What you'll learn
Human activity changes the environment faster than natural processes do, and Human and Social Biology asks you to explain those changes biologically and in terms of their effect on human health. This topic covers pollution of water, air and land and the mechanism by which each causes harm, eutrophication, sewage and waterborne disease, deforestation, the greenhouse effect and climate change, ozone depletion, pest control and pesticides, waste disposal, and conservation and sustainable use. It also covers the particular pressures on Caribbean environments.
The marks are in the mechanism and in the link to health. A question asking why untreated sewage in a river endangers people wants the pathway — not the observation that pollution is undesirable.
Key terms and definitions
Pollution — the addition to the environment of substances or energy that harm living organisms.
Pollutant — the substance or form of energy causing the harm.
Eutrophication — enrichment of water with nutrients, causing excessive algal growth and a subsequent fall in dissolved oxygen.
Biological oxygen demand — the quantity of oxygen consumed by microorganisms breaking down organic matter in water.
Deforestation — the permanent removal of trees from an area of forest.
Greenhouse effect — the trapping of outgoing heat radiation by gases in the atmosphere.
Bioaccumulation — the build-up of a persistent substance in an organism's tissues.
Biomagnification — the increase in concentration of such a substance at successive levels of a food chain.
Biodegradable — able to be broken down by decomposers.
Conservation — the protection and careful management of natural resources and habitats.
Sustainable development — use of resources that meets present needs without preventing future generations from meeting theirs.
Potable water — water that is safe to drink.
Core concepts
Water pollution: sewage and health
Untreated sewage harms in two distinct ways, and a full answer gives both.
First, it contains organic matter. Decomposing bacteria multiply rapidly as they feed on it, and their respiration consumes dissolved oxygen. The biological oxygen demand rises, the oxygen concentration falls, and fish and other aerobic organisms die or move away.
Second, sewage carries pathogens from human faeces. Where it contaminates drinking water it transmits cholera, typhoid, dysentery and hepatitis A, and it carries the eggs of parasitic worms. This is the health link Human and Social Biology cares about most: the disease is transmitted by the faecal–oral route, and the control measures follow directly — treat sewage before discharge, protect and chlorinate water supplies, and separate sewage pipes from water pipes.
Proper sewage treatment removes solids by settling, then uses bacteria in aeration tanks to break down the remaining organic matter, and finally disinfects the effluent. Treated effluent has a low biological oxygen demand and few pathogens.
Water pollution: eutrophication
Fertiliser run-off works by a longer route. Rain washes excess nitrates and phosphates from farmland into rivers and lakes. These nutrients allow algae to multiply rapidly, forming a bloom at the surface. The bloom blocks light from plants beneath, and those plants die. Decomposers feed on the dead algae and plants, multiply greatly, and their respiration removes the dissolved oxygen — and fish die.
Give the steps in order: nutrients, algal bloom, light blocked, plants die, decomposers multiply, oxygen falls, fish die. The step most often omitted is that it is the decomposition of the algae, not their presence, that removes the oxygen.
Persistent chemicals and the food chain
Some pollutants are dangerous in very small quantities because decomposers cannot break them down. Pesticides such as DDT, and heavy metals such as mercury and lead, enter a food chain at low concentration. Each organism retains what it absorbs, a process called bioaccumulation, and because every predator eats many prey the concentration rises at each trophic level — biomagnification. Top predators therefore carry the highest concentrations and suffer most.
In humans the effects are specific and examinable: lead damages the nervous system, particularly in children; mercury causes nerve and kidney damage; and both accumulate because the body cannot excrete them readily. This explains why a chemical can be at a harmless concentration in river water and still poison people who eat fish from that river.
Air pollution and respiratory health
Carbon monoxide from incomplete combustion binds to haemoglobin more strongly than oxygen does, forming carboxyhaemoglobin and reducing the blood's oxygen-carrying capacity. It causes headaches, dizziness and, at high concentration, death.
Sulfur dioxide and oxides of nitrogen dissolve in atmospheric water to form acid rain, which lowers the pH of soil and lakes, damages leaves, releases toxic aluminium ions from soil and kills fish and trees. Both gases also irritate the airways and worsen asthma and bronchitis.
Smoke and particulates irritate the lining of the respiratory tract, paralyse the cilia that sweep mucus upwards, and so increase the risk of infection, chronic bronchitis and emphysema. The mechanism — cilia paralysed, mucus accumulates, bacteria multiply — is the same one examined for tobacco smoke, and it is worth learning once and using in both places.
Ozone depletion
Chlorofluorocarbons, formerly used in refrigerators and aerosols, break down ozone in the upper atmosphere. Because the ozone layer absorbs much of the incoming ultraviolet radiation, its thinning allows more UV to reach the surface, raising the incidence of skin cancer and cataracts and suppressing immune function.
Ozone depletion and the greenhouse effect are different problems with different causes, and confusing them is among the commonest errors in this topic.
The greenhouse effect and climate change
Certain atmospheric gases — chiefly carbon dioxide and methane — allow incoming solar radiation to pass but absorb the longer-wavelength radiation the warmed Earth emits. This natural greenhouse effect keeps the planet habitable; the concern is its enhancement as concentrations rise.
Carbon dioxide rises through the combustion of fossil fuels and through deforestation, which removes trees that would have absorbed it. Methane comes largely from cattle, rice cultivation and decaying waste in landfill.
The consequences follow as a chain: higher average temperatures melt ice and expand sea water, so sea levels rise; rainfall patterns shift and intense storms become more frequent; and species unable to adapt or migrate decline. For Caribbean territories the direct threats are coastal flooding, saltwater intrusion into freshwater supplies, damage to the reefs that protect coastlines, and coral bleaching when sea temperatures rise. Health effects include a wider range for mosquito-borne diseases such as dengue and chikungunya as warm, wet conditions extend.
Deforestation
Trees are cleared for timber, farmland, grazing and building, and the consequences compound.
Removing trees reduces photosynthesis, so less carbon dioxide is absorbed, while burning the felled timber releases more. Habitats are destroyed, so biodiversity falls. Roots no longer bind the soil, so rain washes it away as soil erosion, and because the canopy no longer intercepts rainfall, water reaches the ground faster and flooding becomes more likely. Nutrients are leached from exposed soil, which loses fertility. Trees also return water vapour by transpiration, so clearance can reduce local rainfall.
On steep Caribbean hillsides these effects are severe: soil lost from a slope cannot be replaced quickly, and sediment washed into coastal waters smothers coral and seagrass.
Pest control and waste disposal
Pesticides raise crop yields but kill organisms other than the target pest, including pollinators and the natural predators of pests, and persistent ones biomagnify. Pests may also become resistant, so stronger or more frequent application is needed. Biological control — introducing a natural predator — avoids chemical residues but risks the introduced species becoming a pest itself.
Waste must be considered by whether it is biodegradable. Food and paper decompose; plastics, glass and metals largely do not, so they persist in landfill or as litter, harming animals that swallow or become entangled in them. Landfill takes land and can leach into groundwater; incineration reduces volume but releases gases and particulates; recycling conserves raw materials and energy but requires collection and sorting.
Providing safe drinking water
Because so much of the harm above reaches people through water, treatment of the supply matters as much as treatment of the sewage. Water is made potable by screening out debris, allowing solids to settle, filtration through sand or gravel to remove fine particles and many microorganisms, and finally chlorination to kill remaining pathogens. Fluoride is sometimes added to reduce tooth decay.
In rural Caribbean districts where piped treated water is unavailable, households may rely on rainwater catchment or wells. Both are vulnerable: a catchment collects whatever is on the roof, and a well near a pit latrine can be contaminated by seepage. Simple measures — covering storage tanks, siting latrines well away from and below wells, and boiling water during an outbreak — break the faecal–oral route and are commonly asked for as control measures.
Conservation and sustainable use
Conservation is active management, not simply leaving land alone. Methods include protected areas such as national parks and marine reserves; reforestation; fishing quotas, closed seasons and net mesh-size limits so immature fish escape; captive breeding of endangered species; recycling; and sewage treatment with controlled fertiliser use to reduce pollution at source.
Behind these lies sustainable development: using a resource no faster than it renews. A fishery harvested at the rate the population reproduces can continue indefinitely; one harvested faster collapses. Explaining that balance earns more than listing measures.
Worked examples
Example 1 (5 marks). Untreated sewage is discharged into a river used by a village downstream for drinking water. Explain two ways in which this endangers the villagers.
First, the sewage contains pathogens from human faeces, so drinking the water transmits diseases such as cholera and typhoid by the faecal–oral route. Second, decomposing bacteria feeding on the organic matter multiply and respire, using up dissolved oxygen so that fish die; the villagers may lose a source of food and protein, and the water becomes foul.
Example 2 (6 marks). Fertiliser is applied to fields beside a lake. Some weeks later the fish are dead. Describe the sequence of events.
Rain washed excess nitrates and phosphates into the lake. These nutrients allowed algae to multiply rapidly, forming a bloom at the surface. The bloom blocked light from the plants below, which died. Decomposers fed on the dead algae and plants and multiplied greatly, and their respiration used up the dissolved oxygen. With too little oxygen the fish suffocated.
Example 3 (4 marks). Mercury discharged at low concentration into a bay is later found at high concentration in large fish. Explain how this happens and why it matters to people.
Mercury is not broken down, so each organism retains what it absorbs — bioaccumulation. Because each predator eats many prey, the concentration increases at every trophic level, which is biomagnification, and large predatory fish carry the highest levels. People who eat those fish take in enough mercury to cause nerve and kidney damage, even though the concentration in the water was low.
Example 4 (4 marks). Explain why clearing forest from a steep Caribbean hillside increases the risk of flooding and damages coastal reefs.
Without roots to bind it and without a canopy to intercept rainfall, soil is washed off the slope and rain reaches rivers faster, so rivers rise quickly and flood. The eroded soil is carried into coastal waters, where the sediment settles on coral and seagrass, blocking light and smothering them.
Common mistakes and how to avoid them
Saying algae use up the oxygen. It is the decomposers feeding on dead algae whose respiration removes it.
Confusing the greenhouse effect with ozone depletion. Different gases, different parts of the atmosphere, different harm.
Writing that pollution is bad for the environment. Marks are for named pollutants, routes and effects.
Omitting the health link. This is Human and Social Biology: say which disease or which organ is affected.
Treating the greenhouse effect as wholly harmful. The natural effect makes the Earth habitable; it is the enhanced effect that causes concern.
Listing conservation measures without explaining them. Say why a quota or closed season works — it allows the population to reproduce faster than it is harvested.
Forgetting that deforestation both reduces absorption and increases release. Give both.
Exam technique for Impact of Human Activities on the Environment
Answer in a chain of cause and effect, using connectives such as "so" and "therefore". Eutrophication and biomagnification are both marked step by step, and a missing step is a lost mark.
Name specific examples. "A greenhouse gas" is weaker than "carbon dioxide"; "a disease" is weaker than "cholera"; "a heavy metal" is weaker than "mercury".
Read whether the question asks about effects on organisms, on humans, or on the economy, and answer the one asked. Candidates often supply the wrong one.
For conservation questions, acknowledge why the damaging activity happens — income, food, employment — then explain how the harm can be reduced. A balanced answer scores better than one arguing the activity should stop.
Watch the command word: state wants a short answer, explain a mechanism, discuss more than one side.
Quick revision summary
- Sewage: organic matter → decomposers respire → oxygen falls → fish die; plus pathogens causing cholera, typhoid, dysentery by the faecal–oral route.
- Eutrophication: nitrates and phosphates → algal bloom → light blocked → plants die → decomposers multiply → oxygen falls → fish die.
- Bioaccumulation then biomagnification concentrate persistent pesticides and heavy metals up a food chain; lead damages nerves, mercury nerves and kidneys.
- Carbon monoxide binds haemoglobin, reducing oxygen transport.
- Acid rain from sulfur dioxide and nitrogen oxides lowers pH, kills trees and fish, and worsens asthma.
- Particulates paralyse cilia, so mucus accumulates and infection follows.
- CFCs deplete ozone, admitting UV → skin cancer and cataracts. Distinct from the greenhouse effect.
- Enhanced greenhouse effect: more CO₂ and methane → warming, rising sea level, saltwater intrusion, coral bleaching, wider range for dengue.
- Deforestation: less photosynthesis, more CO₂ released, habitat and biodiversity loss, erosion, flooding, leaching, reduced rainfall.
- Pesticides harm non-target species and select for resistance; biological control avoids residues but carries its own risk.
- Conservation: protected areas, reforestation, quotas and closed seasons, captive breeding, recycling, sewage treatment.
- Sustainable development uses a resource no faster than it renews.