Ecosystems

Levels of organisation

• Individual — one organism.
• Population — all members of one species in a given area.
• Community — all populations of all species in a given area.
• Ecosystem — community + abiotic environment, interacting.

Biotic and abiotic factors

• Biotic (living): predators, prey, competitors, pathogens, food availability.
• Abiotic (non-living): temperature, light intensity, water/moisture, pH, oxygen / carbon dioxide concentration, mineral content.

A change in any factor can shift the size of populations.

Food chains and food webs

Arrows in a food chain point in the direction of energy flow (from prey to predator).

Example: Grass to Rabbit to Fox.

• Producer — autotroph that fixes light energy (e.g. grass).
• Primary consumer — eats the producer (herbivore).
• Secondary / tertiary consumer — predators higher up.
• Decomposers — bacteria and fungi that break down dead material and recycle minerals.

Pyramids of biomass

A pyramid of biomass shows the dry mass of organisms at each trophic level. Biomass decreases moving up because:

• Not all organisms at one level are eaten.
• Not all parts are digested (faeces).
• Energy is lost as heat from respiration and movement.

Roughly only ~10% of biomass is transferred to the next trophic level.

WJEC exam tip

When asked to describe the effect of removing a species from a food web, trace the consequences: predator population falls (lost food), prey population rises (no longer eaten), and effects propagate to species linked by other arrows. Always mention BOTH directions of impact.