20. Human influences on ecosystems
- Syllabus
- 0610–2026–2027
- Section
- 20
- Level
- —
Exam analysis
Most tested topics
20.1 Food supply
- Syllabus
- 0610–2026–2027
- Topic
- 20.1
- Level
- —
Explain five ways humans increase food production
Each method increases production by expanding workable land, improving efficiency, supplying crop resources, reducing losses or selecting more productive organisms.
| Method | How it increases food production |
|---|---|
| agricultural machinery | allows larger areas of land to be cultivated and makes farm work more efficient |
| chemical fertilisers | supply mineral ions needed for plant growth, increasing crop yield |
| insecticides | kill insect pests, reducing crop damage and improving quality and yield |
| herbicides | kill weeds, reducing competition with crops for resources |
| selective breeding | produces crop plants or livestock with chosen features that improve production |
For each method, link the action to its production effect: method → biological or practical change → higher quality, yield or efficiency.
Keep this card to the five named methods. Insecticides target insect pests; herbicides target weeds; fertilisers supply mineral ions rather than killing pests.
Weigh large-scale crop monocultures
A monoculture is the large-scale growth of one crop species over an area.
| Advantages | Disadvantages |
|---|---|
| specialist machinery can be used efficiently | habitats and biodiversity are reduced |
| planting, treatment and harvesting are simpler and more uniform | pests or disease can spread rapidly through a similar crop |
| a large, consistent crop can give a high yield | soil nutrients may be depleted and soil erosion may increase |
A complete comparison links scale and uniformity to both sides: they improve efficient production, but reduce biological variety and make one threat capable of damaging much of the crop.
Monoculture means one crop species, not one individual plant. Increased yield is an advantage; increased vulnerability to pests or disease is a disadvantage.
Weigh intensive livestock production
Intensive livestock production keeps many animals under controlled conditions to maximise food output efficiently.
| Advantages | Disadvantages |
|---|---|
| high and consistent yield | disease and parasites can spread rapidly in crowded conditions |
| efficient use of land and farm resources | waste can pollute water and contribute to eutrophication |
| food can be produced more cheaply and reliably | feed and energy inputs can be costly, and antibiotic use can select resistant bacteria |
Assess intensive production by balancing output and resource efficiency against crowding, waste, disease and input costs.
Do not treat intensive production as automatically better or worse. State a specific advantage or disadvantage and explain the link to production, animals or the environment.
20.2 Habitat destruction
- Syllabus
- 0610–2026–2027
- Topic
- 20.2
- Level
- —
Define biodiversity by species count
Biodiversity is the number of different species that live in an area.
Count different species, not the total number of individual organisms. An area with more species has greater biodiversity by this definition.
Biodiversity includes all species in the area, not only plants or only animals, and it is not the population size of one species.
Explain why humans destroy habitats
Habitats are destroyed when land or water is changed so that its organisms can no longer live there as before.
| Human reason | How habitat is destroyed |
|---|---|
| housing | land is cleared or built over |
| crop production | natural habitat is replaced by farmland |
| livestock production | land is cleared for grazing or animal facilities |
| extraction of natural resources | logging, mining or removal of materials damages the habitat |
| freshwater and marine pollution | pollutants change water conditions and harm organisms |
State both the human use and the habitat change. The required reasons cover land expansion, resource extraction and water pollution.
Trace habitat damage through a food web
Humans can damage habitats by altering food chains and food webs.
If human activity reduces or removes one species, organisms that feed on it have less food, while organisms it consumed may increase. These changes spread through connected feeding relationships.
The resulting population changes can disrupt energy transfer, reduce biodiversity and make the habitat less able to support its original community.
Do not stop at 'the food chain changes'. Name the affected organism and trace at least one consequence to another trophic level.
Explain five effects of deforestation
Deforestation removes trees and the habitat, roots and photosynthetic tissue they provide.
| Required effect | Causal explanation |
|---|---|
| reduced biodiversity | habitat and food sources are lost, so fewer species can survive |
| extinction | populations may fall to zero when organisms cannot move or adapt |
| loss of soil | without roots binding soil and canopy protection, erosion by water or wind increases |
| flooding | less interception and water uptake increases surface run-off |
| more atmospheric carbon dioxide | less photosynthesis removes less carbon dioxide; burning or decomposition of felled trees can release more |
These effects reinforce one another: soil and flood damage further reduce habitat quality, while biodiversity loss disrupts food webs.
Deforestation increases, rather than decreases, atmospheric carbon dioxide and flood risk. Explain each direction through a changed process.
20.3 Pollution
- Syllabus
- 0610–2026–2027
- Topic
- 20.3
- Level
- —
Compare sewage and fertiliser pollution in water
Untreated sewage and excess fertiliser both alter aquatic ecosystems and can reduce dissolved oxygen, but they enter the system differently.
| Pollutant | Initial change | Effects on the ecosystem |
|---|---|---|
| untreated sewage | adds organic waste, microorganisms and mineral ions | decomposers increase and respire aerobically; dissolved oxygen falls, so oxygen-requiring organisms may die; pathogens may spread |
| excess fertiliser | nitrate and other ions wash into the water | producers grow rapidly; later decomposition and aerobic respiration reduce dissolved oxygen, so oxygen-requiring organisms may die |
In both cases, connect the pollutant to biological activity, then to oxygen concentration and organism survival.
Fertiliser does not kill fish by directly using oxygen. The oxygen is used during increased aerobic respiration by decomposers.
Explain why persistent plastics harm ecosystems
Non-biodegradable plastics are not broken down by decomposers, so they persist and accumulate in aquatic and terrestrial ecosystems.
| Aquatic ecosystems | Terrestrial ecosystems |
|---|---|
| animals may become entangled, injured or unable to move and breathe | animals may become trapped or strangled |
| swallowed plastic can block the digestive system, reduce feeding and cause death | ingestion can block digestion and reduce nutrition |
| microplastics can enter food chains and pass to consumers | plastic and associated chemicals can accumulate in organisms and food chains |
| persistent litter damages habitats | persistent litter can cover or damage habitats and soil environments |
The key property is persistence: non-biodegradable does not mean harmless. Large pieces and microplastics can cause different but connected effects.
Link greenhouse gases to sources and climate change
Methane and carbon dioxide are air pollutants that increase the enhanced greenhouse effect and contribute to climate change.
| Gas | Important human-related sources |
|---|---|
| carbon dioxide | burning fossil fuels; deforestation through burning and reduced removal by photosynthesis |
| methane | cattle and other livestock; rice farming; decomposition of waste in landfill |
Higher greenhouse-gas concentrations cause more outgoing infrared radiation to be absorbed and re-radiated in the atmosphere. Less energy escapes to space, strengthening the greenhouse effect and raising global temperatures.
The resulting climate change can alter rainfall and habitats, increase extreme conditions, melt ice and raise sea level.
The natural greenhouse effect makes Earth warm enough for life; pollution enhances it. Weather is a short-term condition, while climate change is a long-term shift in patterns.
Trace the six steps of eutrophication
Eutrophication is a causal sequence in which extra mineral ions ultimately reduce dissolved oxygen in water.
- The availability of nitrate and other ions increases.
- Producers grow more rapidly.
- After producers die, their decomposition increases.
- Decomposers carry out more aerobic respiration.
- The concentration of dissolved oxygen decreases.
- Organisms that require dissolved oxygen die.
The link between steps 3 and 5 is respiration: decomposers use dissolved oxygen while breaking down dead producers.
Keep the order exact. Producers first grow, then die and are decomposed; decomposers—not nitrate ions directly—remove the dissolved oxygen through aerobic respiration.
20.4 Conservation
- Syllabus
- 0610–2026–2027
- Topic
- 20.4
- Level
- —
Define a sustainable resource by replacement rate
A sustainable resource is produced as rapidly as it is removed from the environment, so it does not run out.
The test is a rate comparison: replacement rate must be at least equal to removal rate. If removal is faster, the stock decreases and use is not sustainable.
A resource is not sustainable merely because it is natural or causes little pollution. Its rate of replacement must match its rate of use.
Recognise forests and fish stocks as manageable resources
Forests and fish stocks are biological resources that can be conserved and managed sustainably.
| Resource | Basis for sustainable management |
|---|---|
| forests | trees can regrow or be replanted while removal is controlled |
| fish stocks | fish reproduce while catches are controlled so the population can replace removals |
Sustainable management does not mean no use. It means controlling use so replacement can keep pace with removal.
Explain six causes of endangerment and extinction
A species becomes endangered when its population falls so low that extinction becomes likely; extinction occurs when no members remain.
| Cause | How it lowers population survival |
|---|---|
| climate change | conditions and habitats shift faster than some species can adapt or move |
| habitat destruction | food, shelter and breeding sites are removed |
| hunting | individuals are killed faster than they are replaced |
| overharvesting | too many organisms are removed for populations to recover |
| pollution | toxins or changed environmental conditions reduce survival or reproduction |
| introduced species | new predators, competitors or pathogens harm native populations |
A cause must be linked to reduced survival or reproduction. Endangered means at high risk; extinct means the species has no living individuals.
Match endangered-species conservation methods to roles
Conservation combines information, protection, behaviour change and stored or captive populations.
| Method | Conservation role |
|---|---|
| monitoring species and habitats | reveals population trends, locations and threats |
| protecting species and habitats | reduces killing and preserves food, shelter and breeding sites |
| education | changes behaviour and builds support for sustainable practices |
| captive breeding programmes | increase numbers under protection and can supply individuals for reintroduction |
| seed banks | store seeds from plant species for future growth and preserve genetic material |
Captive breeding and seed banks support wild conservation; they do not remove the need to protect the original habitat.
Explain four ways to conserve forests
Forest conservation controls removal, protects habitat and replaces harvested trees.
| Method | How it conserves forests |
|---|---|
| education | reduces damaging practices and supports sustainable use |
| protected areas | restrict clearance and safeguard habitats |
| quotas | limit the amount of timber removed so regrowth can keep pace |
| replanting | replaces harvested trees and restores forest cover |
Replanting alone is insufficient if trees are still removed faster than they grow. Conservation depends on replacement and controlled removal together.
Explain six ways to conserve fish stocks
Fish-stock management must leave enough breeding adults and young fish for reproduction to replace the catch.
| Method | How it protects the stock |
|---|---|
| education | reduces demand for unsustainable catches and improves compliance |
| closed seasons | prevents fishing during breeding or recovery periods |
| protected areas | provide places where fish can feed, breed and grow without capture |
| controlled net types and mesh size | reduce by-catch and allow small, immature fish to escape |
| quotas | limit the number or mass removed |
| monitoring | measures stock size and catch so controls can be adjusted |
A larger minimum mesh size lets small fish escape; reducing mesh size would catch more immature fish and undermine replacement.
Explain why conservation programmes matter
Conservation protects both species and the ecosystem functions and resources on which organisms and people depend.
| Reason | What is maintained |
|---|---|
| maintain or increase biodiversity | the number of species and their interactions |
| reduce extinction | populations and species that would otherwise be lost |
| protect vulnerable ecosystems | habitats that are easily damaged or slow to recover |
| maintain nutrient cycling | movement and reuse of mineral nutrients through ecosystems |
| maintain resource provision | supplies of food, drugs, fuel and useful genes |
Conservation is not only about saving one visible species. Protecting ecosystems can preserve many species, interactions and resources together.
Compare AI and IVF in captive breeding
Artificial insemination and in vitro fertilisation help selected captive animals reproduce without natural mating.
| Artificial insemination (AI) | In vitro fertilisation (IVF) |
|---|---|
| sperm is collected, checked or stored, then inserted into the female reproductive system at a fertile time | eggs and sperm are collected and fertilisation occurs outside the body |
| fertilisation occurs inside the female | embryos begin developing in culture before transfer to a female's uterus |
| semen from selected or distant males can be used | embryos can be selected, stored or transferred between breeding programmes |
Both methods can increase breeding opportunities and help managers choose parents from different populations to preserve genetic variation.
AI places sperm into the female; it does not create an embryo outside the body. IVF creates embryos outside the body before implantation.
Explain the genetic risks of a shrinking population
When population size decreases, fewer alleles remain and genetic variation is reduced.
A small population has fewer possible mates, so related individuals are more likely to breed. Inbreeding increases the chance that harmful recessive alleles are inherited together and expressed.
With less variation, fewer individuals may have features that help them survive a new disease, environmental change or other threat. Reproduction may also become difficult, so extinction risk rises further.
The required explanation is reduced genetic variation and its consequences; knowledge of genetic drift is not required.