Human impact: WJEC A level Biology revision notes

Lesson 1 of 21

What is the difference between an endangered species and an extinct one?

These notes cover topic 3.6, Human impact on the environment, in WJEC A level Biology Unit 3: Energy, and the Environment. Each lesson follows the WJEC specification, and the exam tips come from BioWelsh analysis of WJEC past papers.

A is when no individuals of that are alive anywhere in the world. A is when so few individuals remain that it is in danger of becoming . The difference matters: an can still be saved, an one cannot.

is not new. It has been happening since life began, and the record lets biologists estimate the , which is about one in every million each year, written as 10⁻⁶ per year. Human activity has pushed the rate far above this, to an estimated 1 000 to 10 000 times the background rate in tropical areas alone. Very large animals, known as , were among the first to go: a giant flightless bird in New Zealand and a giant ground sloth in South America both disappeared as humans spread, although changes in climate may have contributed. Geological evidence shows five in the history of life, when most were lost after a catastrophic change such as a fall in global temperature, a shortage of dissolved oxygen in the oceans or , and many biologists argue that a sixth is now under way, caused by human activity.

Two extinctions are recorded in detail because they happened recently. The passenger pigeon of North America, shown below, was once one of the most numerous birds on Earth and became in 1914. The thylacine, a marsupial carnivore from Tasmania, followed in 1936.

A museum specimen of a pigeon mounted on a small piece of wood against a plain pale wall, with a printed label just visible at the edge. The bird has a blue-grey head and back, a warm orange-pink breast, a red eye, dark spots on the folded wing and a long pointed tail. — A mounted specimen of a passenger pigeon, a North American bird that became extinct in 1914. Photo: James St. John, CC BY 2.0. Resized.

Both were hunted heavily, and both lost large areas of at the same time.
Passenger pigeons bred in enormous , so once numbers fell the survivors could no longer breed successfully.
Neither could be saved once the became very small, which is why aims to act while a is still only .

work needs an agreed way of saying how much trouble a is in. The () assesses plants and animals and places each one in a category, based on its numbers, its rate of decline and how widely it is distributed. The results are published in . The diagram below shows the scale.

Seven coloured boxes in a row, each holding a code with its name written underneath: LC least concern, NT near threatened, VU vulnerable, EN endangered, CR critically endangered, EW extinct in the wild and EX extinct. A red dashed outline surrounds VU, EN and CR and is labelled the three threatened categories. An arrow beneath the row runs from lower risk on the left to higher risk on the right. A note at the foot reads that species with too little information are rated DD, data deficient, those not yet assessed are rated NE, not evaluated, and that ratings are set from numbers, rate of decline and distribution and published in Red Data Lists. — The IUCN categories, from least concern to extinct. Original BioWelsh diagram.

(EX) no individuals survive; in the wild (EW) individuals survive only in captivity or as a cultivated .
The three threatened categories are critically (CR), (EN) and vulnerable (VU). These are the at real risk of .
Near threatened (NT) and least concern (LC) are not classed as threatened, although a near threatened is close to qualifying.
Data deficient (DD) there is not enough information to make an assessment, and not evaluated (NE) the has not yet been assessed. A in either group may still be at risk.
Two of similar animals can sit in different categories: African elephants are currently assessed as vulnerable, while Sumatran elephants are critically .

Lesson 2 of 21

Why do species become endangered?

become when the and factors they depend on are taken away, so that they reproduce less successfully and numbers fall. Human activity is now the main cause, and is the most important cause worldwide: forests are cleared for farming and timber, wetlands are drained for crops, and are removed to make fields large enough for modern machinery. The five groups below cover almost every example an exam question uses.

The main reasons species become endangered
Cause	How it puts a species at risk	Example
	Individuals less suited to the conditions reproduce less successfully. Human activity changes habitats faster than new allow to adapt	driven to faster than before humans had such influence
	Food, shelter and breeding sites are removed, so the falls	, drainage of wetlands and removal
Overhunting and	Individuals are removed faster than they can be replaced by reproduction	Trophy hunting, traditional medicines, the bushmeat trade and
	An competes with native for the same resources, or eats them, and may bring new disease	The grey squirrel outcompeting the red squirrel in the UK
Pollution	Toxic substances kill organisms directly or build up along food chains	Oil spills at sea and persistent industrial chemicals

matter more than their size suggests. The diagram below compares a landscape of small fields separated by with the same land after the have been removed.

Two panels side by side. The left panel, headed mixed fields with hedgerows, shows two green lines of bushes across a pale green field, labelled hedgerows between small fields, with a note that hedges give nest sites, food and shelter and act as corridors so animals can move between fields. The right panel, headed one large field one crop, shows the same area with no hedges and evenly spaced vertical rows of a single crop, labelled one crop across the whole field, with a note that there is one habitat only, so fewer plant and animal species, and that removing hedges also removes the corridors. A line underneath reads that larger fields suit large machinery but the loss of hedgerows lowers biodiversity at every trophic level. — How removing hedgerows changes farmland habitat. Original BioWelsh diagram.

A provides a for birds, reptiles, mammals and insects, with nesting sites, food and shelter.
Light intensity and water availability vary across a hedge, so many different plants can grow in it, and many different animals can live there.
act as , letting animals move between areas without crossing open ground, which helps to maintain biodiversity.
When are removed, this specialised is destroyed. Herbivores and other consumers fall in number, and so do the at higher that feed on them.

Changes in how a crop is grown can be just as damaging as clearing land. Many farmers now sow crops in autumn rather than in spring, so by the time ground-nesting birds are ready to breed the crop is already too tall and dense for them to nest in. The photograph below shows a lapwing, one of the birds that has declined for this reason, along with the skylark.

A bird standing on short, dry grass. It has a white face and throat, a black cap with a long thin crest curving upwards, a black breast band, a white belly, pinkish legs, and a back and folded wing that shine dark green, pink and bronze in the sunlight. — A northern lapwing, a ground-nesting farmland bird that has declined as crops are sown in autumn. Photo: yrjö jyske, CC BY 2.0. Resized.

Lapwings nest on open ground in a shallow scrape, so they need short vegetation in spring.
An autumn-sown crop is tall and dense by the spring, so suitable nesting sites disappear.
The still looks green and healthy, which is why degradation can be missed when only the area of farmland is measured.

Overhunting takes several forms. Trophy hunting is defended on the grounds that only old or sick animals are targeted and that the fees fund , but it removes the largest individuals from small . Traditional medicines drive the trade in tiger body parts, pangolin scales and rhinoceros horn, and the bushmeat trade kills primates and other mammals for food. do damage because they arrive without their usual predators, parasites and competitors: rats brought ashore by ships ate the eggs of the dodo on Mauritius, the North American signal crayfish is outcompeting the smaller native white-clawed crayfish in the UK, and the grey squirrel has replaced the red squirrel over most of the country. An can also damage the economy, for example by fouling boats and harbours. Pollution harms in other ways again. Oil released when a tanker runs aground floats on the surface and prevents oxygen dissolving in the water, animals surfacing through it become covered in oil, and birds are chilled to death because their feathers clump together and can no longer trap air for insulation. Persistent industrial chemicals are a slower problem: polychlorinated biphenyls (PCBs) accumulate along food chains, act as neurotoxins, and hormone disruptors, and are still detected in waste water decades after their use was banned.

Lesson 3 of 21

Why does breaking a habitat into fragments put species at risk?

is not always destroyed completely. More often a large is cut into smaller pieces by roads, fields and housing. This is called , and it endangers even when the total area left is quite large.

The first problem is that the is split into , which that are no longer joined. Added together the numbers may look healthy, but if the groups cannot interbreed each one behaves as a separate, small . In a small there is little , so the is small. Close relatives breed together more often, harmful appear more frequently, and there are fewer available if conditions change, so the becomes less able to adapt and more likely to die out. also about by chance in a small , which is called , and this loses still more .

The diagram below shows a wood that has been fragmented. Three fragments, P, Q and R, have been separated from the main wood in different ways.

A large beige rectangle labelled developed land contains four green blocks of woodland. On the left is a tall block labelled main forest. Block R, upper middle, is joined to it by a short green bar labelled corridor. Block P, lower middle, is separate, with a measuring line to the main forest labelled near. Block Q, on the right, is separate, with a longer measuring line labelled far. Text underneath explains that individuals and pollen can move between R and the main forest, that a few individuals still cross to P, that Q is the most isolated gene pool, and that the smaller and more isolated the gene pool, the greater the effect of genetic drift and the risk of extinction. — One wood broken into fragments by development. Original BioWelsh diagram.

Fragment R is still joined to the main wood by a strip of trees. This is a , so animals can move between the two areas and continues.
Fragment P is separated by developed land but lies close to the main wood, so a few individuals may still cross.
Fragment Q is the furthest away and the most isolated, so is least likely.
The risk of is therefore greatest in Q, then P, and least in R. The reasoning, not just the order, earns the marks.

Fragments also suffer from . Conditions at the edge of a wood are windier, brighter, warmer and drier than deep inside it, so that need the sheltered interior only survive in the core. The diagram below shows why small fragments are worse than their area suggests.

Three squares of decreasing size, each drawn as a pink band around a pale green inner square labelled core. A note at the top says the shaded band is the 35 m strip around the edge, where conditions differ from the core. The squares are labelled 600 m by 600 m, 300 m by 300 m and 150 m by 150 m, and beneath each is the percentage of its area that is edge: 22.0%, 41.2% and 71.6%. Text at the foot gives the calculation, percentage edge equals total area minus core area, divided by total area, times 100, and notes that light, temperature, humidity and wind at the edge differ from conditions inside, so the smaller the fragment the greater the share of it that is affected. — How the proportion of core habitat falls as a fragment gets smaller. Original BioWelsh diagram.

In each fragment the shaded band around the outside is the edge, and the inner square is the core .
In the largest fragment the edge band takes up about 22% of the area, so most of the wood is still core .
In the middle fragment the same width of edge takes up about 41%, and in the smallest it takes up about 72%.
Cutting one large into several small ones therefore removes far more core than the area lost to the road or field.
work uses this reasoning: corridors such as , strips of woodland or tunnels under roads restore , and a reserve that is round rather than long and thin has a greater proportion of core .

Lesson 4 of 21

How are gene pools conserved in the wild?

is the sensible management of the biosphere so that habitats are maintained and biodiversity is enhanced, while human activity continues. Its biological aim is to keep high, so that can still adapt when conditions change.

happens along two routes, shown in the diagram below. Conserving a in its natural is called . Conserving it away from its is called .

A flow chart with a box at the top labelled conservation. Two arrows lead down to boxes labelled in the wild, in situ, and in captivity, ex situ. Under in situ are bullet points: protected sites, nature reserves, SSSIs and special areas of conservation; legislation protecting habitats, game, birds' eggs and wild flowers; international agreements restricting trade in ivory and whaling; education and ecotourism. Under ex situ are: zoos and botanic gardens running captive breeding programmes; sperm banks and seed banks; rare breed societies; reintroduction to former habitats. A box across the bottom reads both aim to keep genetic diversity high, so that populations can adapt as conditions change. — The two routes by which gene pools are conserved. Original BioWelsh diagram.

methods protect the , so the whole is conserved and the keeps evolving in its natural surroundings.
methods protect individuals, gametes or seeds in controlled conditions, which is safer for a on the edge of but conserves fewer .
Both routes aim to maintain , and most programmes use them together.

Protected sites are the main method. In the UK these from local nature reserves, some only a few hectares in size, to national nature reserves and larger designations such as () and special areas of (SACs). Different designations carry different levels of legal protection, and sites are managed and monitored by wardens. Legislation supports this work: national and European law has protected habitats, restricted and , and made it an offence to hunt protected game, collect birds' eggs, pick wild flowers or take protected plants. International agreements restrict trade in products such as ivory, and restrict whaling, although some countries continue to hunt whales under scientific exemptions.

Education changes what people do. Global organisations run public awareness campaigns, and in Wales a government body advises on how activities affect wildlife and their habitats, proposes management schemes and establishes nature reserves. goes further. It is responsible travel to natural areas that conserves the environment and improves the well-being of local people, and done properly it contributes money to , employs local people, educates visitors about the local environment and culture, and works with local communities to manage natural areas. The income gives a a reason to protect a rather than hunt it.

Lesson 5 of 21

How are gene pools conserved in captivity?

When a wild is too small or its too damaged, individuals can be conserved away from the wild. These methods are together known as , because what is really being stored is a collection of .

Specialised zoos and botanic gardens run programmes for . The photograph below shows a giant panda, one of the most closely associated with this work.

A giant panda sitting on a broad stone ledge with one front paw resting on the rock, leaning slightly forward. It has a white head and body with black eye patches, black ears, black forelegs and a black band over the shoulders. Conifer branches and a wire mesh fence are behind it. — A giant panda in a zoo, one of the species most associated with captive breeding programmes. Photo: Jeff Kubina, Public domain.

Captive animals are protected from predators, poaching and food shortages, so a far higher proportion of young survive.
Records of every mating are kept, so that parents can be chosen deliberately to avoid breeding close relatives together, which keeps as high as possible in a small .
Popular attract visitors and publicity, which funds work on that attract less attention.
store the gametes of threatened animals and of economically important breeds. Moving a large animal between zoos is expensive and stressful, but a sample can be sent anywhere in the world, so from one male can enter breeding programmes in many other collections.

do the same job for plants. Seeds are dried and stored in highly controlled conditions, often at very low temperature in liquid nitrogen, as shown in the photographs below.

A tall, narrow concrete and metal wedge projecting out of a deep snow-covered hillside at dusk. The upper face is filled with a lit panel of angular reflective pieces, there is a closed door at the base reached by steps with metal handrails, and a floodlight and a snow sculpture of a bear stand to the left. — The entrance to a global seed vault, built into a mountainside in the Arctic. Photo: Dag Endresen, CC BY 3.0. Resized.

A close view of rows of pale yellow paper packets standing upright in metal trays. Each packet carries a printed label with a reference number, a species name, the country of collection and a barcode, and a ballpoint pen rests between two of the rows. — Labelled packets of seed in the store of a seed bank. Photo: R. C. Johnson, CC BY 2.5. Resized.

Seeds are small and can be stored in enormous numbers, so a holds far more than a garden of living plants.
Stores are placed in secure, cold and stable locations, and each sample is labelled with where and when it was collected.
Seeds still degrade over time, so samples are periodically thawed and germinated, the plants are grown to maturity, and a fresh generation of seed is collected.
conserve traditional varieties of crops as well as wild , so they are also a source of food plants if a catastrophe destroys modern varieties.

do the same for domesticated animals, keeping older, less commercial breeds because they carry useful characteristics such as hardiness or wool quality that modern commercial breeds have lost. The point of all this work is : returning a to a it once occupied. Red kites were brought back to mid Wales, and the chough to Cornwall, after successful breeding programmes. is not simple, because animals raised in captivity may have no fear of predators and no idea how to forage, so systems are now used to train them before release.

Lesson 6 of 21

Why should we conserve species?

Conserving costs money and restricts what people can do with land, so exam questions often ask for justification. There are four main arguments, and the strongest answers give a biological reason rather than only an emotional one.

Ethical reasons: each is a unique combination of and , adapted to its environment, and is considered to have value in its own right.
Agriculture and horticulture: crops and livestock were bred from wild . breeding increases uniformity and loses rarer , so wild relatives act as a for characteristics such as disease or tolerance of cold.
Adapting to change: if the environment changes, some give an advantage. Individuals carrying them are selected for, which prevents the of the . that are lost cannot be recovered.
Medicines: many drugs come from other organisms. Quinine, used to treat , has been extracted from the bark of the tree Cinchona since the sixteenth century.

Tropical rainforests and coral reefs cause particular concern, because they are large, complex communities that hold a huge share of the world's . The photograph below looks up into the canopy of tropical rainforest.

A view from the base of a very tall rainforest tree, looking up along a broad, dark, mottled buttress root and the trunk above it, to layers of green leaves and branches of the canopy with small patches of bright sky between them. — Looking up the trunk of a tall tree into the canopy of tropical rainforest. Photo: T. R. Shankar Raman, CC BY 3.0. Resized.

Rainforests cover roughly 10% of the Earth's land area but are thought to contain at least half of all .
Most of those have never been studied, so their chemical properties are unknown.
A plant may therefore become before anyone discovers that it produces a useful drug, which is a strong argument for protecting habitats rather than individual .
All four arguments come back to the same biological idea: conserving a conserves its , which is all the present in a , and once an is gone it cannot be made again.

Lesson 7 of 21

How does modern farming conflict with conservation?

Agriculture has to feed a growing human , so the quantity and quality of food produced must keep rising. The ways that yields have been increased often conflict with maintaining habitats and biodiversity, and the specification expects you to be able to weigh the two against each other. Over the last seventy five years more land has been cultivated, use of and pesticide has increased, and farms have become far more mechanised. Machinery works efficiently only in large fields, so were removed, and the larger fields were used for .

is the practice of growing a single crop, breed or variety over a large area. Mixed cropping provided many different microhabitats, so many different plants and animals could live there. A provides only one , so it reduces diversity.

Why growing the same crop in the same field damages the land
Practice	Effect on the soil or crop	Consequence
Growing one crop repeatedly	The roots are always the same length, so they take the same mineral ions from the same depth of soil	Yields fall, so far more inorganic is used
Growing one crop repeatedly	The crop is always susceptible to the same pests, so pest numbers build up	More insecticides, herbicides and fungicides are used
by cattle	Hooves compact the soil, driving out the air and stopping water draining through	Roots cannot penetrate the soil, so grass for grazing cannot grow

Attitudes have changed. Government, farmers and consumers now value the countryside as a and as a place to visit, not only as a source of food, and farmers are encouraged to take biodiversity into account while managing their land. Field margins, restored and pesticide use are all attempts to resolve the conflict rather than to choose one side of it.

Lesson 8 of 21

What are the consequences of deforestation?

is the large-scale removal of forest. Timber is used for building and fuel and to make paper and packaging, and land is cleared for farming, for grazing cattle and for growing crops such as oil palm. High value trees are targeted individually, and felling them damages many others. New roads built to move the timber out open up still more forest.

The photographs below show timber being taken out of a forest, and an oil palm plantation of the kind that replaces cleared rainforest.

A red logging lorry loaded with several very large cut trunks, driving along a narrow road through tall dark forest. — A lorry carrying felled trunks out of a redwood forest in California. Photo: Tomas Sennett, Public domain.

Rows of palm trees of even height with large arching fronds, stretching to the horizon, with low green ground vegetation in front of them. — An oil palm plantation of the kind that replaces cleared rainforest. Photo: Irvin calicut, CC BY 3.0. Resized.

felling of high value trees removes the largest, oldest trees, which provide most of the .
A plantation looks green and productive, but it is a , so very few forest can live in it.
Roads built for extraction fragment what remains, which adds the problems of fragmentation to those of .

The most immediate effects are on soil and water. The diagram below compares the flow of water through forested land and through the same land after clearance.

Two blocks of ground side by side, each with a row of blue rain arrows above it. The left block, headed forested land, has three trees with spreading roots, a dark band at the surface labelled humus, green arrows up the trunks labelled much water returns by transpiration, and a small blue arrow at the edge labelled little run off, with a note that roots bind the soil and it holds water like a sponge. The right block, headed cleared land, has two small stumps, a dashed grey arrow labelled little transpiration, slow evaporation, a thick blue arrow across the surface labelled water runs off the surface, and a blue box inside the soil labelled waterlogged, cold soil, with a note in red that topsoil is washed away, soil erosion. A line at the foot reads that cleared land gives more run off and flooding lower down, topsoil and its nutrients lost, and wet cold soil with less oxygen, so roots grow poorly, denitrifying bacteria thrive and the soil loses its nitrates. — What happens to rainfall before and after a forest is cleared. Original BioWelsh diagram.

Tree roots bind the soil together. Without them, heavy rain washes the topsoil down the slope, which is . Topsoil holds the nutrients, so what is left is poor for crops.
Plants, and leaf litter act like a sponge, soaking up heavy rain and releasing it slowly. Once they are gone, water runs off the surface quickly, and the lowland below floods.
Trees transpire and return water to the atmosphere. Without them water evaporates from bare soil instead, which is a slower process, so less rain falls locally and the area moves towards desert conditions, a process called .
Deforested soil stays wetter. Water fills the air spaces, so less oxygen is available for roots, and the soil warms up more slowly, so and root activity are .
Cold, damp, anaerobic soil favours bacteria, so is converted to nitrogen gas and the soil loses its fertility.

also changes the carbon cycle. Fewer trees less carbon dioxide is removed by . Cut trees are usually burned, which releases carbon dioxide by combustion, and any material left to rot releases it through the of . It is worth noticing that removing the trees also removes their own , so a question can ask you to explain a decrease in carbon dioxide as well as an increase. (If the processes that move carbon between the air and living things are unfamiliar, read the Populations and ecosystems lesson on the carbon cycle.) follows quickly: it is estimated that at least half of the Earth's live in tropical rainforest, which covers only about a tenth of the land area. The graph below shows what has happened to one rainforest primate as the area of oil palm in its region has grown.

Line graph with two series plotted against year from 1970 to 2020 on a shared axis labelled value, see key. A brown line marked with crosses, for orangutans in thousands, falls steadily from 200 in 1970 through 160 in 1990 and 125 in 2000 to 62 in 2020. A green line marked with circles, for area of oil palm in thousand square kilometres, rises steadily from 3 in 1970 through 16 in 1990 and 30 in 2000 to 62 in 2020. The two lines meet at about 62 in 2020. — Numbers of a rainforest primate and the area of oil palm plantation in the same region, 1970 to 2020. Original BioWelsh graph.

Orangutan numbers fall steadily across the whole period, from about 200 000 to about 62 000.
The area of oil palm rises over the same period, from about 3 thousand km² to about 62 thousand km².
The two changes are correlated, which is not by itself proof that one causes the other.
The causal link is the biology: oil palm plantations replace the forest canopy that the orangutans feed and nest in, so the of the area falls.

Lesson 9 of 21

How can forests be used sustainably?

Wood is a renewable resource, so forests can be used without being destroyed. forestry taking timber at a rate that allows the forest to continue, and replanting and regenerating what is removed. The oldest method is : a small area of forest is cut and burned, crops are grown on soil fertilised by the ash, and when the soil is no longer people move on and the area regenerates. This is on a small scale, but not when it is done to large areas of rainforest, because there is no surrounding forest left to recolonise the cleared ground.

In Britain, has been used for thousands of years. A tree trunk is cut close to the ground, leaving a stool a few centimetres high, and new shoots grow from buds in the stool. The photograph below shows a stool with its new poles.

A low cut tree stump surrounded by dozens of straight pale brown poles growing upwards from its edges, in wet woodland with bare ground beneath. — A coppiced stool, with many new poles growing from a tree cut close to the ground. Photo: Naturenet, CC BY 2.5. Resized.

The tree is not killed, so the same stool produces poles again and again for many years.
Poles are cut on , which different areas are cut in different years, so timber of different widths is always available.
A long time increases , because many years pass between cuts in the same area.
Because adjacent areas are at different stages of regrowth, a variety of habitats develops, which favours diverse wildlife.

Instead of clearing every tree at once, removes only some trees and leaves the rest standing. The diagram below puts and side by side.

Two panels side by side. The left panel, headed coppicing, shows a low cut stump labelled stool with several straight new poles growing from it, then an arrow labelled years leading to two thicker trees with full crowns labelled poles thicken. A note says the trunk is cut close to the ground and regrows from the stool, so the same tree is harvested again and again. The right panel, headed selective cutting, shows three full-grown trees still standing with two small stumps between them, each labelled felled. A note says only some trees are taken, so the canopy, the soil and the habitats of the wood are left in place. — Two ways of harvesting timber without destroying the wood. Original BioWelsh diagram.

In , the trunk is cut close to the ground and new poles grow from the stool, thickening over the years, so the same tree is harvested again and again.
In , only some trees are felled and the rest are left standing, so the canopy, the soil and the habitats of the wood stay in place.
Because most roots remain, they continue to hold the soil, nutrients stay in the forest and less soil is washed into nearby waterways.
Clear felling, by contrast, exposes the whole area at once, so is severe on steep slopes and the is destroyed.

A third method suits large forests. In , a narrow band of forest is harvested and then left uncut for decades. Light reaches ground level and stimulates growth, seeds arrive from the neighbouring trees on either side, and restores the , so the is maintained and new niches appear while timber is still taken.

Good forestry also uses the land efficiently. Trees are planted the optimum distance apart, because trees planted too close compete with each other and grow tall and thin, producing poor quality timber. Pests and diseases are controlled so that the trees grow well, which fewer trees need to be felled for the same amount of usable timber, and cutting a similar number of trees each year allows the forest to be maintained while timber is still extracted. Which is replanted matters too: in the UK most woodland that remains was planted rather than natural, and plantations of conifers support far fewer than mixed native woodland, so replanting native trees allows native woodland to regenerate and preserves biodiversity. Elsewhere, replanting with a fast-growing from another continent has caused problems, because trees whose shallow roots absorb water very efficiently lowered both the water content and the content of the soil, and biodiversity fell with them.

Lesson 10 of 21

Why does overfishing reduce fish stocks?

is the situation in which fish are harvested at a greater rate than the rate at which they reproduce. Numbers then fall year after year, however hard the fleet works, because the cannot replace what is removed. Fishing became far more effective during the twentieth century: larger boats, powered trawlers and equipment such as radar and sonar meant that far more fish could be caught, and that the last shoals could still be found when stocks were low.

The graph below shows annual landings of cod from one North Atlantic fishery. The fishery had been worked for centuries before the period shown.

Line graph with crosses of cod landed in thousand tonnes against year from 1950 to 2020. Landings rise from 180 in 1955 to a peak of 800 in 1968, which is labelled peak catch, then fall steeply to 250 by 1980 and to 40 by 1992. A vertical dashed line at 1992 is labelled fishery closed. After 1992 the line stays low, rising only from 10 in 1995 to 30 in 2020. — Annual cod landings from one North Atlantic fishery, 1955 to 2020, with the year of closure marked. Original BioWelsh graph.

Landings rise steeply through the 1960s as new technology is introduced, peaking at about 800 thousand tonnes.
From about 1970 landings fall sharply, even though fishing effort stayed high. Catching more each year had removed the adults that would have bred.
The dashed line marks 1992, when the fishery was closed completely to allow stocks to recover.
Landings after the closure remain tiny, which shows that a collapsed may take decades to recover, or may not recover at all.

Part of the damage is caused by the size of the net mesh. Nets with a very small mesh catch young fish before they have become sexually mature. Those fish never breed, so fewer individuals are left to reproduce and the falls further. With fewer individuals spread over a large area it is also harder for survivors to find a mate, and because fewer individuals contribute their , the of the decreases. There is a genuine conflict here, because reducing the catch protects the fish but reduces the income of fishing communities, so a balance has to be struck between catching enough fish for people to make a living and leaving enough breeding stock to replenish the .

Two ways of fishing and the damage each causes
Method	How it works	Damage
	Nets are suspended from floats and stretched between two boats to catch pelagic fish, which live in surface waters	With thousands of miles of net in use, non-target such as dolphins and marine turtles become trapped
	A large net is dragged through the water to catch fish living in mid water or near the bottom	Heavy equipment dragged along the seabed destroys the habitats of molluscs such as clams and of other seabed organisms

one affects the rest of the food web. The photographs below show a trawler crew handling their net, and Antarctic krill, a small shrimp-like crustacean about 50 mm long.

Black and white photograph of two men in oilskins bending over a large tangled net on a wet deck, with winch machinery to one side and rough sea behind. — Fishermen handling a trawl net on the deck of a trawler at sea. Photo: Richard Stone, Public domain.

A small, almost transparent pale blue crustacean in a dish of water, with a dark round eye, long feathery front limbs, a green patch inside its body and a segmented tail. — Antarctic krill, a shrimp-like crustacean about 50 mm long that feeds on phytoplankton. Photo: ingvildriska, CC BY 4.0. Resized.

Trawlers also catch small fish that people do not eat but which are important prey for larger fish, so removing them contributes to the decline of the that feed on them.
Krill are primary consumers, feeding on phytoplankton, and they form swarms many miles across.
Krill are the main food of whales, and they supplement the diets of seals, penguins, squid and fish, so heavy harvesting of krill affects the rest of that food web.
The ecological balance in the Antarctic had already been upset by the overexploitation of whales, so the loss of krill adds to an existing problem.

Lesson 11 of 21

How can fishing be regulated so that stocks recover?

Because is a problem of rates, every method works by reducing the rate at which fish are removed, or by increasing the rate at which they are replaced. Knowing which of the two a method affects makes it easy to explain. The most direct is a legal minimum , shown in the diagram below.

Two squares of netting side by side. The left one, headed small mesh, is drawn with small holes and holds a large blue fish labelled adult caught and a small fish labelled young fish caught too, with a note that young fish are caught before they can breed, so fewer are left to reproduce and stocks fall. The right one, headed large mesh, has larger holes and holds the adult fish, while a green arrow shows a small fish passing out through the netting, labelled young fish escape, with a note that young fish pass through and grow to maturity, so they can breed and replace the catch. — How the mesh size of a net changes which fish are caught. Original BioWelsh diagram.

A small mesh retains fish of all sizes, including immature fish that have not yet bred.
A larger mesh lets immature fish swim through and escape, while still catching the adults.
Those young fish survive to become sexually mature and interbreed, so the stock is replenished.
The rule is enforced by legislation that prevents undersized fish being landed and sold.

Measures used to reduce overfishing
Measure	How it protects the stock
Larger net	Immature fish escape and survive to breed, so the stock is replenished
	A limit is set on the mass of each that may be landed, so the total removed is controlled
	Fishing is banned during the breeding season, so fish can reproduce undisturbed
	Fishing is prohibited in defined areas, which act as reserves where fish breed and grow
Limits on fleet size and days at sea	Fewer boats fishing for fewer days reduces the total catch
Consumer choice	Buying only fish certified as coming from a source reduces demand for fish caught in damaging ways
	Farmed fish supply the market, so fewer wild fish need to be caught

None of these is perfect. encourage discards, where fish caught above the are thrown back dead, and only work if they are enforced, so the position of fishing boats is monitored by satellite and fines are imposed for non-compliance. Scientists judge whether a policy is working by monitoring two things about each . The first is the level of exploitation, measured as the of fish caught, which is a measure of the death rate. The second is the reproductive capacity, measured as the of fish that are reproducing, which is a measure of the .

If the death rate is greater than the , the will decrease.
If the is greater than the death rate, the will increase.
In a stable, stock, and death rate are equal.
This is why both criteria have to be measured. A stock can be caught at a modest rate and still decline if too few mature adults are left to breed.

The graph below shows the percentage of monitored fish in one sea area that met both targets in each year of a monitoring programme. The label marks the year the fishing policy was introduced.

Line graph with crosses of populations meeting both targets as a percentage against year from 2004 to 2024. The values rise from 12% in 2005 and 15% in 2009, then more steeply through 24% in 2013 and 38% in 2017, to 48% in 2021 and 51% in 2023. A label on the graph reads policy introduced in 2010. — Percentage of monitored fish populations meeting both sustainability targets, 2005 to 2023. Original BioWelsh graph.

In 2005, before the policy, only about 12% of the monitored met both targets, and by 2009 the figure had reached only 15%.
After the policy was introduced in 2010 the line rises much more steeply, through 24% in 2013 and 38% in 2017, to about 51% in 2023.
The policy is therefore working, because a greater proportion of is now being fished sustainably.
It is only partly effective: about half of the still fail at least one target, and that were never monitored are not included in the figures at all.

Lesson 12 of 21

Is fish farming a solution or a new problem?

, or , rears fish to maturity in ponds, lakes and managed enclosures, or in cages in estuaries and sea lochs. Predation is prevented and food is supplied, so a far higher proportion of the young survive than in the wild, and over 95% of the salmon and trout eaten in the UK is farmed. Rearing fish rather than pork, poultry or beef has real advantages: fish convert their food into protein more efficiently, because they do not use energy to maintain a constant body temperature and are supported by water rather than by a skeleton, a greater proportion of a fish's body is edible, and has a lower than farming mammals.

Farming fish in cages in open water causes problems of its own, shown in the diagram and photograph below.

A large pale blue rectangle of open water contains a darker blue rearing pen holding rows of fish. An arrow from a box on the left labelled uneaten food and faeces points into the pen. An arrow out of the pen to the left points to a box labelled escaped fish and parasites, such as sea lice. An arrow out of the pen to the right points to a green box labelled extra nitrate and phosphate feed algae, an algal bloom, and a red arrow leads down from it to a box labelled decomposers use up oxygen, fish die. A line underneath reads that fish convert food into protein efficiently and a larger part of the body can be eaten, but dense stocking spreads disease and the waste enriches the water. — What leaves a fish farm cage and where it goes. Original BioWelsh diagram.

Several large circular floating pens made of dark pipework with netting stretched over them, on choppy blue water, with forested hills behind. — Circular cages of a salmon farm moored in a sheltered inlet. Photo: Peter Whyte, CSIRO, CC BY 3.0. Resized.

Farmed fish are stocked densely, so disease passes quickly through the and can spread to wild fish nearby.
are used to keep the stock healthy, which encourages the development of - bacteria.
Pesticides used to kill parasites such as sea lice are also toxic to local marine invertebrates.
Uneaten food, faeces and dissolved waste pass straight into the water around the cages.

That last point causes the most damage. Nitrogen and phosphorus in the feed are lost as uneaten food, as organic matter in faeces, and as inorganic products such as ammonium and phosphate ions. In one set of published figures, of every 86 kg of nitrogen fed per tonne of fish produced, 4 kg was lost as uneaten food, 12 kg in faeces and 37 kg as inorganic products. (If the full sequence is unfamiliar, read the Populations and ecosystems lesson on eutrophication.)

The extra and phosphate allow algae at the surface to grow rapidly, producing an .
The bloom blocks light from reaching the plants in deeper water, so they cannot photosynthesise and they die.
, which are bacteria and fungi, respire aerobically as they break down the dead material, and use up the dissolved oxygen.
The water becomes deoxygenated, so fish and other organisms that need oxygen die. In the anaerobic conditions that follow, bacteria may reduce by .

Escaped fish are a separate problem. Farmed fish have been selected for very rapid growth, so escapees compete successfully with wild fish for food, and mates, they can transmit parasites and other infections, and if they interbreed with wild fish they set up fast-growing that may push the wild type towards . Coastal fish and shellfish farms can also destroy the they sit in: the shrimp industry in particular has been blamed for salinisation of soil and groundwater, and for clearing the mangroves shown in the photograph below.

A wide mudflat at low tide covered with hundreds of short dark woody roots projecting upwards from the sand, with shallow water behind and a single small mangrove seedling. — Mangrove roots exposed at low tide on a tropical shore. Photo: Vyacheslav Argenberg, CC BY 4.0. Resized.

Mangroves are salt-tolerant trees whose dense roots trap sediment along tropical coasts.
They protect coastal communities from tropical storms, and they are nurseries for many fish .
Clearing them for ponds removes that protection and that at the same time.
Environmental such as methyl mercury, PCBs and dioxins are more concentrated in farmed fish than in wild fish, although at concentrations judged not to outweigh the health benefits of eating fish.

Lesson 13 of 21

What does sustainability mean, and how do human choices change it?

As the human grows, the pressure it puts on the environment grows with it. using resources in a way that meets present needs without preventing future generations from meeting theirs. A practice can continue indefinitely; an unsustainable one eventually destroys the resource it depends on.

Sustainable and unsustainable use of the same resource
Resource	Unsustainable use	Sustainable use
Forest	Clear felling large areas for farmland, with no replanting	or on a long , with native replanting
Fish stock	Catching fish faster than they reproduce, with small mesh nets	, larger mesh sizes, and
Soil	Repeated and , so fertility and structure are lost	, restored and stocking levels that the land can support
	Pumping groundwater from faster than rain can refill them	Water , reclaimed waste water and

Achieving needs changes in human attitudes as much as new technology. People have to accept that the resources of the biosphere are limited, and that a choice made by one person is multiplied by billions. Informed choices are the practical result: buying fish certified as sustainably caught, or timber from managed forests, shifts demand towards who protect the resource; eating less meat reduces the area of land needed for crops and grazing; and using water efficiently, insulating homes and reducing the use of all reduce pressure on the global systems described later in this topic. Technological innovation contributes too, because can reduce the use of and can provide where rainfall is not enough. Both are covered later in this topic, and both bring problems of their own, which is exactly why decisions need evidence rather than enthusiasm.

Lesson 14 of 21

How is the environment monitored?

describes the quality of the environment. A single set of measurements establishes the current status. Repeating those measurements in the same way shows the pattern of change, so that trends can be identified and predictions made. Without monitoring there is no evidence on which to base a decision. Air quality monitoring measures the concentrations of air pollutants, because of the health effects associated with exposure to them, and because air pollution is carried by the wind, the direction and strength of the wind must be taken into account when the source of a pollutant is identified. Soil monitoring looks at structure and , water-holding and drainage capacity, pH, organic particles, earthworms and other organisms, which together show whether the soil can support plant growth and whether it is being damaged by compaction or contamination.

The graph below shows nitrogen dioxide concentration measured at different distances from a motorway. The dashed line marks the guideline annual recommended by the World Health Organisation, 40 µg m⁻³.

Line graph with crosses of nitrogen dioxide in micrograms per cubic metre against distance from the motorway from 0 to 500 m. The curve starts at 68 at the roadside, falls steeply through 49 at 50 m and 38 at 100 m, then flattens to 18 at 500 m. A horizontal dashed red line at 40 is labelled WHO guideline, 40 micrograms per cubic metre, and the curve crosses it at about 90 m. — Mean nitrogen dioxide concentration with distance from a motorway, against the World Health Organisation guideline. Original BioWelsh graph.

The highest concentration is at the motorway itself, about 68 µg m⁻³.
Concentration falls steeply over the first 100 m, then much more gradually.
The guideline value of 40 µg m⁻³ is met at a distance of about 90 m from the road.
The pattern supports the hypothesis that vehicle exhausts are the main source, because the highest levels are where there are most vehicles.

Living organisms can be used as well as instruments. An is one whose presence, absence or tells you about conditions in a . The photograph below shows a bushy lichen growing on a branch.

A tuft of pale green, finely branched lichen growing on a dark wet branch on the forest floor, with fallen pine needles around it. — A bushy lichen growing on a fallen branch, of the kind that is most sensitive to air pollution. Photo: US Fish and Wildlife Service Midwest Region, CC BY 2.0. Resized.

Lichens absorb water and dissolved substances over their whole surface, so they take up air pollutants directly.
Bushy lichens are the most sensitive, leafy lichens are less sensitive and crust-like lichens are the most tolerant.
The of lichen growing on rocks and trees therefore indicate the relative level of air pollution in an area.
Lichens on soil can also indicate the type and severity of heavy metal contamination.

Three ways of monitoring water quality
Type of monitoring	What is measured	What it shows
Chemical	Concentrations of pollutants, including and substances that behave like oestrogens	Substances that have a feminising effect on aquatic organisms, and may affect human development and the timing of puberty
Biological	The presence and of , such as mayfly larvae and brown trout	Well oxygenated, high quality water. A sharp decline in a sensitive can be an early indication of acid rain
Microbiological	Counts of bacteria and viruses, especially coliform bacteria and E. coli	Contamination by sewage. Water may look clean but still carry a high bacterial count, so high counts trigger tests for specific

Data from monitoring feed into an . This is a document that aims to predict the environmental effects of a proposed project, such as a road or a wind farm, before it is built. It describes the project and the site, the alternatives that were considered, and the environment that would be affected, including , animals, plants, air, soil, water, landscape and cultural heritage. An assessment must also describe the effects of the proposal and the , which the ways in which negative effects will be avoided. Evidence is required to show that reducing the harm has been considered: wind turbines can be built at times when birds are not nesting, and a proposed road can be routed away from the breeding ponds of protected amphibians.

Lesson 15 of 21

How should scientific evidence guide political decisions?

Decisions about the environment are made by governments and local authorities, not by scientists. The specification expects you to understand why those decisions need to be informed by knowledge based on sound scientific principles, and how monitoring data become reliable enough to use. The diagram below shows the four stages that take measurements through to a decision.

Four boxes in a row joined by short links. The first is headed monitor, measure air, soil and water quality. The second is headed analyse, means, trends and statistical tests. The third is headed assess, environmental impact assessment, with mitigation. The fourth is headed decide, a political decision based on the evidence. A line underneath reads that each stage must be open to checking, so a decision can be defended and revisited when new data arrive. — From measurements to an evidence-based decision. Original BioWelsh diagram.

Monitor: air, soil and water quality are measured, using the same method each time so that the results can be compared.
Analyse: and trends are calculated, and a statistical test decides whether a difference is greater than would be expected by chance. The test needs a , which states that there is no difference between the groups being compared.
Assess: the findings go into an , which sets out the likely effects of the proposal and the planned.
Decide: the political decision is taken in the light of that evidence. Every stage must be open to checking, so that a decision can be defended and revisited when new data arrive.

The statistical step is what turns a difference into evidence. A test produces a calculated value, which is compared with a read from a table for the appropriate number of , usually at a probability level of 0.05.

If the calculated value is greater than the , the is rejected.
Rejecting the there is a difference between the , and that the difference is not due to chance.
If the calculated value is less than the , the is accepted, and any difference is treated as chance .
A probability level of 0.05 that a difference as large as this would arise by chance in fewer than 5 samples in 100.

A statistically result is still only as good as the design that produced it. Suppose nitrogen dioxide is measured before and after a motorway junction is closed, and the concentration is significantly higher after the closure. Before concluding that the closure raised pollution, ask what else changed.

Trees beside the road absorb nitrogen dioxide, and they absorb far more while they are in leaf in spring and summer than after their leaves fall.
If the road was open during the summer and closed in the autumn, the seasonal change in absorption could have produced the difference on its own.
This reduces confidence in the conclusion, because the effect of the closure cannot be separated from the effect of the season.
The design is improved by repeating the closure experiment at other times of year, or over a longer period, so that seasonal effects apply equally to both conditions.

Authorities then use the evidence to justify a decision, and they must use it honestly. If emissions were significantly higher while the junction was closed, keeping the junction open can be justified as giving better air quality, and if there was no difference, the honest conclusion is that closing the junction changes nothing. Planning brings together engineers, mathematicians, economists, archaeologists, geologists and materials scientists as well as biologists, and the legal framework requires them to preserve habitats and enhance biodiversity. Each specialist has to extend their thinking into areas outside their own field, and the decision has to be based on the most reliable evidence available.

Lesson 16 of 21

What are planetary boundaries?

The concept of was introduced in 2009 as a way of defining a for humanity. Nine global processes have been identified that regulate the stability of planetary systems and the interactions of air, land and sea. For each one, scientists have estimated a boundary: a limit below which the system can continue to operate without extreme responses.

These systems are precarious. The response to a change is not linear, and a may be reached. At a , a small further change has a very large and unpredictable effect on the environment. Exceeding a boundary is therefore likely to result in sudden and catastrophic change to environmental conditions.

The status of the nine systems is usually drawn as a circular diagram, shown below.

A circular diagram of nine wedges radiating from a centre, with a solid black circle around them and a larger red dashed circle outside that. Nine labels are placed around the diagram: climate change at the top, then clockwise biosphere integrity, land-system change, biogeochemical flows, fresh water use, ocean acidification, ozone depletion, atmospheric aerosols and novel entities. Four wedges are pink and reach past both circles: climate change, biosphere integrity, land-system change and biogeochemical flows. Three wedges are green and stay inside the black circle, the shortest being ozone depletion, along with ocean acidification and fresh water use. Two wedges are grey and stay inside the black circle: atmospheric aerosols and novel entities. A note says the length of each wedge shows how far that system has moved from its safe level. The key reads pink crossed, high risk; green inside the boundary, safe; grey not quantified; the solid line is the edge of the safe operating space; the dashed line is the zone of uncertainty where risk is rising; and beyond the dashed line the risk is high. — The status of the nine planetary systems. Original BioWelsh diagram.

The inner green circle is the , where planetary systems are stable, changes are reversible and there is no detrimental impact on global systems.
The ring between the inner green circle and the outer red circle is the , an area of increasing risk.
A wedge that extends beyond the red circle shows a boundary that has been crossed. There the risk is high and events become unpredictable.
Grey wedges are systems for which a boundary has not yet been quantified, because the effects are too variable to measure reliably.

The nine planetary boundaries, in three groups
Group	Boundaries
Global systems	, ,
Systems relating to the biosphere	, use, ,
Non-natural phenomena	loading, introduction of

Two of the nine are called : and . Crossing a would drive the Earth into a new and unpredictable state with severe consequences for the whole biosphere, so they matter more than the others. At the time of writing, four boundaries have been crossed: , , and . is the only boundary whose crossing has been deliberately avoided, by international action, and and have not been quantified. The study of is interdisciplinary, and its conclusions rest on very large amounts of research and data collection, in the hope that political, economic and other decisions will take account of the knowledge these boundaries represent.

Lesson 17 of 21

Which boundaries have been crossed, and why?

Four boundaries have been crossed. Two of them are the , and , and the other two, and , are closely to farming. The boundary is set by the concentration of carbon dioxide in the atmosphere, proposed as 350 parts per million by volume. Enough have already been added that even if emissions stopped immediately the average global temperature would continue to rise for decades, and the effects on wind, ocean currents, rainfall patterns and precipitation are already being documented.

The graph below shows the concentration of atmospheric carbon dioxide measured since 1960, with the proposed boundary marked.

Line graph with crosses of carbon dioxide concentration in parts per million against year from 1960 to 2030. Values rise from 317 in 1960 through 346 in 1985, 354 in 1990 and 390 in 2010 to 425 in 2024, with the line becoming steeper over time. A horizontal dashed red line at 350 is labelled planetary boundary, 350 ppm, and the plotted line crosses it between 1985 and 1990. — Mean atmospheric carbon dioxide concentration since 1960, with the climate change planetary boundary marked. Values rounded from NOAA records. Original BioWelsh graph.

The concentration rises throughout, from about 317 parts per million in 1960 to about 425 in 2024.
The line becomes steeper over time, so the rate of increase is itself increasing.
The boundary of 350 parts per million was crossed in about 1988.
A sea level rise of up to 7 m is predicted by 2100 unless drastic action is taken. Even if the temperature rise is held to 2 °C, sea level is still predicted to rise by 2 m.
International agreements have tried to address this, the first major one, agreed in 1997, setting targets for reducing emissions.

used to be called the biodiversity loss and boundary. The name was changed to emphasise human impact on how function: individual may not be on their own, but together they produce such as recycling minerals and absorbing carbon dioxide, on which the whole biosphere depends. Human activity causes environmental change too fast for to produce organisms suited to the new conditions, so fail to reproduce enough to maintain the . Marine and coastal habitats show this clearly, and the photograph below shows a reef where some corals have lost their colour, which is known as .

An underwater view of a shallow reef in clear blue water, with sunlight streaming down. Many branching and plate-shaped corals in the foreground are pale cream or white, while a few others are yellow and brown, and small fish swim above. — A reef on which many corals have lost their colour, which is called bleaching. Photo: NOAA, Public domain.

Coral reefs are biodiversity hot spots, with up to 1 000 per m².
Corals contain photosynthetic organisms called , which give the corals their colour and feed them with the products of .
Warm water causes the to leave, so the coral turns white and loses its food supply.
In many cases the do not return even if conditions improve, so the corals die and the that depend on the reef are lost.

Coastal communities are affected too. Mangroves, salt marshes and seagrass meadows absorb carbon dioxide up to 50 times more efficiently than the same area of tropical forest, but as sea level rises they are flooded with salt water, water loss by kills the plants, and the animals that depend on them follow.
converting land to human use, largely by expanding farming and raising livestock, clearing forest for oil palm and soya for , and urban development. The boundary measures the misuse of land that results in little food being produced. Crossing it had been predicted for 2050, but data from 2015 suggested it had already happened. Concentrating farming in the most productive areas and reducing meat consumption globally would both reduce the area under cultivation.
are the cycling of elements between the and parts of an . Agricultural have been used so intensively that the boundaries for both the nitrogen cycle and the phosphorus cycle have been crossed, and the polluting events that follow, such as , damage further. One long-term aim has been to transfer nitrogen-fixing into crop plants so that less is needed, but this has so far not been possible. (For how bacteria move nitrogen between the air, the soil and living things, read the Populations and ecosystems lesson on the nitrogen cycle.)

Lesson 18 of 21

Which boundaries have not been crossed, and which cannot yet be measured?

Two of the global boundaries have not been crossed. One of them, , is the only boundary whose crossing has been deliberately avoided by human action, so it is worth knowing well. About 90% of the world's ozone is in a layer in the stratosphere, 10 to 50 km up. Normally ozone and oxygen are in equilibrium there: 3O₂ ⇌ 2O₃. Halogenated hydrocarbons, such as the () once used as propellants in spray cans, as solvents, as refrigerator coolants and in making disposable food and drink containers, alter the position of this equilibrium and favour the breakdown of ozone.

The diagram below shows how a molecule destroys ozone.

Four boxes joined by arrows in a cycle. A grey box at the top left reads CFC molecule in the stratosphere, and an arrow down from it, labelled ultraviolet light, leads to a yellow box reading chlorine free radical Cl. A diagonal arrow labelled attacks leads from that box up to a blue box reading ozone O3, and an arrow down from it, labelled broken down to, leads to a blue box reading oxygen O2. A red arrow runs back from the oxygen box to the chlorine free radical box, labelled the same chlorine free radical is released again. A box across the foot reads that one chlorine free radical can break down about 100 000 ozone molecules before it is removed. — How chlorofluorocarbons destroy ozone in the stratosphere. Original BioWelsh diagram.

molecules are heavier than air, so they take two to five years to climb to the stratosphere.
In the stratosphere, ultraviolet light breaks chlorine out of the molecules.
Each chlorine can go on to break down about 100 000 ozone molecules, because the radical is regenerated each time.
Ozone absorbs ultraviolet light of 280 to 315 nm, known as . With less ozone, more reaches the surface.

This matters biologically because that of is strongly absorbed by DNA. More at the surface more , and the numbers of skin cancers and cataracts increased as the ozone layer thinned. Depletion was first observed in the 1970s, and a hole appeared in the layer over Antarctica each spring once the concentration of reached a . The response was political and international: the use of in spray cans was banned in 1978, and an international agreement in 1987 banned their manufacture and placed obligations on nations to reduce their use. The ozone layer continues to rebuild, and this is the clearest example in the specification of political decision making informed by sound science.

The boundary has not been crossed either, although it is being approached. The diagram below shows what happens when carbon dioxide dissolves in sea water.

Four boxes in a vertical column joined by downward arrows, headed how extra carbon dioxide acidifies the ocean. The first reads carbon dioxide dissolves in sea water. The second reads carbonic acid H2CO3. The third reads hydrogen ions H plus and hydrogencarbonate HCO3 minus. The fourth reads that more hydrogen ions mean a lower pH, and that hydrogen ions react with the calcium carbonate of shells and exoskeletons, with the equation CaCO3 plus H plus gives Ca 2 plus plus HCO3 minus. A line at the foot reads that ocean pH has fallen from about 8.16 to 8.03, a rise of about a third in hydrogen ion concentration. — How dissolved carbon dioxide lowers ocean pH and weakens shells. Original BioWelsh diagram.

Carbon dioxide dissolves in water and reacts with it to form carbonic acid: H₂O + CO₂ → H₂CO₃.
Carbonic acid releases a : H₂CO₃ → H⁺ + HCO₃⁻. Hydrogen carbonate ions then break down further: HCO₃⁻ → H⁺ + CO₃²⁻.
The extra hydrogen ions lower the pH. In the seventeenth century ocean pH was about 8.16; it is now about 8.03.
Because the pH scale is logarithmic, a fall of one pH unit is a tenfold increase in concentration, so this modest-looking change is a rise of about a third.

The consequences follow from the chemistry. A low pH leaches calcium carbonate out of the shells of molluscs and corals and out of the exoskeletons of arthropods: CaCO₃ + H⁺ → Ca²⁺ + HCO₃⁻. The shells soften, so the organisms become vulnerable to physical and chemical attack, and fish are particularly vulnerable to low pH because the structure and functioning of their gills is damaged. Both of these boundaries are described as avoidable: was avoided by banning the chemicals responsible, and can be avoided by reducing the combustion of , which is the source of the carbon dioxide dissolving in the oceans.

Two other boundaries concern non-natural phenomena, and neither has been given a numerical value at all. are the microscopic particles put into the atmosphere by the combustion of fuels and by the dust created by digging and quarrying, and their concentration has doubled since the Industrial Revolution.

The photograph below shows a city skyline under heavy smog, which is what a high aerosol concentration looks like.

An aerial view across a wide river towards a city of tall buildings, whose outlines fade into thick brown and grey haze, with a bridge in the foreground. — A city skyline almost hidden by heavy smog. Photo: Chester Higgins, Jr., Public domain.

of 10 µm diameter, called , forms an aerosol and lodges in the lungs when it is inhaled, making respiratory problems worse and causing deaths from lung disease.
, with a diameter of 2.5 µm, is small enough to transfer into the capillaries and be carried around the body, which increases the likelihood of death from cardiovascular disease.
The main source of both is motor vehicles, especially those with diesel engines.
Particles deposited on leaves reduce the absorption of light for , so crop yields fall.

No boundary has been set for because different have opposite effects: sulphates reflect sunlight, which cools the surface, while other particles such as soot absorb sunlight and re-radiate it, which warms the surface. The balance of the two is not clear, and are so variable from place to place that no can be suggested. , previously called the chemical pollution boundary, have the same difficulty. The change of name reflects the increasing use of new technologies and materials, so the boundary now covers organic pollutants, radioactive materials, nanomaterials and micro-plastics as well as chemicals. Some substances have proved so toxic that they are already widely restricted, including DDT and PCBs, and an estimated 100 000 manufactured chemicals are in use worldwide in millions of different products. They may interact with each other and produce additional harmful effects, so the effects cannot be quantified and no boundary has been identified, although the overall concentration worldwide is still considered to be within the safe zone.

Lesson 19 of 21

Why is the fresh water use boundary a problem, and how can it be avoided?

Liquid water is critical for the survival of living organisms, and most plants and mammals need regular access to . has a low concentration of dissolved salts, below 0.05% by mass. The use boundary has not yet been crossed, and is thought to be avoidable, but the amount of used has to be . Very little of the world's water is usable: about 97% is saline, and of the 2.5% that is fresh, most is frozen in ice sheets, ice caps and glaciers, with the Antarctic ice sheet alone holding about 61% of the on Earth. What is left is unevenly distributed, so one country may hold a fifth of the world's supply for a small while another holds a tenth for thirty times as many people.

Demand has risen steadily. The graph below shows the volume of withdrawn worldwide each year.

Line graph with crosses of water withdrawn in thousand cubic kilometres per year against year from 1970 to 2020. The values rise steadily from 1.4 in 1970 through 2.4 in 1990 and 3.6 in 2010 to 4.0 in 2020. — Volume of fresh water withdrawn worldwide each year, 1970 to 2020. Original BioWelsh graph.

Withdrawal rises from about 1.4 thousand km³ per year in 1970 to about 4.0 thousand km³ per year in 2020.
That is an increase of about 2.6 thousand km³ per year, which is close to a threefold rise in fifty years.
Agriculture consumes more than any other human activity, so most of this increase is irrigation.
Where rain cannot refill them, underground are being pumped out faster than they are replaced, so the supply cannot continue at this rate.

Changing landscapes: draining wetlands, and change the flow of and so affect the water cycle.
Agriculture: irrigation places a heavy strain on local supplies, and water pumped from deep will not be refilled in the foreseeable future.
Increased demand: have grown, and people have moved to warm climates that have little of their own.
Pollution: makes water unusable. Blooms of the cyanobacterium Anabaena flos-aquae, which form when concentration is high, release a neurotoxin that is to birds and many mammals.
: as glaciers melt, availability rises at first and then falls, causing drought. Thermal expansion of the oceans raises sea level, so groundwater in coastal regions becomes too salty for drinking and irrigation.

The photograph below shows the consequence of taking too much water from the rivers feeding an inland sea, which has dried out until ships are stranded far from the water.

A rusting white and blue fishing boat sitting upright on flat, pale, salt-crusted ground that stretches to the horizon, with a man in blue overalls walking past on a dry track in the foreground. — A ship stranded on the dried bed of an inland sea whose rivers were diverted for irrigation. Photo: Staecker, Public domain.

The rivers that fed this sea were diverted to irrigate crops, so far less water reached it.
As the volume fell, the remaining water became more saline, and the fish and the fishing industry that depended on them were lost.
Salt and dust from the exposed bed are blown across the surrounding farmland, damaging it further.
The effects are not confined to the water itself: a whole regional and economy changed.

Supply can be increased as well as demand . Water , for example not irrigating non-food crops such as and cotton, water measures, reclamation of waste water for irrigation and industry, and capturing urban runoff to recharge groundwater all help, and delivers water containing directly to the roots of individual plants rather than spraying it over a large area. removes minerals from saline water to produce . It is one of the few sources of water that does not depend on rainfall, and about 1% of the world's depends on it. Solar stills distil sea water using heat from the Sun; the main industrial method is , shown in the diagram below.

Two containers side by side, each divided by a vertical purple bar labelled partially permeable membrane, with pure water on the left of the bar and sea water, drawn with dots for dissolved salts, on the right. The left container is headed osmosis: a red arrow points from the pure water through the membrane into the sea water, whose level is higher, and a note reads that water moves down its water potential gradient into the sea water, so that level rises. The right container is headed reverse osmosis: a dark piston pressed down by an arrow labelled pressure sits on the sea water, a red arrow points from the sea water back through the membrane into the pure water, and a note reads that pressure forces water the other way, against its water potential gradient. A line at the foot reads that reverse osmosis needs energy, because water is pushed from a low to a high water potential. — Osmosis and reverse osmosis compared. Original BioWelsh diagram.

In , water moves down its gradient, from the dilute into the more concentrated sea water, across a .
In , pressure is applied to the sea water, forcing water molecules to move the other way, from the sea water into the .
Water is therefore moving against its gradient, which is why the system requires energy. The membrane also removes harmful contaminants, but it removes useful minerals as well.
The costs are real: energy consumption is high, so the water is expensive, and the places with the biggest shortages are often poor, far from the sea or at high elevation. Burning to run the process adds carbon dioxide to the atmosphere, and the left behind is denser than sea water, so it sinks and damages the seabed unless it is diluted before it is returned to the ocean.

Lesson 20 of 21

Can biofuels help us to stay inside the climate change boundary?

A is a fuel made by a biological process, such as anaerobic digestion, rather than by the geological processes of long-term heat and compression that formed . This is a technological response to the boundary, and the specification covers it as an example of an informed choice rather than as detailed biochemistry.

The argument for is about where the carbon came from. The carbon dioxide released when a burns was removed from the atmosphere very recently by the crop that made it, and growing the next crop removes it again. release carbon that was locked away hundreds of millions of years ago, so burning them adds carbon dioxide that was not in circulation. In Europe, producing and using has been found to reduce emissions by 60 to 90% compared with the production and use of .

Three biofuels
Biofuel	How it is made	How it is used
	Fermentation of the carbohydrates in sugar or starch crops such as maize, sugar beet, sugar cane and sweet sorghum	Mixed with petrol, usually up to 15%, which most car petrol engines are designed to take
	Vegetable oils, from soya, rapeseed, oil palm or algae, are reacted with an alcohol, commonly methanol, to produce methyl esters	Usually blended with diesel, for example 5% with 95% diesel
	Bacterial digestion of biodegradable waste, such as animal and human waste or energy crops, giving roughly 60% methane and 40% carbon dioxide	Burned for cooking, heating and electricity generation; the solid residue is dried and used as fuel or

is the commonest , and the flow diagram below shows how it is produced.

A flow diagram headed making bioethanol from a sugar or starch crop. A box at the top reads sugar or starch crop, crushed, and three arrows lead from it to boxes labelled bagasse, fibrous waste, on the left, glucose and fructose, in the middle, and sucrose crystallised, on the right. From the middle box an arrow labelled anaerobic fermentation by yeast leads down to a box labelled ethanol, and from there an arrow labelled distillation leads to a box labelled pure ethanol. A red arrow from the bagasse box runs across to the same final stage, labelled burning the bagasse supplies the heat for distillation. — How bioethanol is produced. Original BioWelsh diagram.

The plant material is crushed, and the stored starch is digested by carbohydrases to release sugars.
Sucrose is crystallised out, leaving molasses, a mixture rich in glucose and fructose.
Yeast ferments the glucose and fructose anaerobically to produce a mixture containing .
The mixture is heated and pure is distilled off. The heat comes from burning the , the fibrous waste from the original plant material, so the process does not need a separate fuel.

Food versus fuel: land used to grow food has been turned over to crop production for , so there is less food to eat or to export.
Land use: growing crops has driven , with the and biodiversity loss that follow.
: energy crops are usually grown as , which reduces diversity.
Water: crops require large volumes of irrigation water, which reduces water availability.
Emissions: burning produces more oxides of nitrogen than diesel, and these are both and air pollutants.

is produced in three stages. First, macromolecules in the waste are digested by amylases, and to sugars, amino acids, and . Next, in acetogenesis, an aerobic process, short chain are produced, especially ethanoic acid, and the oxygen is used up. Finally, in , an anaerobic process, methane is produced: C₆H₁₂O₆ → 3CH₄ + 3CO₂. Small digesters matter, because about 0.5 kg of waste per day can provide enough fuel for a family's cooking. So are not simply good or bad. They reduce the use of and so help with the boundary, but they push on the and use boundaries at the same time, and production depends on planting and efficient technical systems. That judgement is exactly the kind of informed choice this topic is about.

Lesson 21 of 21

How does this topic fit together?

Every part of this topic follows the same chain of reasoning: a human activity changes a or a global system, the change affects , and a decision has to be made about what to do. The tree below shows how the ideas connect.

How the ideas in human impact connect

Zoom 100%

Zoom in to read each branch. Swipe or scroll sideways to explore.

Human impact on the environment
- at risk
  - and , categories
  - , hunting, , pollution
  - Fragmentation: small and
- of
  - In the wild: reserves, legislation, education,
  - In captivity: zoos, , ,
- Agricultural exploitation
  - Farming: loss, ,
  - and forestry
  - , regulation and
- and decisions
  - Monitoring: air, soil, water and
  - Statistics, and impact assessments
- - Crossed: , , ,
  - Not crossed: , , use
  - Not quantified: ,
  - Technologies: and

These are the mistakes that cost the most marks in this topic.

Describing as 'low numbers' without saying at risk of .
Listing methods without explaining how each one raises numbers.
Writing 'competition' without naming the and the resource.
Treating a correlation between two graphs as proof of cause.
Giving only one side of a conflict when the question asks about production and .
Leaving the out of the sequence for .
Writing about in questions that are not about them.
Stating a conclusion from statistics without saying whether the is accepted or rejected.

Related learning

Populations & ecosystems
, birth and death rates, the carbon and nitrogen cycles and are all used here.
Photosynthesis
removes the carbon dioxide that and combustion release.
Respiration
by releases carbon dioxide and removes the oxygen lost during .
Microbiology
use in links to the development of - bacteria.
Classification & biodiversity
Measuring biodiversity and classifying underpin the assessment of which are threatened.
Experimental skills
Null hypotheses, critical values and are practical analysis skills tested in Unit 5.

Specification & sources

https://www.wjec.co.uk/media/mxbh1zaf/wjec-gce-biology-spec-from-2015-e-101025.pdf
https://openstax.org/books/biology-2e/pages/47-1-the-biodiversity-crisis
https://openstax.org/books/biology-2e/pages/47-2-the-importance-of-biodiversity-to-human-life
https://openstax.org/books/biology-2e/pages/47-3-threats-to-biodiversity
https://openstax.org/books/biology-2e/pages/47-4-preserving-biodiversity
https://openstax.org/books/biology-2e/pages/44-5-climate-and-the-effects-of-global-climate-change
https://openstax.org/books/biology-2e/pages/46-3-biogeochemical-cycles
https://gml.noaa.gov/ccgg/trends/data.html
WJEC GCE A Level Biology Unit 3 mark schemes, Summer 2017 (Q7), 2018 (Q3 and Q8), 2019 (Q2 and Q5), 2022 (Q2 and Q7), 2023 (Q6), 2024 (Q3) and 2025 (Q6 and Q7), with examiner reports: depth and exam wording
`u3.human-impact__photo-aral-ships.jpg` · Staecker · Public domain · https://commons.wikimedia.org/wiki/File:Aralship2.jpg
`u3.human-impact__photo-clearcut.jpg` · Tomas Sennett · Public domain · https://commons.wikimedia.org/wiki/File:WITHIN_A_MILE_OF_MAGNIFICENT_REDWOODS_ARE_SITES_OF_SOME_OF_THE_WORST_LOGGING_AND_CLEAR_CUTTING_-_NARA_-_542854.jpg
`u3.human-impact__photo-coppice.jpg` · Naturenet · CC BY 2.5 · https://commons.wikimedia.org/wiki/File:Coppice_stool2.JPG
`u3.human-impact__photo-coral-bleaching.jpg` · NOAA · Public domain · https://commons.wikimedia.org/wiki/File:Coral_reef_with_healthy_corals_and_corals_with_signs_of_bleaching.png
`u3.human-impact__photo-fish-cages.jpg` · Peter Whyte, CSIRO · CC BY 3.0 · https://commons.wikimedia.org/wiki/File:CSIRO_ScienceImage_7413_Atlantic_salmon_marine_cages.jpg
`u3.human-impact__photo-krill.jpg` · ingvildriska · CC BY 4.0 · https://commons.wikimedia.org/wiki/File:Euphausia_superba_349737046.jpg
`u3.human-impact__photo-lapwing.jpg` · yrjö jyske · CC BY 2.0 · https://commons.wikimedia.org/wiki/File:Northern_Lapwing,_Finland_(51128772342).jpg
`u3.human-impact__photo-lichen.jpg` · US Fish and Wildlife Service Midwest Region · CC BY 2.0 · https://commons.wikimedia.org/wiki/File:Lichen_on_a_fallen_tree_branch_(45032004931).jpg
`u3.human-impact__photo-mangrove.jpg` · Vyacheslav Argenberg · CC BY 4.0 · https://commons.wikimedia.org/wiki/File:Mangrove_tree_roots,_Koh_Chang,_Thailand.jpg
`u3.human-impact__photo-oil-palm.jpg` · Irvin calicut · CC BY 3.0 · https://commons.wikimedia.org/wiki/File:Oil_palm_Plantation.JPG
`u3.human-impact__photo-panda.jpg` · Jeff Kubina · Public domain · https://commons.wikimedia.org/wiki/File:Giant_Panda_2004-03-2.jpg
`u3.human-impact__photo-passenger-pigeon.jpg` · James St. John · CC BY 2.0 · https://commons.wikimedia.org/wiki/File:Ectopistes_migratorius_(passenger_pigeon).jpg
`u3.human-impact__photo-rainforest.jpg` · T. R. Shankar Raman · CC BY 3.0 · https://commons.wikimedia.org/wiki/File:Borndipt.JPG
`u3.human-impact__photo-seed-store.jpg` · R. C. Johnson · CC BY 2.5 · https://commons.wikimedia.org/wiki/File:Seedbank.jpg
`u3.human-impact__photo-seed-vault.jpg` · Dag Endresen · CC BY 3.0 · https://commons.wikimedia.org/wiki/File:Svalbard_Global_Seed_Vault_-_panoramio.jpg
`u3.human-impact__photo-smog.jpg` · Chester Higgins, Jr. · Public domain · https://commons.wikimedia.org/wiki/File:EAST_RIVER_AND_MANHATTAN_SKYLINE_IN_HEAVY_SMOG_-_NARA_-_548365_(restored).jpg
`u3.human-impact__photo-trawler.jpg` · Richard Stone · Public domain · https://commons.wikimedia.org/wiki/File:Grimsby_Trawlers-_Everyday_Life_With_the_Fishermen,_Grimsby,_Lincolnshire,_England,_UK,_1945_D24801.jpg
https://creativecommons.org/licenses/by/4.0/
https://creativecommons.org/licenses/by/3.0/
https://creativecommons.org/licenses/by/2.5/
https://creativecommons.org/licenses/by/2.0/
https://creativecommons.org/publicdomain/mark/1.0/

What is the difference between an endangered species and an extinct one?

Why do species become endangered?

Why does breaking a habitat into fragments put species at risk?

How are gene pools conserved in the wild?

How are gene pools conserved in captivity?

Why should we conserve species?

How does modern farming conflict with conservation?

What are the consequences of deforestation?

How can forests be used sustainably?

Why does overfishing reduce fish stocks?

How can fishing be regulated so that stocks recover?

Is fish farming a solution or a new problem?

What does sustainability mean, and how do human choices change it?

How is the environment monitored?

How should scientific evidence guide political decisions?

What are planetary boundaries?

Which boundaries have been crossed, and why?

Which boundaries have not been crossed, and which cannot yet be measured?

Why is the fresh water use boundary a problem, and how can it be avoided?

Can biofuels help us to stay inside the climate change boundary?

How does this topic fit together?

Worksheets

Worksheet 1: 3.6 Human impact (154 marks)

Check your understanding.

Which statement correctly distinguishes an endangered species from an extinct one?

Human impact on the environment

What is the difference between an endangered species and an extinct one?

Why do species become endangered?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 11 marks in total. This is a revision priority, not a prediction of your exam.

Why does breaking a habitat into fragments put species at risk?

How are gene pools conserved in the wild?

How are gene pools conserved in captivity?

Why should we conserve species?

How does modern farming conflict with conservation?

What are the consequences of deforestation?

How can forests be used sustainably?

Why does overfishing reduce fish stocks?

How can fishing be regulated so that stocks recover?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 18 marks in total. This is a revision priority, not a prediction of your exam.

Is fish farming a solution or a new problem?

What does sustainability mean, and how do human choices change it?

How is the environment monitored?

How should scientific evidence guide political decisions?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 19 marks in total. This is a revision priority, not a prediction of your exam.

What are planetary boundaries?

Which boundaries have been crossed, and why?

Which boundaries have not been crossed, and which cannot yet be measured?

Why is the fresh water use boundary a problem, and how can it be avoided?

Can biofuels help us to stay inside the climate change boundary?

How does this topic fit together?

Worksheets

Worksheet 1: 3.6 Human impact (154 marks)

Check your understanding.

Why do species become endangered?

How can fishing be regulated so that stocks recover?

How should scientific evidence guide political decisions?