B4 Community-level systems — OCR Gateway Biology (J257/02)

Ecosystems and food webs

Definitions:

• Population: all individuals of one species in one area.
• Community: all populations of all species in one area.
• Ecosystem: community + abiotic (non-living) environment.
• Habitat: the place where an organism lives.
• Niche: the role an organism plays in its ecosystem (what it eats, when it is active, what eats it).

Food chains and food webs:

• Arrows show direction of energy transfer (not "who eats who" — energy moves from prey to predator).
• Producer: photosynthetic organism (plant/alga) — makes organic molecules from sunlight + CO₂ + H₂O.
• Primary consumer: eats producer (herbivore).
• Secondary consumer: eats primary consumer.
• Tertiary consumer: eats secondary consumer.
• Decomposers (bacteria, fungi): break down dead organic matter, returning minerals to soil.

Trophic levels: Position in a food chain. Only ~10% of energy passes to the next level — the rest is lost as heat (respiration), undigested material (egestion), or is used for growth that is not eaten.

Pyramids of biomass: show dry mass of organisms at each trophic level. Always a pyramid shape (energy lost at each level). More reliable than pyramids of number.

Nutrient cycles

The carbon cycle

Carbon moves through the environment via:

• Photosynthesis: CO₂ absorbed from atmosphere → organic compounds in plants.
• Respiration: organic compounds → CO₂ released by all organisms.
• Feeding: carbon passes from plant → animal through food chains.
• Decomposition: decomposers break down dead matter → CO₂ released.
• Combustion: burning fossil fuels → CO₂ released.
• Fossilisation: organic matter buried → coal/oil/gas over millions of years.

The nitrogen cycle

Nitrogen makes up 78% of atmosphere but plants cannot use N₂ gas directly. The cycle involves:

1. Nitrogen fixation: Nitrogen-fixing bacteria (in soil/root nodules of legumes, e.g. Rhizobium) convert N₂ → ammonium ions (NH₄⁺). Lightning also fixes nitrogen.
2. Nitrification: Nitrifying bacteria in soil convert NH₄⁺ → nitrites → nitrates (NO₃⁻). Plants absorb nitrates via roots.
3. Assimilation: Plants use nitrates to make amino acids, proteins, chlorophyll. Animals obtain nitrogen by eating plants.
4. Decomposition: Decomposers (bacteria/fungi) break down proteins in dead matter/excreta → ammonium ions.
5. Denitrification: Denitrifying bacteria (in waterlogged soils) convert nitrates → N₂ released back to atmosphere.

Biodiversity and sampling

Biodiversity: the variety of living organisms in an area. Includes species richness (number of species) and species evenness (relative abundance).

Why biodiversity matters:

• Ecosystem stability: more species → more resilient to change.
• Genetic resources for medicine and food.
• Ecosystem services: pollination, water purification, oxygen production.

Threats to biodiversity: Habitat destruction, over-exploitation, pollution, invasive species, climate change.

Sampling techniques (PAG B4.1):

Random sampling — to avoid bias:

1. Place grid over area.
2. Use random number table/calculator to generate coordinates.
3. Place quadrat at each coordinate.

Quadrats (for plants or slow-moving animals):

• Count number of individuals of each species (frequency/density).
• Or estimate % cover per species.
• Larger quadrats suit sparse organisms; smaller for dense ones.

Transects (for studying change across a gradient, e.g. seashore zonation):

• Lay measuring tape across the area.
• Line transect: record organisms touching the tape.
• Belt transect: place quadrats at intervals along the tape; more data.

Mark-release-recapture (for mobile animals):

1. Capture sample (n₁); mark with non-toxic paint/tag; release.
2. After time, capture second sample (n₂); count how many are marked (m).
3. Population estimate (Lincoln Index): N = (n₁ × n₂) ÷ m.

Assumptions for mark-release-recapture:

• Marked individuals mix randomly with population.
• Marking does not affect survival (marks not visible to predators).
• No births, deaths, immigration, or emigration between samples.
• Population is closed.

Abiotic and biotic factors

Abiotic (non-living): temperature, light intensity, pH, salinity, water availability, mineral ion concentration.

Biotic (living): food availability, predators, competitors, pathogens, mutualistic species.

Interdependence: organisms in a community depend on each other. Removing one species cascades through the food web. Example: sea otter removal → urchin population explodes → kelp forest destroyed → many species lose habitat.

Common OCR examiner traps

1. Food chain arrows go FROM prey TO predator (direction of energy flow), not "eaten by."
2. Only ~10% energy transfer between trophic levels — this is why food chains rarely have more than 5 levels.
3. Mark-release-recapture denominator is marked in second capture (m), not total second capture. N = n₁ × n₂ ÷ m.
4. Nitrification is not nitrogen fixation. Nitrogen fixation: N₂ → NH₄⁺. Nitrification: NH₄⁺ → NO₃⁻.
5. Biodiversity ≠ number of individuals — it's about number/variety of species (and evenness).