Ecology — section overview

Ecology B7 studies how organisms interact with each other and their environment. It links to environmental issues, food webs, and the cycling of materials.

Key ecological concepts

Term	Definition
Population	All individuals of one species in an area
Community	All the populations of different species in an area
Ecosystem	Community + the non-living environment
Habitat	Place where an organism lives
Abiotic factor	Non-living factor (e.g. temperature, light, water, pH, mineral ions)
Biotic factor	Living factor (e.g. predators, prey, parasites, food availability)

Feeding relationships

Food chain: Producer → Primary consumer → Secondary consumer → Tertiary consumer

Producer = plant/alga that photosynthesises (makes its own food)

Trophic levels: energy is lost at each level (as heat via respiration, excretion, uneaten parts)

Pyramid of biomass: shows dry mass at each trophic level — always pyramid shaped. Pyramid of numbers: shows number of organisms — can be inverted (e.g. one oak tree supports many caterpillars).

Efficiency: typically only 10% of energy transferred between trophic levels.

Sampling techniques

• Quadrat: count organisms in a small area; repeat for reliable estimate
• Transect: place line across habitat; record organisms at regular intervals — shows change across a gradient
• Mark-release-recapture: capture, mark, release; re-capture later → estimate population:

$$N = \frac{n_1 \times n_2}{m}$$

Where: $N$ = total population, $n_1$ = first catch, $n_2$ = second catch, $m$ = marked individuals recaptured.

Material cycling

Carbon cycle:

• CO₂ removed by photosynthesis → locked in organic molecules
• CO₂ returned by respiration, decomposition, combustion

Water cycle:

• Evaporation → condensation → precipitation → transpiration (plants) → runoff

Nitrogen cycle:

• N₂ fixed by nitrogen-fixing bacteria → ammonium → nitrites → nitrates (nitrification by bacteria)
• Plants absorb nitrates → proteins → animals eat plants → decomposers → ammonium
• Denitrifying bacteria convert nitrates → N₂ (gas)

Biodiversity and threats

Biodiversity = variety of species in an area. High biodiversity = stable ecosystem.

Threats: habitat destruction, pollution, climate change, invasive species, overexploitation.

Conservation: SSI (Sites of Special Scientific Interest), nature reserves, seed banks, breeding programmes, rewilding.

Common exam mistakes in B7

1. Pyramids of numbers can be inverted — but pyramids of biomass are always pyramid-shaped
2. Mark-release-recapture assumptions — population closed; marks don't affect survival; uniform mixing; marks don't wash off
3. Nitrogen cycle — confusion about which bacteria do what — nitrogen-fixing (N₂ → NH₄⁺), nitrifying (NH₄⁺ → NO₂⁻ → NO₃⁻), denitrifying (NO₃⁻ → N₂)