Ecosystems: Components, Food Webs and Global Biomes

An ecosystem is a natural system in which living organisms interact with each other and with their non-living environment. Ecosystems exist at a range of scales — from a small garden pond to an entire continent-spanning biome.

Every ecosystem has two types of components:

Component type	Definition	Oak woodland examples
Biotic	All living parts of the ecosystem	Oak trees, bluebells, rabbits, sparrowhawks, earthworms
Abiotic	All non-living parts of the ecosystem	Soil, rainfall, temperature, sunlight, air

These components are interdependent: change one and you alter the whole system. Plants absorb minerals from soil; decomposers break down dead matter and return nutrients to the soil; animals depend on plants or other animals for energy. The boundary between the ecosystem and its surroundings is not always sharp — energy enters as sunlight and leaves as heat; water and nutrients cycle through.

The small-scale UK ecosystem examined throughout this lesson is an oak woodland, a temperate deciduous ecosystem found widely across England and Wales. It illustrates all the key interactions the spec requires.

Within any ecosystem, organisms are classified by how they obtain energy:

Role	Definition	Oak woodland examples
Producers	Plants that convert sunlight into energy via photosynthesis	Oak trees, bluebells, brambles, grasses
Primary consumers	Herbivores that eat producers	Caterpillars, rabbits, wood mice, deer
Secondary consumers	Carnivores that eat primary consumers	Sparrowhawks, foxes, weasels
Tertiary consumers	Top predators that eat secondary consumers	Tawny owls, pine martens
Decomposers	Organisms that break down dead organic matter	Earthworms, fungi, bacteria

Decomposers are as important as producers. They break down dead leaves, wood, and animal remains into inorganic nutrients — nitrates and phosphates — which return to the soil and are absorbed again by plant roots. Without decomposers, nutrients would lock up permanently in dead organic matter and the nutrient cycle would stop.

Energy flows in one direction through a food chain: it cannot be recycled. Nutrients, by contrast, cycle repeatedly through the system.

A food chain shows how energy transfers between organisms in a single linear sequence. Each position in the chain is a trophic level.

Oak woodland example:

Oak leaves → Caterpillar → Blue tit → Sparrowhawk

A food web is a more realistic representation: it maps all the overlapping and interconnected food chains within an ecosystem.

Term	Definition
Food chain	A single linear sequence of energy transfer
Food web	Multiple interlinking food chains
Producer	First trophic level; makes its own food by photosynthesis
Predator	Organism that hunts and eats another
Prey	Organism that is hunted and eaten

In a real oak woodland food web, an oak tree simultaneously feeds caterpillars, aphids, deer, and squirrels. Each of those is eaten by different predators. Interdependence becomes clear: if one species is removed, organisms that depended on it face food shortages, while those it preyed upon may increase unchecked. A food web diagram contains more information than a single food chain, and exam questions sometimes ask you to draw one — show at least three linked chains crossing.

Nutrients move continuously through the ecosystem in a nutrient cycle. There are three main stores:

• Biomass — nutrients held in living organisms (mainly plants and trees)
• Litter — nutrients in dead organic matter on the ground surface
• Soil — nutrients dissolved in soil water, available for plant root uptake

How the cycle works in an oak woodland:

1. Oak trees absorb nitrates and phosphates from the soil through roots
2. Nutrients are stored in leaves, trunk, and branches (the biomass store)
3. When leaves fall in autumn, they form leaf litter on the ground
4. Decomposers (fungi, bacteria, earthworms) break down litter into inorganic nutrients
5. Nutrients return to the soil and the cycle restarts in spring

The cycle below shows the three stores and the transfers that link them.

Leaching — the washing of nutrients downward through the soil by rainfall — removes nutrients from the upper layers. In very wet upland areas, leaching can be severe, leaving soils nutrient-poor despite high vegetation cover. Decomposer activity must replenish what leaching removes.

The speed of the nutrient cycle varies enormously between biomes. In a tropical rainforest, decomposition takes a few weeks and nutrients cycle rapidly. In Arctic tundra, frozen ground slows decomposition to almost nothing, and nutrients accumulate in the litter store for years.

Ecosystems are in dynamic equilibrium — they adjust continuously to maintain a working balance. Changing one component triggers knock-on effects throughout the system, demonstrating interdependence.

Example: Introduction of grey squirrels to UK oak woodlands

Stage	Effect
Grey squirrels introduced from North America in the 1870s	Established rapidly across England and Wales
Grey squirrels outcompete red squirrels for food and spread squirrelpox virus	Red squirrel numbers collapsed; now <140,000 in the UK, mainly Scotland
Grey squirrels strip bark from young oak trees in spring	Trees weakened and killed; woodland regeneration reduced
Fewer mature oaks reduce leaf litter, canopy cover and acorn supply	Ground flora composition changes; insect populations affected
Changes in invertebrate populations affect bird food supply	Woodland birds including woodpeckers and nuthatches decline

No single change stays isolated. A second example: Dutch elm disease (caused by a fungal pathogen, spread from the late 1960s) killed an estimated 25 million elm trees in the UK. The white-letter hairstreak butterfly, which breeds almost exclusively on elm, declined severely. Bark-feeding beetles that depended on elm lost habitat. Both examples illustrate that removing one species from a food web does not simply reduce numbers — it restructures the entire system.

A biome is a large-scale global ecosystem characterised by a distinctive climate, vegetation and wildlife. Biome distribution is determined primarily by temperature and precipitation, which are controlled by latitude and, secondarily, by distance from the ocean and prevailing wind patterns.

Biome	Location	Annual rainfall	Temperature	Key vegetation
Tropical rainforest	5°N–5°S (equatorial)	2,000–3,000 mm+	26–28°C year-round	Dense, multi-layered evergreen forest
Tropical grassland (savanna)	5–15°N and S	500–1,500 mm (seasonal)	20–30°C; wet/dry seasons	Grass with scattered flat-topped trees
Hot desert	15–30°N and S (sub-tropical)	Under 250 mm	35–45°C days; cold nights	Sparse cacti, succulents, dry scrub
Temperate deciduous forest	40–60°N	500–1,500 mm	4–20°C; four seasons	Oak, beech, ash; seasonal leaf fall
Boreal forest (taiga)	55–70°N	300–600 mm	–15 to +20°C; short summer	Spruce, fir, pine (coniferous)
Tundra	Above 70°N (Arctic)	Under 250 mm	–30 to +10°C	Mosses, lichens, dwarf shrubs

The map below shows how biomes form latitude bands, broadly mirrored north and south of the equator. The pattern follows temperature and rainfall, which are governed by latitude.

Moving from the equator towards the poles, vegetation becomes progressively less dense as temperature drops and growing seasons shorten. The pattern is not perfectly symmetrical because oceans, mountains, and atmospheric circulation modify it — deserts occur where descending dry air (Hadley cell) suppresses rainfall, while monsoon circulations create the seasonal wet periods that define savanna.

1. Confusing biotic and abiotic components

Soil is abiotic. Bacteria living in the soil are biotic. Students frequently categorise soil bacteria as abiotic because they associate them with the soil rather than as living organisms. If it is alive — or was once alive — it is biotic.

2. Omitting decomposers from nutrient cycling

Exam answers describing nutrient cycling often include only producers and consumers, skipping decomposers entirely. Without decomposers, the cycle cannot continue. Always include decomposers and name their role explicitly.

3. Drawing a food chain when asked for a food web

A food chain is a single linear sequence; a food web shows multiple interlinking chains. If asked to draw or describe a food web, include crossing pathways between at least three chains.

4. Overstating the impact of removing one species

The impact of losing a species depends on how many other species depend on it. Not all species losses are equally disruptive. Use hedged language: the ecosystem is disrupted and describe the specific chain of effects, rather than claiming immediate total collapse.

5. Mixing up small-scale ecosystem and global biome

The spec requires both: a specific small-scale UK ecosystem example (e.g. oak woodland) and a global biomes overview. Keep these distinct in extended writing. A question asking about a UK ecosystem does not want an answer about tropical rainforests.