Ecosystems: small-scale and global biomes

An ecosystem is a community of living organisms (the biotic parts) interacting with the non-living environment (the abiotic parts) in a defined area. Ecosystems exist at every scale — from a freshwater pond to the whole Earth.

Components and their interactions

• Producers convert sunlight into chemical energy (plants, algae, some bacteria) via photosynthesis. They are the foundation of every food web.
• Consumers eat producers (primary consumers) or other consumers (secondary, tertiary).
• Decomposers (fungi, bacteria, detritivores) break down dead matter, releasing nutrients back into the soil for producers.
• Abiotic components: sunlight, temperature, rainfall, soil pH, nutrients, water chemistry.

The interactions show up in two key cycles:

1. Energy flow is one-way — sunlight enters at the producer level and is lost as heat at every transfer (~90 % is lost between trophic levels). This is why food chains rarely have more than 4–5 levels.
2. Nutrient cycling is circular — carbon, nitrogen and other nutrients pass from soil to plants to animals to decomposers and back to soil. Disturbance (e.g. clear-cutting) breaks the cycle.

Food chains, food webs and the trophic pyramid

• A food chain is one feeding pathway: oak leaves → caterpillar → blue tit → sparrowhawk.
• A food web is all chains in an ecosystem combined; more realistic and more resilient — if one species is lost, others can fill the gap.
• The trophic pyramid shows energy or biomass at each level. Energy losses mean each layer is much smaller than the one below.

Disturbance and balance

A small change in one component cascades through the whole web. Examples:

• Reintroducing wolves to Yellowstone (1995). Elk numbers fell, willow regrew along streams, beaver populations recovered, river channels stabilised. A single predator restored a whole landscape.
• Removing rabbits from a UK heathland lets coarse grasses dominate; heather and rare wildflowers decline.
• Eutrophication — fertiliser runoff into a pond → algae bloom → light blocked → submerged plants die → oxygen falls → fish die.

Small-scale UK ecosystem case study — a freshwater pond

You should know a small-scale UK ecosystem in detail. Use a freshwater pond as a workable example:

• Producers — algae, pondweed, water lilies. Use sunlight; release oxygen.
• Primary consumers — water fleas, snails, tadpoles.
• Secondary consumers — small fish (sticklebacks), dragonfly nymphs.
• Tertiary consumers — herons, kingfishers.
• Decomposers — pond-edge bacteria, fungi on dead leaves.
• Abiotic — sunlight, water temperature, pH, oxygen levels (lower in summer; can crash if eutrophicated).

If a heron pair leaves, fish populations rise, eat too many invertebrates, leading to algal explosion as grazing pressure on algae from invertebrates falls. The whole system shifts.

Global distribution of biomes

A biome is a global-scale ecosystem characterised by climate and dominant vegetation. Their distribution follows latitude and the global atmospheric circulation:

• Tropical rainforest — equatorial belt (Amazon, Congo, SE Asia). Rising air → high rainfall, year-round growth.
• Tropical grassland (savanna) — 5°–15° N/S. Wet/dry seasons; iconic East African plains.
• Hot deserts — ~20°–30° N/S. Sinking air at the high-pressure belt → little rainfall (Sahara, Atacama, Australian Outback).
• Temperate deciduous forest — 30°–60° N (UK, eastern North America, central Europe). Mild, moist; trees lose leaves in winter.
• Temperate grassland — interior of large continents (prairies, steppes); too dry for forest.
• Mediterranean — west coasts at ~30°–40°. Hot dry summers, mild wet winters; chaparral, garrigue.
• Boreal forest (taiga) — 50°–70° N (Canada, Scandinavia, Siberia). Cold winters; coniferous evergreens.
• Tundra — ~60°+ N. Permafrost, very short growing season, low-growing plants.
• Polar (ice) — Arctic and Antarctic.

Altitude mimics latitude: climbing a mountain you pass through similar bands.

Examiner tip

For 6-mark and 9-mark questions on ecosystems, always link biotic and abiotic components and show a chain of cause and effect. Use named species/places. Marks reward specifics — "if X is removed, Y rises, which means Z". Generic answers about "things eat each other" score Level 1.