What you'll learn
Human activity changes ecosystems faster than natural processes do, and CSEC Biology expects you to explain those changes in biological terms rather than simply to disapprove of them. This topic covers the main types of pollution and the mechanisms by which each causes harm, deforestation and its consequences, the greenhouse effect and climate change, the depletion of the ozone layer, the effects of sewage and fertiliser on water, conservation and sustainable use, and the particular pressures on Caribbean environments. The marks lie in the mechanism: a question asking why fertiliser run-off kills fish wants the sequence of events, not the observation that pollution is bad.
Key terms and definitions
Pollution — the addition to the environment of substances or energy that cause harm to living organisms.
Pollutant — the substance or form of energy causing that harm.
Eutrophication — the enrichment of water with nutrients, leading to excessive growth of algae and a subsequent fall in dissolved oxygen.
Biological oxygen demand — the quantity of oxygen consumed by microorganisms as they break down organic matter in water.
Deforestation — the permanent removal of trees from an area of forest.
Greenhouse effect — the trapping of heat in the atmosphere by gases that absorb outgoing radiation.
Greenhouse gas — a gas that contributes to this trapping, such as carbon dioxide or methane.
Bioaccumulation — the build-up of a persistent substance in an organism's tissues over time.
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.
Biodiversity — the variety of different species present in an area.
Non-biodegradable — describing a material that decomposers cannot break down.
Core concepts
Water pollution: sewage and eutrophication
Two different pollutants produce a similar result in water, and distinguishing them earns marks.
Untreated sewage contains organic matter. When it enters a river, decomposing bacteria multiply rapidly as they feed on it, and their respiration consumes dissolved oxygen. The biological oxygen demand rises, oxygen concentration falls, and fish and other animals that need well-oxygenated water die or move away. Sewage also carries pathogens, so it presents a direct risk of disease such as cholera or typhoid where drinking water is affected.
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 grow and multiply rapidly, forming a bloom that covers the surface. The bloom blocks light from plants below, which then die. Decomposers feed on the dead algae and plants, multiply, and their respiration removes dissolved oxygen — and once again fish die. This whole sequence is eutrophication, and examiners expect the steps in order: nutrients, algal bloom, light blocked, plants die, decomposers multiply, oxygen falls, fish die.
Note that the algae themselves do not remove the oxygen by being present. It is their decomposition that does, which is the step candidates most often omit.
Water pollution: persistent chemicals
Some pollutants are harmful in very small quantities because they are not broken down. Pesticides and heavy metals such as mercury and lead may enter a food chain at low concentration in water, but each organism retains what it absorbs. Concentration therefore rises at each trophic level, so top predators accumulate the highest levels and suffer most. This is why a chemical can be present at a harmless concentration in water and still kill birds of prey or large fish.
Oil spills coat the surface of the sea, preventing gas exchange and light penetration, and they clog the feathers of seabirds so that the birds lose insulation and buoyancy. In the Caribbean, where fishing and tourism both depend on coastal water, the economic consequences follow closely on the biological ones.
Air pollution
Carbon monoxide from incomplete combustion binds to haemoglobin more strongly than oxygen does, reducing the blood's capacity to carry oxygen. Sulfur dioxide and oxides of nitrogen dissolve in atmospheric water to form acid rain, which lowers the pH of soil and of lakes, damages leaves, releases toxic aluminium ions from soil, and kills fish and trees. Smoke and particulates irritate the lungs and worsen respiratory illness.
Chlorofluorocarbons, once 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, cataracts and damage to crops. Note carefully that ozone depletion and the greenhouse effect are different problems with different causes — one of the commonest confusions in this topic.
The greenhouse effect and climate change
Some atmospheric gases, chiefly carbon dioxide, methane and water vapour, allow incoming solar radiation to pass but absorb the longer-wavelength radiation the warmed Earth emits. This trapping keeps the planet warm enough for life; the concern is its enhancement, as rising concentrations trap more heat than before.
Carbon dioxide concentrations rise through the combustion of fossil fuels and through deforestation, which removes trees that would otherwise have absorbed it. Methane comes largely from cattle, rice cultivation and decaying waste in landfill.
The consequences are examined as a chain of reasoning: higher average temperatures cause ice to melt and sea water to expand, so sea levels rise; weather patterns shift, altering rainfall and the frequency of intense storms; and species whose habitats change faster than they can adapt or migrate decline. For small island states, rising sea level and coastal erosion are direct threats to land and fresh water, and coral bleaching — the loss of the algae living within coral tissue when water becomes too warm — damages reefs that protect coastlines and support fisheries.
Deforestation
Trees are removed for timber, for farmland, for grazing and for building. The consequences run together.
Removing trees reduces photosynthesis, so less carbon dioxide is taken from the air, while burning the felled timber releases more. Habitats are destroyed, so biodiversity falls and some species are lost entirely. Tree 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 the exposed soil, which becomes less fertile. Trees also return water vapour to the air by transpiration, so their removal can reduce local rainfall.
On steep Caribbean hillsides these effects compound: soil lost from a slope cannot be replaced quickly, and the sediment washed into coastal waters smothers coral and seagrass.
Conservation and sustainable use
Conservation is not simply leaving an area untouched; it is management. Methods include establishing protected areas such as national parks and marine reserves; replanting felled forest; setting quotas and closed seasons in fisheries and controlling net mesh size so immature fish escape; captive breeding of endangered species for release; recycling to reduce demand for raw materials; and sewage treatment and controlled fertiliser use to reduce pollution at source.
Behind these is the idea of sustainable development: using a resource at a rate that allows it to renew itself. A fishery harvested at the rate at which the population reproduces can continue indefinitely; one harvested faster collapses. Explaining that balance, rather than listing measures, is what distinguishes a strong answer.
Worked examples
Example 1. Fertiliser is applied to fields beside a river. Two weeks later, fish in the river are found dead. Explain the sequence of events.
Rain washed excess nitrates and phosphates from the fields into the river. These nutrients allowed algae to grow and multiply rapidly, forming a bloom at the surface. The bloom blocked light from plants beneath, and those plants died. Decomposing bacteria fed on the dead algae and plants and multiplied greatly, and their respiration used up the dissolved oxygen in the water. With too little oxygen remaining, the fish suffocated and died.
Example 2. Explain why a pesticide present at a very low concentration in a lake can kill fish-eating birds.
The pesticide is not broken down, so each organism that absorbs it retains it in its tissues. Small organisms take it in from the water, and each predator consumes many prey, so the concentration increases at every trophic level. Fish-eating birds are at the top of the chain and therefore accumulate the highest concentration, which may be high enough to kill them even though the concentration in the water is harmless.
Example 3. A hillside is cleared of trees. Explain two consequences for the soil.
Tree roots no longer bind the soil particles together, and the canopy no longer intercepts rainfall, so rain strikes the surface directly and washes soil away — soil erosion. Second, with no leaf litter being added and with rain passing rapidly through the exposed soil, mineral ions are leached out, so the soil becomes less fertile.
Example 4. Distinguish between the enhanced greenhouse effect and the depletion of the ozone layer.
The enhanced greenhouse effect is the increased trapping of outgoing heat radiation by gases such as carbon dioxide and methane, leading to a rise in average global temperature. Ozone depletion is the breakdown of ozone in the upper atmosphere by chemicals such as chlorofluorocarbons, which allows more ultraviolet radiation to reach the surface. They have different causes, different mechanisms and different consequences.
Common mistakes and how to avoid them
Saying algae use up the oxygen. It is the decomposers feeding on dead algae and plants whose respiration removes the oxygen. Include that step or the explanation is incomplete.
Confusing the greenhouse effect with ozone depletion. Different gases, different regions of the atmosphere, different harm. Keep them in separate paragraphs when both appear in a question.
Writing that pollution is "bad for the environment". Marks are for mechanisms and named organisms. Say which substance, by what route, and what it does.
Treating the greenhouse effect as entirely harmful. The natural greenhouse effect keeps the Earth warm enough to be habitable; it is the enhanced effect that causes concern. Using the word "enhanced" protects the mark.
Listing conservation methods without explaining them. A question asking how a fishery can be managed sustainably wants the reason a quota or closed season works — allowing the population to reproduce faster than it is harvested.
Forgetting that deforestation both reduces absorption and increases release. Fewer trees photosynthesise, and burning the timber emits carbon dioxide. Strong answers give both.
Exam technique for Human Impact on the Environment
Most questions here reward a chain of reasoning, so write in sequence and use connectives that show cause: "so", "therefore", "which means that". Eutrophication in particular is marked step by step, and a step skipped is a mark lost.
Where a question asks for a named example, give one. "A greenhouse gas" is weaker than "carbon dioxide"; "a heavy metal" is weaker than "mercury".
Read whether the question asks for effects on organisms or effects on humans, and answer the one asked. Many candidates lose marks by describing economic consequences when the biology was wanted, or the reverse.
For questions on conservation, balance is rewarded. Acknowledge why an activity happens — timber, food, employment — and then explain how the harm can be reduced, rather than arguing the activity should simply stop.
Finally, watch the command word. State wants a short answer; explain wants a mechanism; discuss wants more than one side. Answering a "discuss" question with a list is the single commonest way to lose marks in this topic.
Quick revision summary
- Sewage in water: decomposers multiply, respire, oxygen falls, fish die; also carries pathogens.
- Eutrophication: nitrates and phosphates → algal bloom → light blocked → plants die → decomposers multiply → oxygen falls → fish die.
- Persistent pesticides and heavy metals are not broken down, so concentration rises along a food chain and top predators are worst affected.
- Acid rain from sulfur dioxide and oxides of nitrogen lowers soil and lake pH, damaging trees and killing fish.
- Carbon monoxide binds haemoglobin and reduces oxygen transport.
- CFCs deplete the ozone layer, admitting more UV — a different problem from the greenhouse effect.
- Enhanced greenhouse effect: more CO₂ and methane trap more outgoing radiation → warming, rising sea level, shifting weather, coral bleaching.
- Deforestation: less photosynthesis, more CO₂ released, habitat and biodiversity loss, soil erosion, flooding, leaching, reduced local rainfall.
- Conservation: protected areas, replanting, quotas and closed seasons, captive breeding, recycling, sewage treatment.
- Sustainable development uses a resource no faster than it renews.