Human impact: WJEC A level Biology cram notes

Lesson 1 of 21

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

Cram notes for topic 3.6, Human impact on the environment, in WJEC A level Biology Unit 3.

A is when no individuals are left alive anywhere in the world. A is when so few individuals remain that it is in danger of becoming .

The is about one in a million each year, written as 10⁻⁶ per year. Human activity has raised the rate in tropical areas to between 1 000 and 10 000 times this. The passenger pigeon became in 1914 and the thylacine in 1936.

The rates by their numbers, rate of decline and , and publishes the results in . The scale is shown below.

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) and in the wild (EW) come first; EW survive only in captivity or cultivation.
The three threatened categories are critically (CR), (EN) and vulnerable (VU).
Near threatened (NT) and least concern (LC) are not threatened categories.
Data deficient (DD) and not evaluated (NE) may still be at risk; there is simply not enough information.

Lesson 2 of 21

Why do species become endangered?

become when the and factors they need are removed, so they reproduce less successfully and numbers fall.

: human activity changes habitats faster than and selection can produce adapted organisms.
: , drainage of wetlands and removal. provide nesting sites, food and , so removing them reduces biodiversity.
Overhunting and : trophy hunting, traditional medicines and the bushmeat trade remove individuals faster than they are replaced.
: an may outcompete native , may have no natural predator here, and may bring disease, which reduces biodiversity and can damage the economy.
Pollution: oil spills prevent oxygenation of surface water and destroy the insulation of birds' feathers; persistent chemicals such as PCBs accumulate along food chains.

Farming practice matters too: autumn sowing the crop is too tall in spring for ground-nesting birds such as the skylark and the lapwing to nest in.

Lesson 3 of 21

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

Fragmentation splits a into . The total number may look healthy, but groups that cannot interbreed behave as several small .

Each small has a small and little , so it is less able to adapt to change and more likely to become .

The diagram below shows three fragments separated from a 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.

R is joined by a corridor, so with the main wood continues and the risk is lowest.
P is separated by developed land but is close to the wood, so some individuals may still cross.
Q is the most isolated, so is least likely and the risk of is greatest.
make small fragments worse than their area suggests, because a greater proportion of the fragment is windier, brighter and drier edge rather than core .

Lesson 4 of 21

How are gene pools conserved in the wild?

is the sensible management of the biosphere to maintain habitats and enhance biodiversity while allowing human activity. protects a in its natural .

Protected sites: local and national nature reserves, and special areas of , managed and monitored by wardens.
Legislation: protecting habitats and preventing , , hunting of game, collection of birds' eggs and picking of wild flowers.
International agreements: restricting trade in products such as ivory, and restricting whaling.
Education: public awareness campaigns, and government bodies that advise on how activities affect wildlife.
: responsible travel that funds , employs local people, educates visitors and works with local communities.

Lesson 5 of 21

How are gene pools conserved in captivity?

protects individuals, gametes or seeds away from the natural . These methods are together called .

Zoos, botanic gardens and are the main examples, shown below.

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.

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.

Zoos and botanic gardens run programmes; records of matings let parents be chosen to keep high.
store gametes, which can be sent around the world far more easily than the animals themselves.
store seeds in controlled conditions, often in liquid nitrogen. Seeds degrade, so samples are periodically germinated and fresh seed is collected.
keep older domesticated breeds that carry useful characteristics such as hardiness.

The aim is to a former , as with the red kite in mid Wales. Captive-bred animals may have no fear of predators and no ability to forage, so they are trained before release.

Lesson 6 of 21

Why should we conserve species?

Conserving a conserves its , which is all the in that . Once an is lost it cannot be recovered.

Ethical: each is a unique combination of and and is considered to be intrinsically valuable.
Agriculture and horticulture: breeding loses rarer , so wild relatives are a for characteristics such as disease .
Adapting to change: if conditions change, individuals carrying useful are selected for, preventing .
Medicines: many drugs come from other organisms, for example quinine from the bark of Cinchona. may become before their chemistry is investigated.

Rainforests and coral reefs cause particular concern because they hold large, complex communities. Rainforest covers about a tenth of the land area but holds at least half of all .

Lesson 7 of 21

How does modern farming conflict with conservation?

Food production has been intensified by cultivating more land, using more and pesticide and using larger machinery. were removed to make large fields, which are used for .

is growing a single crop, breed or variety over a large area. It provides only one , so it reduces diversity.

Growing the same crop repeatedly takes the same minerals from the same depth of soil, so yields fall and more is used.
The same crop is susceptible to the same pests, so pest numbers build up and more pesticides are used.
by cattle compacts the soil, so water cannot drain and roots cannot penetrate it.

Attitudes have changed, and farmers are now encouraged to take biodiversity into account while managing their land.

Lesson 8 of 21

What are the consequences of deforestation?

is the large-scale removal of forest for timber, fuel, paper, farmland, grazing and crops. Roads built to extract the timber open up more forest.

The main consequences follow from losing the trees and their roots, shown below.

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.

: roots no longer bind the soil, so heavy rain washes the topsoil away.
Flooding: plants, and leaf litter no longer soak up rain, so water runs off and lowland areas flood.
Less rainfall: water returns to the atmosphere by evaporation rather than transpiration, which is slower, so the area moves towards desert conditions.
Poorer soil: wet soil has less oxygen and warms slowly, so root activity falls, and cold damp conditions favour bacteria, so is lost.
and biodiversity loss: rainforest covers about 10% of the land area but holds at least half of all .

Carbon dioxide rises because less is used in , more is released by combustion of the felled trees and more by decomposition. Carbon dioxide can also fall locally, because the trees no longer respire.

Lesson 9 of 21

How can forests be used sustainably?

forestry takes timber at a rate that allows the forest to continue, and replants and regenerates what is removed.

: on a small scale, because the surrounding forest recolonises the cleared ground, but not on a large scale.
: the trunk is cut close to the ground, leaving a stool from which new poles grow. A long time increases and creates varied habitats.
: only some trees are felled, so roots still hold the soil, nutrients stay in the forest and less soil is washed into waterways.
: a narrow strip is cut and left for decades. Light reaches the ground, seeds arrive from neighbouring trees and restores the .
Good practice: optimum spacing avoids that produces tall thin trees, pests and diseases are controlled, and a similar number of trees is cut each year.

Replanting native preserves biodiversity. Planting a fast-growing from another continent can lower the water and content of the soil, and biodiversity falls with them.

Lesson 10 of 21

Why does overfishing reduce fish stocks?

is harvesting fish at a rate greater than the rate at which they reproduce, so the cannot replace what is removed.

Larger boats, trawlers, radar and sonar made fishing far more effective. The graph below shows the collapse of one cod fishery.

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 to a peak of about 800 thousand tonnes.
Landings then fall sharply even though fishing effort stayed high, because too few adults were left to breed.
The fishery was closed in 1992 to allow the stock to recover.
Landings have stayed very low since, so a collapsed may take decades to recover or may never recover.

Small mesh nets catch immature fish before they breed, so fewer individuals reproduce and falls.
traps non-target such as dolphins and marine turtles.
drags heavy equipment over the seabed and destroys the habitats of molluscs and other organisms.
Removing small fish such as capelin removes prey for larger fish; harvesting Antarctic krill removes the main food of whales and part of the diet of seals, penguins, squid and fish.

Lesson 11 of 21

How can fishing be regulated so that stocks recover?

Every reduces the rate at which fish are removed or increases the rate at which they are replaced.

Larger net mesh sizes, so young fish escape, survive and interbreed.
limiting the mass of fish brought ashore, although fish caught above the may be discarded dead.
during breeding, and where fishing is prohibited, monitored by satellite with fines for non-compliance.
Limits on the size of the fleet and the number of days at sea.
Consumer choice: buying only fish certified as coming from a source.
, so fewer wild fish are caught.

Stocks are monitored using two criteria: the of fish caught, which measures the death rate, and the of reproducing fish, which measures the . If the death rate is greater than the the decreases; in a stock the two are equal.

The graph below shows the percentage of monitored meeting both targets.

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.

The percentage rises from about 12% in 2005 to about 51% in 2023, most steeply after the policy was introduced in 2010, so the policy is working.
About half of the still fail at least one target, so it is only partly effective.
that are not monitored do not appear in the figures at all.

Lesson 12 of 21

Is fish farming a solution or a new problem?

rears fish in ponds, lakes, enclosures or cages, where predation is and food is supplied. Fish convert food into protein efficiently, more of the body is edible, and the is lower than farming mammals.

Farming fish in cages in open water causes the problems shown 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.

Fish are densely stocked, so disease spreads quickly and can pass to wild fish; are used, which encourages - bacteria.
Pesticides used against parasites such as sea lice are toxic to marine invertebrates.
Uneaten food, faeces and inorganic waste add nitrogen and phosphorus to the water, causing .
Escaped fish have been selected for rapid growth, so they outcompete wild fish for food, and mates, transmit parasites and interbreed with wild .

Extra and phosphate allow algae to grow rapidly, producing an at the surface.
The bloom blocks light, so plants in deeper water cannot photosynthesise and they die.
respire aerobically as they break down the dead material and use up the dissolved oxygen.
The water is deoxygenated, so fish and other oxygen-requiring die.

Shrimp farming has been blamed for salinisation of soil and groundwater and for destroying mangroves, which protect coastal communities from tropical storms.

Lesson 13 of 21

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

using resources so that present needs are met without preventing future generations from meeting theirs. A rate of use does not exceed the rate of replacement.

Forests: or on a long with native replanting, rather than clear felling.
Fish: , larger mesh sizes, and , rather than catching fish faster than they reproduce.
Soil: , restored and suitable stocking levels, rather than repeated and .
: , reclaimed waste water and , rather than pumping dry.

Achieving needs changes in attitudes as well as technology. Informed choices include buying certified fish and timber, eating less meat and using water and energy efficiently. Technologies such as and help, but bring problems of their own.

Lesson 14 of 21

How is the environment monitored?

establishes the current status of the environment, and repeating it in the same way shows trends and allows predictions.

Air quality: concentrations of pollutants are measured because of their health effects. Pollution is carried by the wind, so wind must be considered when a source is identified.
Soil: structure, , water-holding and drainage capacity, pH, organic particles and earthworms.
Water: chemical monitoring of pollutants, biological monitoring using , and microbiological counts of coliform bacteria and E. coli.
: the of lichen present indicate the level of air pollution, because lichens absorb pollutants over their whole surface.

The graph below shows nitrogen dioxide concentration at different distances from a motorway, with the World Health Organisation guideline of 40 µg m⁻³ marked.

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 concentration is highest at the road, about 68 µg m⁻³, and falls steeply over the first 100 m.
The guideline of 40 µg m⁻³ is met at about 90 m from the road.
The highest levels are where there are most vehicles, which supports the hypothesis that exhausts are the main source.

Monitoring data feed into an , which predicts the environmental effects of a proposed project. It describes the project, the alternatives considered, the environment affected, the effects and the , such as building wind turbines when birds are not nesting.

Lesson 15 of 21

How should scientific evidence guide political decisions?

Political decisions about the environment need to be based on knowledge gained using sound scientific principles, so monitoring data are tested statistically before they are used.

A states that there is no difference between the being compared.
A statistical test gives a calculated value, which is compared with a from a table at a probability level of 0.05.
If the calculated value is greater than the , reject the : there is a difference and it is not due to chance.
If it is smaller, accept the and treat the difference as chance .

A result still has to be checked against other evidence. If trees beside a road absorb more nitrogen dioxide in summer than in autumn, a seasonal effect could explain a difference between a road being open and closed, which reduces confidence in the conclusion. Repeating the experiment at other times of year, or over a longer period, improves the design.

Planning teams include engineers, economists, geologists and others as well as biologists, and the legal framework requires them to preserve habitats and enhance biodiversity.

Lesson 16 of 21

What are planetary boundaries?

, introduced in 2009, define a for humanity. Nine global systems regulate the stability of the planet and the interactions of air, land and sea, and each has an estimated .

The response to change is not linear. At a a small change has a very large and unpredictable effect, so exceeding a boundary is likely to cause sudden and catastrophic environmental change.

The status of the nine systems is shown on the circular diagram 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 circle is the : systems are stable, changes are reversible and there is no detrimental impact on global systems.
Between the inner circle and the outer circle is the , an area of increasing risk.
Beyond the outer circle the risk is high, the boundary has been crossed and events are unpredictable.
Grey wedges are boundaries that have not been quantified.

Three are global: , and .
Four relate to the biosphere: , use, and .
Two concern non-natural phenomena: and .
and are the two .

Lesson 17 of 21

Which boundaries have been crossed, and why?

The boundary is a carbon dioxide concentration of 350 parts per million by volume. Even if emissions stopped now, temperature would keep rising for decades, and a sea level rise of up to 7 m is predicted by 2100 unless drastic action is taken.

The graph below shows measured carbon dioxide concentration with the 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 from about 317 parts per million in 1960 to about 425 in 2024.
The line gets steeper, so the rate of increase is rising.
The boundary of 350 parts per million was crossed in about 1988.

: renamed from biodiversity loss to emphasise functioning. together provide such as recycling minerals and absorbing carbon dioxide.
Coral reefs hold up to 1 000 per m². Warm water makes the leave, so corals bleach and, if the do not return, die.
: forests cleared for farming, livestock, oil palm and soya. The boundary measures land use that results in little food being produced.
: intensive use has crossed the boundaries for both the nitrogen and the phosphorus cycles, causing .

Lesson 18 of 21

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

About 90% of ozone is in the stratosphere, 10 to 50 km up, where ozone and oxygen are in equilibrium: 3O₂ ⇌ 2O₃.

destroy ozone as shown below.

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.

are heavier than air and take two to five years to reach the stratosphere.
Ultraviolet light releases chlorine from the molecules.
Each chlorine destroys about 100 000 ozone molecules, because it is regenerated each time.
Less ozone more , of 280 to 315 nm, reaches the surface. is strongly absorbed by DNA, so skin cancers and cataracts increased.

in spray cans were banned in 1978 and their manufacture was banned by international agreement in 1987. is the only whose crossing has been deliberately avoided.

has not been crossed either. The reactions are shown below.

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 and forms carbonic acid: H₂O + CO₂ → H₂CO₃, which releases a : H₂CO₃ → H⁺ + HCO₃⁻.
Hydrogen carbonate breaks down further: HCO₃⁻ → H⁺ + CO₃²⁻, so the pH falls.
Ocean pH has fallen from about 8.16 in the seventeenth century to about 8.03. The pH scale is logarithmic, so one pH unit is a tenfold change in concentration.
Hydrogen ions leach calcium carbonate from shells and exoskeletons: CaCO₃ + H⁺ → Ca²⁺ + HCO₃⁻, and fish gill structure and function are damaged.

are microscopic particles from the combustion of fuels and from dust created by digging and quarrying. Their concentration has doubled since the Industrial Revolution.

Heavy smog, shown below, 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.

particles, 10 µm across, lodge in the lungs and worsen respiratory disease.
particles, 2.5 µm across, transfer to the capillaries and increase deaths from cardiovascular disease.
Motor vehicles, especially diesel engines, are the main source of both.
Particles on leaves reduce light absorption for , so crop yields fall.

No boundary has been set because sulphates reflect sunlight and cool the surface while soot absorbs and re-radiates it and warms the surface. The balance is unclear and are very variable.

, previously the chemical pollution boundary, cover organic pollutants, radioactive materials, nanomaterials and micro-plastics. About 100 000 manufactured chemicals are in use, they may interact to produce additional effects, and their effects cannot yet be quantified. The overall concentration is still thought 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?

has a low concentration of dissolved salts, below 0.05% by mass. About 97% of the world's water is saline, and most of the 2.5% that is fresh is frozen, with about 61% of it in the Antarctic ice sheet.

Demand has risen sharply, as the graph below shows.

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.
Agriculture uses more than any other human activity, so most of the rise is irrigation.
pumped faster than rain refills them cannot supply water at this rate indefinitely.

Supply is by draining wetlands, and , which change the water cycle.
Pollution makes water unusable; blooms of Anabaena flos-aquae release a neurotoxin to birds and mammals.
first increases and then decreases supply as glaciers melt, and rising sea level makes coastal groundwater too salty.
Provision can be increased by water , , reclaimed waste water, capturing urban runoff, and .

is the main method of , shown 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 into sea water across a .
In , pressure applied to the sea water forces water the other way, against its gradient, which is why energy is needed.
Energy costs make the water expensive, and burning for that energy releases carbon dioxide.
The left behind is dense, so it sinks and damages the seabed; it is diluted before being returned to the ocean.

Lesson 20 of 21

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

A is made by a biological process such as anaerobic digestion, rather than by the geological processes that formed .

The carbon dioxide released when a burns was removed from the atmosphere recently by the crop, and the next crop removes it again. release carbon laid down hundreds of millions of years ago. Producing and using in Europe reduces emissions by 60 to 90% compared with .

is the commonest ; its production is shown below.

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.

Crushed plant material is digested by carbohydrases so that the stored starch releases sugars.
Sucrose crystallises out, leaving a mixture rich in glucose and fructose.
Yeast ferments the glucose and fructose to give a mixture containing .
The mixture is distilled, using heat from burning the fibrous , to give pure .

: vegetable oils are reacted with an alcohol, commonly methanol, to make methyl esters, usually blended with diesel.
: bacterial digestion of biodegradable waste gives roughly 60% methane and 40% carbon dioxide, ending with , C₆H₁₂O₆ → 3CH₄ + 3CO₂.
Concerns: food versus fuel, for energy crops, , large volumes of irrigation water, and more oxides of nitrogen from burning .

Lesson 21 of 21

How does this topic fit together?

Each part of this topic follows the same chain: a human activity changes a or a global system, are affected, and a decision has to be made.

at risk: and , categories, , hunting, , pollution and fragmentation.
: reserves, legislation, education and ; zoos, , and .
Agricultural exploitation: and loss, and forestry, , regulation and .
and decisions: monitoring of air, soil and water, , statistics, and impact assessments.
: four crossed, ozone deliberately avoided, and use avoidable, and not quantified.

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: cram notes

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?