4d Human Influences on the Environment¶

Part of 4 Ecology and the Environment.

Human activity changes the gases in the air, the nutrients in the soil and water, and the land that is covered by trees. Each topic here is a chain of cause and effect, and a good answer follows the chain one link at a time.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

How air pollution, greenhouse gases and water pollution affect ecosystems, and how eutrophication happens. Biology-only students also learn the effects of deforestation.

Air Pollution¶

Burning fossil fuels releases gases that harm organisms directly and ecosystems indirectly.

	Sulfur dioxide	Carbon monoxide
Source	Burning fossil fuels that contain sulfur, particularly coal and oil	Incomplete combustion of fossil fuels, when oxygen is limited
What it does	Dissolves in water droplets to form dilute sulfuric acid, which falls as acid rain	Binds irreversibly to haemoglobin, so haemoglobin can no longer carry oxygen
Effect on organisms	Makes rivers and lakes too acidic for many organisms, which reduces biodiversity, and washes mineral ions out of the soil, which prevents plant growth	Reduces the oxygen-carrying capacity of the blood, causing tiredness, unconsciousness or death. In pregnancy it can impair the growth of the foetus
Other effect	Corrodes metals and erodes limestone structures

Greenhouse Gases and Global Warming¶

Greenhouse gases absorb infrared radiation from the Earth's surface and radiate some of it back down, which traps heat in the lower atmosphere. This greenhouse effect keeps the planet warm naturally. Human activity has increased the concentration of these gases, which strengthens the effect and causes global warming.

Gas	Main sources
Water vapour	Evaporation from rivers, lakes and oceans
Carbon dioxide	Burning fossil fuels, and deforestation
Methane	Cattle (from digestion), rice paddy fields, landfill
Nitrous oxide	Agricultural fertilisers and vehicle engines
CFCs	Refrigerants and aerosol propellants

The biological consequences follow from the warming:

Temperature and rainfall patterns change, so habitats change.
Melting glaciers and polar ice raise sea level, which can flood coastal habitats.
Species that lose their habitat must move or face extinction.
Seasonal events can shift in timing, which can break links between species, for example when insects emerge before the birds that feed on them.

Water Pollution and Eutrophication¶

Two sources of pollution lead to the same sequence of events. Sewage is rich in organic matter, so bacteria multiply to break it down. Fertilisers applied to farmland can be washed by rain into rivers and lakes (leaching), and the excess nitrate and phosphate ions make algae grow explosively.

The chain that follows, called eutrophication, has to be followed in order:

Extra nutrients enter the water, causing a rapid growth of algae, an algal bloom, on the surface.
The algae block light from reaching the plants below, so those plants die.
Bacteria decompose the dead algae and plants, and multiply.
The bacteria respire aerobically and use up the dissolved oxygen.
With little oxygen, fish and other aquatic animals die, so biodiversity falls.

Sewage treatment works reduce the problem by adding oxygen, through aeration with stirring or air injection, so that bacteria can break the sewage down before it reaches a river.

Exam technique

The oxygen does not run out because the algae use it. The bloom is a cause, and the oxygen is used up by the bacteria that decompose the dead material. Write each step as a result of the one before ("so", "which means"), and name the group of organisms doing it. A list of unconnected facts about algae and fish earns less than the full chain.

Biology-Only Content¶

This content is required for Biology-only students and is not required for Combined Science students.

The section below, on deforestation, is Biology-only.

Deforestation¶

Deforestation is the large-scale removal of trees from an area, usually to make agricultural land or to obtain timber. Trees do several jobs in an ecosystem, so removing them has several effects:

What trees do	Effect of removing them
Take up mineral ions through their roots	Minerals are washed into rivers and lakes (leaching), which adds to eutrophication
Bind soil with their roots and intercept rainfall	Soil is easily washed or blown away (soil erosion), so the land becomes less productive
Release water vapour by transpiration	Less water vapour enters the air (reduced evapotranspiration), which changes local rainfall and the water cycle
Remove carbon dioxide by photosynthesis and store carbon in wood	Burning or rotting trees releases stored carbon, and fewer trees remove carbon dioxide, so atmospheric carbon dioxide rises and the greenhouse effect is strengthened

The ecological impact is both local, through the soil and rainfall, and global, through the balance of atmospheric gases.

Common Confusions¶

Acid rain and global warming: Acid rain is caused by sulfur dioxide, and global warming by greenhouse gases such as carbon dioxide and methane. They are different problems.
Carbon monoxide vs carbon dioxide: Carbon monoxide is toxic because it binds to haemoglobin. Carbon dioxide is a greenhouse gas, and is not poisonous in the same way.
Eutrophication and oxygen: Oxygen falls because bacteria respire as they decompose dead material, not because the algae use it.
Greenhouse effect vs global warming: The greenhouse effect is natural and keeps the Earth warm. Global warming is the extra warming caused by human increases in greenhouse gases.

Check Yourself¶

Explain how sulfur dioxide from a power station can reduce the number of fish in a lake.
Explain why carbon monoxide is poisonous.
Name two greenhouse gases and one human source of each.
A farmer spreads too much fertiliser on a field next to a river. Explain, in order, how this could kill fish in the river.
Explain why a sewage works adds air to the sewage.
(Biology only) Explain two ways in which deforestation can increase the carbon dioxide concentration in the atmosphere.

Answers

Sulfur dioxide dissolves in rain to form acid rain, which runs into the lake and makes it too acidic for many organisms, so the fish die or cannot breed and the number falls.
It binds irreversibly to haemoglobin in red blood cells, so haemoglobin cannot carry oxygen. Tissues receive too little oxygen.
Carbon dioxide, from burning fossil fuels; methane, from cattle or rice paddy fields (or nitrous oxide from fertilisers).
Rain washes the fertiliser into the river, and the extra nitrate causes an algal bloom. The algae block light, so plants die. Bacteria decompose the dead material and use up the dissolved oxygen by respiration. With too little oxygen the fish die.
The air supplies oxygen for bacteria to break down the sewage by aerobic respiration, so the sewage is treated before it enters a river and does not use up the river's oxygen.
Burning or rotting trees releases the carbon stored in them as carbon dioxide. With fewer trees there is also less photosynthesis, so less carbon dioxide is removed from the air.

Key Terms¶

Greenhouse gas: a gas in the atmosphere that absorbs and re-emits infrared radiation, contributing to the warming of the Earth's surface.
Greenhouse effect: the trapping of heat energy in the lower atmosphere by greenhouse gases.
Global warming: a long-term increase in average global temperature caused by elevated greenhouse gas concentrations.
Acid rain: rain made acidic by dissolved pollutants such as sulfur dioxide.
Eutrophication: nutrient enrichment of water causing algal overgrowth and oxygen depletion.
Algal bloom: a rapid and excessive growth of algae, often triggered by high nutrient levels.
Leaching: the washing of dissolved nutrients out of the soil by water.
Soil erosion: the removal of topsoil by wind or water, particularly when roots are no longer present.
Evapotranspiration: combined water loss from soil evaporation and plant transpiration.
Deforestation: the large-scale removal of trees from an area.
Carbon monoxide: a toxic gas produced by incomplete combustion that binds irreversibly to haemoglobin.