Ecosystems & feeding relationships

An ecosystem is all the living organisms (the community) plus the non-living (abiotic) conditions in an area. Energy enters through producers (plants) that photosynthesise.

• Producer → primary consumer → secondary consumer → tertiary consumer.
• Arrows in a food chain show the direction in which biomass (and energy) is transferred — from the eaten to the eater.
• Only about 10% of the biomass passes to the next level — the rest is lost through respiration, excretion and egestion (undigested waste).

Interdependence: organisms depend on each other for food, shelter, pollination and seed dispersal. Removing one species can affect the whole community.

Which way do the arrows point?

What affects a community?

Abiotic (non-living) factors: temperature, light intensity, moisture level, soil pH. Biotic (living) factors: predators, food, competition.

• Competition — for limited resources: plants for light, space, water and mineral ions; animals for food, mates and territory.
• Predation — predator numbers rise and fall a little after prey numbers.
• Mutualism — both species benefit (e.g. nitrogen-fixing bacteria in the root nodules of peas and beans).
• Parasitism — the parasite benefits and the host is harmed (e.g. fleas, tapeworms).

Interdependence: species depend on each other, so a change in one population (e.g. losing a predator) affects others in the community.

The carbon cycle

Carbon is constantly recycled between the air, living things and the ground:

• Photosynthesis removes CO₂ from the air and locks carbon into plants.
• Respiration (by plants, animals and decomposers) returns CO₂ to the air.
• Combustion (burning fossil fuels and wood) releases CO₂.
• Decomposers (bacteria and fungi) break down dead material, returning carbon to the cycle.

Balance: photosynthesis takes CO₂ out; respiration and combustion put it back. Burning fossil fuels faster than photosynthesis removes it raises atmospheric CO₂.

Removing CO₂ from the air

What speeds up decay?

Decomposers (bacteria and fungi) break down dead material and waste, returning carbon dioxide to the air and mineral ions to the soil. Decay is faster when it is:

• Warm — their enzymes work faster (too hot denatures them).
• Moist — microorganisms need water.
• Oxygen-rich — most decomposers respire aerobically. In anaerobic conditions (e.g. waterlogged soil) decay is much slower.

Watch out: cold, dry or airless conditions slow decay — that is why food lasts longer in a fridge or a vacuum pack.

Where does the carbon in a rotting leaf go?

Water & nitrogen cycles

Water cycle: energy from the Sun evaporates water; it condenses into clouds and falls as precipitation, providing fresh water before returning to the sea.

Nitrogen cycle — plants need nitrogen to make proteins, but can't use nitrogen gas directly:

• Nitrogen-fixing bacteria turn N₂ gas into nitrogen compounds (ammonia).
• Decomposers release ammonia from dead matter and waste.
• Nitrifying bacteria convert ammonia → nitrates (which plants absorb).
• Denitrifying bacteria return nitrates to N₂ gas.

Key role of bacteria: without nitrogen-fixing and nitrifying bacteria, plants could not get the nitrates they need to build proteins.

Match each process to what it does

Tap a statement, then its matching answer.

How do you count daisies without counting?

Sampling with quadrats & transects

We estimate population sizes without counting every organism using sampling:

• Quadrats — square frames placed randomly to estimate abundance; count the mean number per quadrat, then scale up to the whole area.
• Transects — a line across an area; used to study how a population changes with distance (e.g. from a pond).
• Sampling must be random to avoid bias; more samples give a more reliable estimate.

Scaling up: mean per quadrat × (total area ÷ quadrat area) estimates the whole population. A larger, random sample makes the estimate more reliable.

Your turn — estimate the population

Why sample randomly?

How do you count daisies without counting?

Biodiversity & why it matters

Biodiversity is the variety of different species in an ecosystem. High biodiversity makes an ecosystem more stable — species are less dependent on any single other species.

• Human actions such as deforestation, pollution and habitat loss reduce biodiversity.
• Conservation measures include protected habitats, breeding programmes and replanting.

Why protect it: greater biodiversity means a more resilient ecosystem, and it safeguards food supplies, medicines and species for the future.

Benefit of high biodiversity

Sort each into its group

Tap each process, then tap which cycle it belongs to.

The big ideas to know

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