Population, community, habitat, ecosystem

Ecology uses a ladder of precise words. Get them exact:

• Habitat — the place where an organism lives (e.g. a rock pool).
• Population — all the organisms of one species in a habitat at one time.
• Community — all the populations of all the species living together.
• Ecosystem — the community plus the non-living (abiotic) physical environment, interacting together.

Which word fits?

Quadrats — estimating population size

You can't count every daisy on a field, so you sample. A quadrat is a square frame (often 0.5 m × 0.5 m) placed at random positions.

• Count the organisms inside several quadrats placed at random coordinates.
• Find the mean number per quadrat.
• Scale up: mean per quadrat × (total area ÷ quadrat area) = estimated population size.

Why random? Random placement avoids bias and makes the sample representative. More quadrats → more reliable estimate.

Your turn — estimate the population

Belt transects — measuring distribution

A transect is a line across a habitat. To study how species change with distance (e.g. up a beach), you lay a tape and place quadrats at regular intervals along it — a belt transect.

Use a transect when a physical factor changes across the habitat (light, water, salinity). Use random quadrats for an overall estimate of one uniform area.

Right tool for the job

Producers, consumers & decomposers

• Producers — plants (and algae) that photosynthesise to make their own food. They start every food chain.
• Consumers — animals that eat other organisms. Primary eat producers, secondary eat primary, tertiary eat secondary.
• Decomposers — bacteria and fungi that break down dead material and waste, recycling nutrients back into the soil.

The stages (producer, primary consumer, secondary consumer…) are called trophic levels.

Misconception buster: the arrow points in the direction that energy and biomass flow — from the food to the feeder. It does not point at what the animal eats.

Food chains link into a food web

Most organisms eat more than one thing, so several chains cross to make a food web. Follow the arrows: each one shows energy passing from prey to predator.

Read the web

Match organism ↔ trophic level

Tap an organism on the left, then its correct role on the right.

Ecological pyramids

• Pyramid of numbers — the number of organisms at each level. Can be an odd shape (one oak tree feeds many aphids, so the base is narrow).
• Pyramid of biomass — the total mass of living material at each level. Almost always a true pyramid.
• Pyramid of energy — the energy stored at each level per year. Always a perfect pyramid.

Key idea: the width of each bar is drawn to scale for the mass (or energy) at that level — not the number of organisms.

Your turn — biomass transfer

Why only ~10% gets through

Only about 10% of the energy at one trophic level ends up stored in the next. The other ~90% is lost because:

• Much is released as heat during respiration (movement, warmth).
• Not all of the prey is eaten (bones, roots) or is indigestible — lost in faeces and urine.
• Some is lost in excretory waste.

Why so short?

The carbon cycle

Carbon moves between the air and living things. Learn the four core processes:

• Photosynthesis removes CO₂ from the air, fixing carbon into glucose.
• Respiration (by plants, animals and decomposers) returns CO₂ to the air.
• Combustion — burning fossil fuels and wood returns CO₂.
• Decomposition — microbes respire dead matter, releasing CO₂; incomplete decay over millions of years forms fossil fuels.

Carbon out of the air

The nitrogen cycle

Plants need nitrogen (for proteins) but can't use nitrogen gas (N₂) — they absorb nitrates through their roots. Four groups of bacteria move nitrogen around:

• Nitrogen-fixing bacteria — turn N₂ from the air into ammonia/ammonium (some live in root nodules of legumes).
• Decomposer bacteria — break down dead matter and urea into ammonium.
• Nitrifying bacteria — convert ammonium → nitrites → nitrates (needs oxygen).
• Denitrifying bacteria — convert nitrates back into N₂ gas (in waterlogged, low-oxygen soil).

Name that bacterium

Which cycle is this process in?

Tap whether each process belongs to the carbon cycle or the nitrogen cycle.

Greenhouse gases & global warming

Burning fossil fuels and deforestation raise the levels of carbon dioxide (CO₂) and methane (CH₄) — the main greenhouse gases.

• They trap heat radiated from the Earth (the greenhouse effect), raising average temperatures — global warming.
• Methane comes largely from cattle, rice paddies and landfill/decomposition.
• Consequences: melting ice, rising sea levels, changing climate and habitats.

Don't confuse: the greenhouse effect (heat trapping → warming) is different from acid rain (sulfur dioxide) and different again from the ozone story — Edexcel 4BI1 wants CO₂/CH₄ → warming.

Sulfur dioxide & acid rain

Burning fossil fuels (especially coal) releases sulfur dioxide (SO₂). It dissolves in rain water to form acids, giving acid rain.

• Acid rain damages leaves and kills trees.
• It makes lakes and rivers acidic, killing fish and other aquatic life.
• It erodes limestone buildings and statues.

Pollutant ↔ problem

Eutrophication

When excess nitrate/phosphate fertiliser (or untreated sewage) washes into a river or lake, it triggers a chain of events:

1. Nutrients cause an algal bloom — algae grow rapidly over the surface.
2. Algae block light, so plants below die.
3. Decomposer bacteria feed on the dead plants and multiply.
4. These bacteria respire aerobically, using up the dissolved oxygen.
5. Fish and other animals die from lack of oxygen.

Misconception buster: the fertiliser doesn't poison the fish directly — the fish suffocate because bacteria remove the oxygen from the water.

Your turn — why the fish die

Deforestation

Cutting down forests (often by slash-and-burn) has several consequences Edexcel expects you to list:

• Less photosynthesis → less CO₂ removed from the air → more global warming.
• Burning trees releases stored carbon as CO₂ (combustion).
• Loss of habitat and reduced biodiversity (species may go extinct).
• Soil erosion and flooding — tree roots no longer bind the soil.
• Disruption of the water cycle — less transpiration, drier local climate.

Your turn — habitat lost

Conservation — protecting biodiversity

Humans can reduce their impact and protect species and habitats:

• Protecting habitats — nature reserves and national parks.
• Replanting forests and managing woodland sustainably.
• Captive breeding and seed/gene banks to save endangered species.
• Reducing pollution — treating sewage, controlling fertiliser use, cutting emissions.
• Recycling nutrients and materials to reduce waste and habitat destruction.

Big picture: conservation keeps ecosystems working — food webs, the carbon cycle and the nitrogen cycle all depend on biodiversity being maintained.

You've covered all of Section 4 ✅

Tap Next for your final score. 🎉