1. Planting a mixture of native tree species creates varied food sources and habitats for microorganisms, plants and animals.
2. As the woodland develops, more niches become available and local species richness can increase.
3. Trees remove carbon dioxide from the atmosphere during photosynthesis and store carbon in wood, roots and soil organic matter.
4. Photosynthesis releases oxygen, so reforestation can lower atmospheric carbon dioxide and raise oxygen over time.
5. Roots bind soil and tree canopies reduce the force of rain, which limits erosion and protects soil habitats and nearby aquatic ecosystems from sediment.
6. Connected woodland can form wildlife corridors, allowing animals to move between populations and maintain gene flow.

Conservation works at two scales

1. Local action protects particular habitats and populations, while coordinated action across countries protects migratory species, shared seas and global gene pools.
2. Maintaining biodiversity requires continued habitat management because protected populations can decline again if the original pressure returns.

9.2.5 Biological factors affecting food security

Food security depends on supply and access

1. Food security exists when a population has reliable access to enough safe, nutritious and affordable food for a healthy diet.
2. A factor lowers food security if it reduces the amount or variety of food, interrupts supply or raises prices beyond what people can afford.
3. Biological explanations should connect the factor to crop or livestock yield and then to availability, variety, price or access.

Population growth increases demand

1. A larger human population requires more total energy, protein, vitamins, minerals and water from the food system.
2. If food production grows more slowly than the population, the amount available per person falls.
3. More farmland and irrigation may be needed, which increases competition for land and fresh water and can damage habitats or soils.
4. Shortages raise prices and make reliable access hardest for people with low incomes.

Meat and fish use more resources

1. Rising meat consumption increases animal farming, so more crops, land and water are used to feed livestock rather than people directly.
2. Energy and biomass are lost between trophic levels, so feeding crops to animals produces less human food energy than eating suitable crops directly.
3. Livestock also require housing, temperature control, medicines and waste management, which increase agricultural inputs and production costs.
4. Greater demand for fish can cause overfishing, reducing breeding populations and making future catches smaller.
5. Fish farming can increase supply, but it requires feed and careful control of disease, waste and escapees to remain sustainable.

Pests and pathogens cut yields

1. Pests eat crops, damage stored food or compete with livestock, so less usable food reaches people.
2. Plant pathogens reduce photosynthesis, growth or reproduction, while animal pathogens reduce growth, fertility and survival.
3. A new pathogen can spread quickly if crops or livestock lack resistance, especially where many genetically similar individuals are kept together.
4. Diseased animals may need to be isolated or culled, and infected crops may need to be destroyed, which reduces supply immediately.
5. Pesticides, medicines, resistant varieties and biosecurity can limit losses, but they add costs and may become less effective as resistance evolves.

Environmental change reduces harvests

1. Human greenhouse-gas emissions increase global temperatures and alter rainfall patterns, raising the risk of drought, flooding and heat stress.
2. Drought closes stomata and limits photosynthesis, while flooding reduces oxygen around roots and can kill crop plants.
3. Warmer conditions can shift the ranges of pests, pathogens and pollinators, exposing farms to new diseases or reducing pollination.
4. Sea-level rise can make coastal soils and irrigation water saltier, reducing water uptake and crop growth.
5. More frequent crop failures make supply less reliable and increase price variation.

Biofuels compete with food crops

1. Land used to grow maize, sugar cane or oilseed for biofuel cannot produce food at the same time.
2. Diverting edible crops into fuel reduces the food entering markets and can raise prices.
3. Clearing new land for fuel crops can remove habitats, release stored carbon and reduce soil quality, which threatens future food production.
4. Waste plant material can make biofuel with less direct competition for fertile farmland, but collection and processing still require energy and money.

Inputs can limit production

1. Fertilisers replace mineral ions removed in harvests, pesticides reduce losses, and fuel and machinery allow large areas to be farmed.
2. Feed, seed, water, labour, medicines, fertiliser, pesticides, energy and machinery all add to the cost of producing food.
3. If input prices rise, farmers may use less of them, which can reduce yield, or pass the cost to consumers through higher food prices.
4. Poorer farmers may be unable to buy high-quality seed, irrigation or disease control, so pests, drought or nutrient shortage cause larger losses.
5. Overusing inputs can also damage long-term production through soil degradation, water pollution, pesticide resistance and loss of pollinators.

Sustainable choices protect future supply

1. Sustainable food production maintains soil, water supplies, pollinators, wild fish stocks and genetic diversity so production can continue in future years.
2. Crop rotation, resistant varieties, efficient irrigation, careful fertiliser use and managed fishing quotas can reduce losses without exhausting the resource base.
3. A method is not sustainable if it raises today's yield by causing soil erosion, resistant pests, collapsed fish stocks or permanent habitat loss.
4. The best response to a food-security question weighs increased present supply against cost, environmental damage and the ability to keep producing food.