Edexcel International GCSE Biology 4BI1: Ecology and the environment – Study Guide

This study guide teaches Topic 4, Ecology and the environment (specification points 4.1–4.18), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The qualification is untiered. Points with a B reference (4.3B, 4.4B, 4.11B and 4.18B) are bold in the specification and are examined on Paper 2 only; everything else can appear on either paper. It applies to the June and November series examined under Issue 3.

Use it with the revision notes and the practice questions. The full course is on the Edexcel IGCSE Biology hub, and you can tick off each point on the printable checklist.

What this topic covers

Spec point	What you must be able to do	Paper
4.1	Use the terms population, community, habitat, ecosystem	1 and 2
4.2	Practical: estimate population size in two areas using quadrats	1 and 2
4.3B	Understand the term biodiversity	Paper 2 only
4.4B	Practical: investigate distribution and measure biodiversity with quadrats	Paper 2 only
4.5	Explain how abiotic and biotic factors affect population size and distribution	1 and 2
4.6–4.7	Name trophic levels; food chains, webs and pyramids of number, biomass and energy	1 and 2
4.8–4.9	Transfer of substances and energy; why only about 10% of energy passes on	1 and 2
4.10	Describe the carbon cycle	1 and 2
4.11B	Describe the nitrogen cycle	Paper 2 only
4.12–4.17	Air pollution, greenhouse gases, sewage and eutrophication	1 and 2
4.18B	Effects of deforestation	Paper 2 only

(a) The organism in the environment

Key terms (4.1)

• Population: all the organisms of one species living in one area at the same time.
• Community: all the populations of different species living in one area.
• Habitat: the place where an organism lives.
• Ecosystem: a community together with its physical (abiotic) environment, interacting.

Estimating population size with quadrats (4.2)

A quadrat is a square frame, often 0.5 m × 0.5 m or 1 m × 1 m. You use it for organisms that do not move, or move slowly, such as plants or limpets.

1. Mark out the area with two tape measures at right angles, forming a grid.
2. Generate random numbers to give coordinates, and place the quadrat at each one. Random placement removes bias in where you sample.
3. Count the individuals of the species inside each quadrat. Decide a rule for plants that lie on the edge (for example, count them only on two sides).
4. Use many quadrats (at least ten) and calculate the mean per quadrat.
5. Scale up: population = mean per m² × total area in m².

To compare two different areas, use the same quadrat size, the same number of quadrats and the same counting rule in both, so that only the area changes.

Worked example 1. You compare dandelions in two areas of a school field. Each area is 30 m × 20 m. You place ten 0.5 m × 0.5 m quadrats at random in each.

Area A counts: 3, 5, 0, 2, 4, 6, 1, 3, 2, 4. Area B counts: 1, 0, 2, 0, 1, 3, 0, 1, 2, 0.

Quadrat area = 0.5 × 0.5 = 0.25 m²
Area of each site = 30 × 20 = 600 m²

Area A: total = 30, mean = 30 ÷ 10 = 3.0 per quadrat
        per m² = 3.0 ÷ 0.25 = 12 per m²
        population = 12 × 600 = 7200 dandelions

Area B: total = 10, mean = 1.0 per quadrat
        per m² = 1.0 ÷ 0.25 = 4 per m²
        population = 4 × 600 = 2400 dandelions

Area A has three times the estimated population of Area B.

Biodiversity (4.3B, 4.4B) – Paper 2 only

Biodiversity is the variety of living organisms in an area. At IGCSE you should think of it in terms of how many different species are present and how evenly their numbers are spread.

To investigate distribution, you lay a tape across a change in conditions (for example, from shade under a tree into open grass) and place a quadrat at regular intervals along it. At each point, record which species are present, or estimate percentage cover. Here regular placement is deliberate, not random.

To measure biodiversity, place quadrats at random in each area and record every species present and the number (or cover) of each. The area with more species, spread more evenly, has the higher biodiversity. For example, a site with five species, each present in similar numbers, is more biodiverse than a site dominated by one species with two rare ones.

Abiotic and biotic factors (4.5)

Abiotic factors are non-living: temperature, light intensity, water availability, pH, mineral ions in soil, oxygen level in water, wind.

Biotic factors are living: food supply, predators, competition (within and between species), disease and parasites.

For a question on this point, link the factor to a process. Low light reduces photosynthesis, so fewer plants grow in shade. More predators increase deaths, so the prey population falls. If a food source is plentiful, more offspring survive, so the population rises until another factor becomes limiting.

(b) Feeding relationships

Trophic levels (4.6)

• Producers: green plants and algae that make food by photosynthesis.
• Primary consumers: herbivores that eat producers.
• Secondary consumers: eat primary consumers.
• Tertiary consumers: eat secondary consumers.
• Decomposers: bacteria and fungi that feed on dead organisms and waste, releasing nutrients.

In a food chain, arrows show the direction of energy transfer, pointing from the food to the organism that eats it. A food web links many food chains. In a food web one animal can sit at two trophic levels, depending on what it eats.

Pyramids (4.7)

• Pyramid of numbers: number of organisms at each level. It can be an odd shape. One oak tree supports thousands of caterpillars, so the bottom bar is narrow.
• Pyramid of biomass: dry mass of organisms at each level. It is almost always a true pyramid, because mass is lost at each step.
• Pyramid of energy transfer: energy passed to each level (for example, in kJ per m² per year). It is always a true pyramid, because energy is lost at every level.

Energy and substances along a food chain (4.8, 4.9)

Substances such as carbon compounds and nitrogen compounds pass along the chain when organisms eat, and are returned by decomposers. Energy enters as light, is captured by producers and passes along the chain in food. It is not recycled.

Only about 10% of the energy at one level reaches the next. The rest is lost because:

• much food is used in respiration and released as heat
• some parts are not eaten (roots, bones, fur)
• some food is not digested and leaves in faeces
• some energy is lost in excretion (urea)
• energy is used in movement and, in birds and mammals, keeping warm.

This is why food chains rarely have more than four or five levels.

Worked example 2. Producers in a meadow capture 24 000 kJ per m² per year. Primary consumers receive 2160 kJ, and secondary consumers receive 216 kJ.

Efficiency = (energy passed to next level ÷ energy in level) × 100

Producers → primary: (2160 ÷ 24 000) × 100 = 9.0%
Primary → secondary: (216 ÷ 2160) × 100 = 10.0%

(c) Cycles within ecosystems

The carbon cycle (4.10)

• Photosynthesis removes carbon dioxide from the air; carbon becomes part of glucose and other compounds in plants.
• Feeding passes carbon compounds to animals.
• Respiration by plants, animals and decomposers releases carbon dioxide back into the air.
• Decomposition: bacteria and fungi break down dead organisms and waste, and their respiration releases carbon dioxide.
• Combustion of fossil fuels and wood releases carbon dioxide.
• Where decomposition is incomplete over long periods, carbon is locked in fossil fuels.

The nitrogen cycle (4.11B) – Paper 2 only

Plants absorb nitrogen as nitrate ions to make proteins. You need the roles of four groups of bacteria (not their names):

• Nitrogen-fixing bacteria (in soil and in root nodules of legumes such as peas and clover) convert nitrogen gas into nitrogen compounds.
• Decomposers break down proteins in dead organisms, faeces and urea into ammonium compounds.
• Nitrifying bacteria convert ammonium into nitrites and then nitrates. They need oxygen, so aerated soil helps.
• Denitrifying bacteria convert nitrates back into nitrogen gas. They work in waterlogged soils with little oxygen, which reduces soil fertility.

(d) Human influences on the environment

Air pollution (4.12)

• Sulfur dioxide comes from burning fossil fuels that contain sulfur. It dissolves in rain to form acid rain. Acid rain damages leaves and kills trees, lowers the pH of lakes so fish and other organisms die, and releases toxic aluminium ions from soil. Sulfur dioxide also irritates lungs.
• Carbon monoxide comes from incomplete combustion, for example car exhaust. It binds to haemoglobin in red blood cells more strongly than oxygen, so less oxygen is carried to tissues. This can cause tiredness, and in high doses death.

Greenhouse gases and global warming (4.13–4.15)

The greenhouse gases are water vapour, carbon dioxide, nitrous oxide, methane and CFCs.

Gas	Human sources
Carbon dioxide	Burning fossil fuels; deforestation
Methane	Cattle, rice paddy fields, landfill sites
Nitrous oxide	Nitrogen fertilisers broken down by soil bacteria; animal waste
CFCs	Old aerosols and refrigerators

The Sun’s radiation passes through the atmosphere and warms the Earth’s surface. The surface emits longer-wavelength (infrared) radiation. Greenhouse gases absorb some of this and re-radiate it, keeping the Earth warm. More greenhouse gases cause an enhanced greenhouse effect, which may lead to global warming. Consequences include melting ice caps and rising sea levels, flooding, changes in rainfall and more extreme weather, changes in species distribution, loss of habitats, and reduced crop yields in some regions.

Water pollution (4.16, 4.17)

Sewage. Untreated sewage adds organic matter to rivers. Bacteria decompose it and multiply. Their aerobic respiration uses up dissolved oxygen, so fish and other aerobic animals die. Sewage can also carry pathogens.

Eutrophication. Fertiliser applied to fields is leached by rain into rivers and lakes.

1. Nitrates and phosphates enter the water.
2. Algae grow rapidly (an algal bloom).
3. The algae block light, so plants below cannot photosynthesise and die.
4. Algae also die.
5. Decomposing bacteria feed on the dead plants and algae and multiply.
6. The bacteria respire aerobically and use up dissolved oxygen.
7. Fish and other aerobic organisms die from lack of oxygen.

Deforestation (4.18B) – Paper 2 only

• Leaching: without tree cover, rain washes mineral ions out of the soil, so it becomes infertile.
• Soil erosion: roots no longer hold the soil, so rain and wind remove it. Soil may block rivers and cause flooding.
• Evapotranspiration: fewer trees means less water vapour released, so less cloud and less rainfall locally; the area can become drier.
• Carbon cycle: fewer trees remove carbon dioxide by photosynthesis. Burning or decomposing the wood releases more.
• Balance of atmospheric gases: carbon dioxide rises and less oxygen is released, adding to the enhanced greenhouse effect.

Common errors

• Writing that energy is “recycled”. Only substances are recycled; energy is lost as heat.
• Arrows in food chains drawn the wrong way (they point towards the eater).
• Giving “10%” without any reasons why energy is lost.
• In eutrophication, saying the algae use up the oxygen. It is the decomposing bacteria respiring that do this.
• Confusing nitrifying and denitrifying bacteria.
• Saying carbon monoxide causes acid rain, or that sulfur dioxide binds to haemoglobin.
• Forgetting to scale by quadrat area in population estimates.

Next steps

Condense this with the revision notes, then test yourself with the practice questions. To find which Biology topics need the most work, try the free diagnostics.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3, first examination June 2019, Pearson Education Limited – Topic 4, Ecology and the environment.