The words you must know

• Habitat — the place where an organism lives.
• Population — all the organisms of one species in an area.
• Community — all the different populations living together in an area.
• Ecosystem — the community plus the non-living (abiotic) factors around it.
• Biodiversity — the variety of different species in an area.

Biotic vs abiotic: biotic factors are living (predators, food, competition); abiotic factors are non-living (light, temperature, pH, water, wind speed).

Food chains & food webs

A food chain shows how energy and substances pass along trophic levels. Arrows point in the direction of energy transfer — from the food to the feeder.

• Producers (green plants) capture light energy by photosynthesis — they start every chain.
• Consumers eat other organisms. A primary consumer (herbivore) eats the producer; a secondary consumer eats the primary consumer, and so on.
• A food web is many food chains linked together.

Arrow direction: the arrow always points towards the animal doing the eating (grass → rabbit → fox), showing which way the energy flows.

Producer or consumer?

In the chain grass → rabbit → fox, tap each organism and sort it.

Energy transfer between levels

Only a small fraction of energy passes to the next trophic level. Energy is lost at each step through:

• Respiration (heat energy released), excretion and egestion (waste), and parts of organisms that are not eaten.

Because so much energy is lost, food chains are usually short (rarely more than 4–5 levels) — there isn't enough energy left to support many more.

Why chains are short: with ~90% of energy lost each step, there is too little left after a few levels to feed a large top predator.

Your turn — energy efficiency

Pyramids of numbers & biomass

• A pyramid of numbers shows the number of organisms at each level. It can look odd — e.g. one big oak tree feeding thousands of insects makes an inverted shape.
• A pyramid of biomass shows the total mass of living material at each level. It is almost always a true pyramid shape, getting smaller towards the top.

Advantage of biomass pyramids: they show the true amount of living material, so they are rarely inverted — unlike pyramids of numbers, which can be misleading.

Why do pyramids narrow?

The carbon cycle

Carbon is constantly recycled between the air and living things:

• Photosynthesis removes CO₂ from the air (plants build it into sugars).
• Respiration, combustion (burning) and decomposition return CO₂ to the air.
• Carbon passes along food chains by feeding, and is released again by excretion, egestion and decay. Over millions of years, dead matter can form fossil fuels (fossilisation).

Balance point: photosynthesis is the only process that removes CO₂. Respiration, decomposition and combustion all add it back.

The nitrogen cycle

Plants need nitrates to make proteins, but they can't use nitrogen gas directly. Microorganisms do the key jobs:

• Nitrogen fixation — bacteria turn nitrogen gas into nitrogen compounds plants can use.
• Decomposition — decomposers break down dead matter and waste, releasing nitrogen compounds (ammonia).
• Nitrification — bacteria convert ammonia into nitrates (needs oxygen — an aerobic process).
• Denitrification — bacteria in waterlogged (anaerobic) soil convert nitrates back into nitrogen gas (a loss to the soil).

Aerobic vs anaerobic: nitrification needs oxygen, so well-aerated soil is good for plants; waterlogging causes denitrification, which removes useful nitrates.

Match each process to what it does

Tap a process on the left, then its matching description on the right.

Sampling with quadrats

We can't count every organism, so we take samples using a quadrat (a square frame of known area):

• Random sampling — place quadrats at random positions to estimate the abundance of a species across a habitat fairly.
• Belt transect — place quadrats along a line to see how a species changes with distance (e.g. from a pond edge inland).

Fair sampling: using several quadrats and taking a mean makes the estimate more reliable; random placement avoids bias.

Your turn — estimating a population

Human impact & conservation

• Global warming — burning fossil fuels and deforestation raise CO₂ levels, trapping heat. Effects include melting ice caps, rising sea levels, flooding, extreme weather and habitat loss.
• Eutrophication — fertiliser run-off and sewage add nitrates to water; algae grow rapidly then die; decomposers use up the oxygen, killing aquatic animals.
• Conservation (positive actions): reforestation and sustainable woodlands, and international treaties to cut global CO₂, protect biodiversity.

Two-way impact: humans can harm ecosystems (global warming, eutrophication) but also help them through reforestation and international agreements.

What removes CO₂?

Community or population?

Why do fish die in eutrophication?

The big ideas to know

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