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Topic 20 · 20.1–20.4

Human influences on ecosystems

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe how humans have increased food production; give the advantages and disadvantages of monocultures and intensive livestock production.
  • Describe biodiversity and the reasons for habitat destruction; explain the undesirable effects of deforestation.
  • Describe the effects of sewage, fertilisers, plastics, methane and carbon dioxide; explain eutrophication EXTENDED.
  • Describe sustainable resources and the conservation of endangered species; explain how forests and fish stocks are conserved, and the risks of a small population EXTENDED.

📚The biology

Food supply

Humans have increased food production using:

agricultural machinery: to farm larger areas of land and improve efficiency
chemical fertilisers: supply mineral ions such as nitrate, to improve yields
insecticides: kill insect pests, to improve quality and yield
herbicides: kill weeds, to reduce competition with the crop
selective breeding: to improve production by crop plants and livestock (topic 18)

A monoculture is a large area growing a single crop (oil palm, wheat, rice).

  • Advantages: machinery can plant, spray and harvest efficiently; high yield of one product; cheap food.
  • Disadvantages: low biodiversity (habitats removed); pests and diseases spread quickly through one species, so more pesticide is used; the same minerals are removed from the soil every year, so more fertiliser is needed; fewer jobs for local people.

Intensive livestock production keeps many animals (chickens, pigs, cattle) indoors in a small space, with controlled food and temperature.

  • Advantages: less energy lost in movement and keeping warm, so more of the food becomes meat, milk or eggs; cheaper products; less land used.
  • Disadvantages: disease spreads easily, so antibiotics are used (resistant bacteria, topic 15); animal welfare concerns; large amounts of waste may pollute rivers; much crop land is needed to grow their feed; livestock produce methane.

Habitat destruction

Biodiversity is the number of different species that live in an area. Habitats are destroyed to make space for housing, crop plants and livestock; by the extraction of natural resources (mining, logging); and by freshwater and marine pollution. By altering food chains and webs (removing a species, or introducing one), humans can also harm habitats.

Deforestation leads to five effects: reduced biodiversity, because there are fewer habitats and food sources; extinction of species found only in that forest; loss of soil, because no roots hold it and rain washes it away; flooding, because less water is taken up and transpired and run-off is faster; and more carbon dioxide in the air, because there is less photosynthesis and burning and decay release carbon dioxide.
The undesirable effects of deforestation — the example of habitat destruction you must be able to explain.

Pollution

Untreated sewage and excess fertiliser entering rivers and lakes add nutrients (nitrate, phosphate) and cause eutrophication; sewage also carries pathogens (e.g. cholera). EXTENDED The process of eutrophication:

Six steps in a cycle-shaped flow chart: 1, fertiliser or sewage adds nitrate and other ions; 2, producers such as algae grow rapidly; 3, algae block light so plants below cannot photosynthesise and die; 4, more decomposition, as decomposers multiply and respire aerobically; 5, dissolved oxygen in the water falls; 6, fish and other organisms that need oxygen die.
EXTENDED Learn the six steps in order: ions → producers → death → decomposition → respiration → oxygen falls.

Non-biodegradable plastics are not broken down by decomposers, so they persist. In water, animals become entangled or swallow plastic (turtles mistake bags for jellyfish) and starve; small fragments enter food chains. On land, plastic litter traps and chokes animals and can release toxic chemicals into the soil.

The Sun's light passes through the atmosphere and is absorbed by the Earth, which warms and emits infrared radiation. Carbon dioxide and methane in the atmosphere absorb some of the infrared and re-radiate it back to Earth; some escapes into space.
More carbon dioxide and methane trap more heat: the enhanced greenhouse effect.
carbon dioxide sources: combustion of fossil fuels (power stations, vehicles), deforestation (burning, and less photosynthesis)
methane sources: cattle and other ruminants, rice paddy fields, decomposition in landfill sites
effects: the enhanced greenhouse effect traps more heat, causing global warming and climate change — melting ice and rising sea levels, more extreme weather, changes to where species can live

Conservation

A sustainable resource is one which is produced as rapidly as it is removed from the environment, so that it does not run out. Forests and fish stocks can be conserved and managed sustainably.

Organisms become endangered or extinct because of climate change, habitat destruction, hunting, overharvesting, pollution and introduced species. Endangered species can be conserved by monitoring and protecting species and habitats, education, captive breeding programmes and seed banks.

EXTENDED forests: education; protected areas (national parks); quotas on the trees felled; replanting
EXTENDED fish stocks: education; closed seasons (no fishing while fish breed); protected areas; controlled net types and mesh size (young fish escape to breed); quotas; monitoring of stocks

EXTENDED Why conserve? To maintain or increase biodiversity, reduce extinction, protect vulnerable ecosystems, and maintain ecosystem functions: nutrient cycling, and resources such as food, drugs, fuel and genes. Captive breeding programmes may use artificial insemination (sperm from a male, perhaps in another zoo, is placed in the female) and in vitro fertilisation (eggs fertilised outside the body and the embryo placed in a female, possibly of a related species), so that animals that will not or cannot mate still reproduce, and genes are mixed between populations.

A large population shown as a circle with many dots in six colours, each colour an allele. After a population crash, a small circle holds six dots in only two colours. The risks: less genetic variation, all may share a weakness, and the population is less able to adapt to new diseases or a changing climate.
EXTENDED A population that shrinks loses alleles, and with them the variation natural selection needs.

✏️Worked example

Untreated sewage enters a river 2 km along its course. The graph shows the dissolved oxygen and the number of bacteria along the river. (a) Calculate the percentage decrease in dissolved oxygen between 0 km and the lowest point. [2] (b) Explain why the number of bacteria rises after the outlet. [2] (c) EXTENDED Explain why the dissolved oxygen falls, and why it rises again further downstream. [4]
A graph against distance along a river from 0 to 12 kilometres. Dissolved oxygen is 9 milligrams per cubic decimetre up to the sewage outlet at 2 kilometres, falls to a minimum of 3 at 4 kilometres, then rises again to about 8 at 12 kilometres. Bacteria are low before the outlet, peak at 3.5 kilometres and fall back to a low level by 10 kilometres.
Illustrative data.

(a) Oxygen falls from 9.0 to 3.0 mg/dm3: \( \tfrac{9.0 - 3.0}{9.0} \times 100 = 67\% \) (66.7%).

(b) Sewage provides organic matter as food for the bacteria (decomposers), so they reproduce rapidly.

(c) The bacteria respire aerobically as they decompose the sewage, using up dissolved oxygen faster than it is replaced. Further downstream the sewage has been broken down, so there are fewer bacteria using oxygen, and oxygen dissolves from the air and is released by photosynthesis of plants and algae.

Check it. Percentage change is always divided by the starting value (9.0), not the final one; dividing by 3.0 would give 200%.
“The algae use up the oxygen.” Algae photosynthesise and release oxygen while they live. The oxygen is used by the bacteria decomposing dead matter.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which gas is released by rice paddy fields and landfill sites and adds to the enhanced greenhouse effect? A: methane. B: nitrogen. C: oxygen. D: sulfur dioxide.
A.
2. (Theory.) Give one advantage and two disadvantages of growing oil palm as a monoculture. [3]
Advantage: efficient use of machinery / high yield of one crop / cheap product. Disadvantages: low biodiversity; pests and diseases spread quickly; the soil loses the same minerals each year, so more fertiliser is needed.
3. (Theory.) Explain why removing trees from a hillside can lead to flooding in a town below. [2]
Fewer roots take up water and less is transpired, and there is nothing to slow the rain, so more water runs off quickly into rivers (carrying soil that blocks them).
4. (Theory.) Describe two effects of discarded plastic bags on marine animals. [2]
Animals become entangled and cannot swim or feed; animals such as turtles swallow them (mistaking them for food), blocking the gut, and starve.
5. (Theory.) Define a sustainable resource, and describe two ways an endangered bird species could be conserved. [3]
A resource produced as rapidly as it is removed, so it does not run out. Any two: protect its habitat (a reserve); monitor numbers; captive breeding and release; educate local people; laws against hunting.
6. (Theory.) EXTENDED Explain how quotas and replanting help to conserve a forest. [2]
Quotas limit the number of trees felled, so no more are removed than can regrow; replanting replaces the trees that are cut, keeping the forest sustainable.
7. (Theory.) EXTENDED Give three reasons for conservation programmes. [3]
Any three: maintain or increase biodiversity; reduce extinction; protect vulnerable ecosystems; maintain nutrient cycling; keep resources such as food, drugs, fuel and genes.
8. (Theory.) EXTENDED Fewer than 80 Javan rhinos survive in the wild. Explain why their small population size puts the species at risk. [3]
Little genetic variation remains (few alleles); if a new disease or environmental change occurs, few or none may have alleles to survive it; the population is less able to adapt, so it may become extinct (inbreeding also brings harmful recessive alleles together).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • IUCN Red List — the status of endangered species worldwide
  • NASA Climate Kids — the greenhouse effect explained