Ecosystems Overview

🌍 What Is an Ecosystem?

An ecosystem is a community of living organisms interacting with each other and with their non-living environment. That single sentence contains three ideas that examiners test constantly, so let's unpack each one.

Living organisms = biotic factors. Everything alive: plants, animals, fungi, bacteria, insects, birds, algae. Every organism that eats, grows, reproduces, and eventually dies. In a temperate oak woodland, that includes the oak trees, the bluebells on the woodland floor, the caterpillars eating the oak leaves, the blue tits eating the caterpillars, the sparrowhawk hunting the blue tits, and the earthworms and fungi decomposing everything that dies.

Non-living environment = abiotic factors. Everything that is not alive but shapes what can live there: temperature, rainfall, sunlight, soil type, geology, topography (slope angle and aspect), pH, wind speed, and humidity. These factors determine which organisms can survive in a place. The tropical rainforest receives over 2,000 mm of rain a year near the equator — those abiotic conditions make it possible for the world's densest canopy to exist. The Sahara Desert receives less than 25 mm in some years — those same abiotic conditions make any life at all a remarkable feat of adaptation.

Interacting. This is the critical word. Biotic and abiotic components do not exist in isolation — they shape each other continuously. Rainfall (abiotic) determines which vegetation (biotic) grows. That vegetation holds the soil together (abiotic). The soil stores nutrients (abiotic) that feed the plants (biotic). Dead plants feed decomposers (biotic) that return nutrients to the soil (abiotic). Remove any one link and the whole system is affected. This is why geography is interested in ecosystems — not as lists of species, but as systems.

Scale: From Puddle to Planet

Ecosystems exist at every scale. A rock pool on a beach is a small-scale ecosystem: abiotic factors include salt concentration, water temperature, and tidal exposure; biotic factors include limpets, periwinkles, anemones, seaweed, and the bacteria decomposing dead material in the crevices. A tropical rainforest is a large-scale ecosystem stretching across multiple countries. The entire Amazon basin — 5.5 million km² — can be treated as a single ecosystem. And at the largest scale of all, the biosphere is the sum of all ecosystems on Earth.

For your exams, you need to be able to work at two scales: the small-scale ecosystem (typically a local UK example like a sand dune, heathland, or woodland) and the large-scale biome (tropical rainforest, hot desert, etc.). Both follow the same rules — the same energy flows, the same nutrient cycles, the same interdependence. The scale changes but the principles do not.

⚡ Energy Flow: From Sunlight to Snake

Every ecosystem needs an energy source. For almost all ecosystems on Earth, that source is the sun. Producers — green plants, algae, and some bacteria — capture sunlight through photosynthesis and use it to convert carbon dioxide and water into glucose. This process transfers solar energy into chemical energy stored in living tissue. It is the foundation of every food chain on Earth.

Food Chains and Food Webs

A food chain shows a single linear pathway of energy transfer: grass → grasshopper → frog → snake → hawk. The arrows represent the direction of energy flow (energy goes from eaten to eater). Each organism in the chain occupies a trophic level: producers are at trophic level 1, primary consumers (herbivores eating producers) are at trophic level 2, secondary consumers are at level 3, and so on.

A food web is a more realistic picture. Most animals eat more than one thing, and most organisms are eaten by more than one predator. A food web shows all the feeding relationships in an ecosystem simultaneously — a complex network of interconnected food chains. This is why the Yellowstone story makes sense: the wolf does not just fit into one chain — it is woven into a web, and pulling on one thread tightens or loosens dozens of others.

The 10% Rule: Why Food Chains Are Short

Here is a fundamental rule about energy in ecosystems: only about 10% of the energy stored at one trophic level is transferred to the next. The other 90% is lost. Where does it go?

• Heat from respiration — organisms use energy to move, maintain body temperature, and carry out all life processes; this energy is released as heat
• Waste products — energy locked in faeces, urine, and excretions that is not passed to the predator
• Parts not eaten — bones, roots, bark, shells that are left behind or eaten by decomposers rather than the next consumer in the chain
• Energy used in growth — energy stored in body parts that die before being eaten (a leaf that falls to the ground rather than being eaten by a caterpillar)

The practical consequence of the 10% rule is that food chains are rarely longer than four or five links. Start with 10,000 kJ of energy in grass. After one transfer: 1,000 kJ reaches the grasshopper. After two: 100 kJ reaches the frog. After three: 10 kJ reaches the snake. After four: just 1 kJ reaches the hawk. At that point, there is simply not enough energy to support a fifth consumer — the food chain ends.

This also has a major implication for human food choices and resource management. Eating beef requires 10 kg of grain to produce 1 kg of beef (roughly a 10× energy loss). Eating the grain directly is ten times more energy-efficient. With a world population of 8 billion, this matters — and it connects directly to the resource management topics you will study later in the course.

🔄 Nutrient Cycling: The Three Stores

Figure 1: The Nutrient Cycle — nutrients move continuously between biomass, litter, and soil stores. Unlike energy, nutrients are not lost — they cycle indefinitely.

Unlike energy, which flows through an ecosystem in one direction and is lost as heat at each trophic level, nutrients cycle. The same atoms of nitrogen, phosphorus, and carbon that were part of a dinosaur are now part of a tree, a mushroom, or your breakfast cereal. They move between three stores in an endless loop.

The Three Stores

• Biomass — the living organisms themselves: the cells, tissues, and bodies of all plants, animals, fungi, and bacteria. This is where most nutrients are locked up in a productive ecosystem. In a tropical rainforest, the biomass store is enormous — the combined weight of all living things in a hectare of forest is greater than almost any other ecosystem on Earth.
• Litter — dead organic matter on or near the surface: fallen leaves, dead branches, animal remains, shed skin, faeces. This is the "waiting room" where nutrients sit after an organism dies, before decomposers break them down.
• Soil — the layer of mineral and organic material beneath the litter. Nutrients are held in the soil in forms that plant roots can absorb. The soil store is where nutrients become available to producers again, completing the cycle.

The Flows Between Stores

Nutrients move between stores through a series of processes:

• Litter → Soil: Decomposition. Bacteria and fungi (decomposers) break down dead organic matter in the litter, releasing nutrients as simple mineral compounds into the soil. The speed of this flow depends heavily on temperature and moisture — warm, wet conditions massively accelerate decomposition.
• Soil → Biomass: Uptake. Plant roots absorb mineral nutrients (nitrogen, phosphorus, potassium) dissolved in soil water. These are used to build new cells, leaves, and woody tissue — the biomass store.
• Biomass → Litter: Leaf fall and death. Leaves fall, organisms die, animals excrete waste. Material transfers from the living biomass into the litter layer.

Inputs and Outputs

The cycle also has external inputs and outputs. Inputs include: weathering of parent rock (releases minerals into soil); atmospheric deposition (nutrients dissolved in rainfall); and nitrogen fixation by bacteria. Outputs include: leaching (heavy rainfall washes nutrients downward through the soil, below root reach — a major problem in tropical rainforests after deforestation); surface runoff; fire (which releases nutrients into the atmosphere as gases and ash); and harvesting (humans removing nutrients when they take crops or timber from the system).

🗺️ Global Biomes: Distribution and Climate

A biome is a large-scale ecosystem defined by its climate and characterised by a distinctive type of vegetation and the animals that depend on it. Biomes are the biggest ecosystems on Earth — they span continents and contain millions of species. Six major terrestrial (land) biomes are essential for your GCSE course.

The Six Major Terrestrial Biomes

Why Biomes Are Where They Are: The Five Drivers

Biomes are not randomly distributed — they follow predictable patterns controlled by five key factors. Understanding these factors allows you to explain any biome's location, and to predict what an area's vegetation would be like from its position on a globe.

🌾 Small-Scale Ecosystem: Sand Dune Succession

Succession is the process by which an ecosystem changes over time as organisms colonise a bare environment, modify it, and create conditions that allow other, more complex communities to establish. It is a long-term demonstration of the interplay between biotic and abiotic factors — each generation of organisms changes the abiotic conditions, enabling the next community to arrive.

Sand dune succession is the classic UK small-scale ecosystem for GCSE geography. It happens wherever wind-blown sand accumulates along a coastline — at Braunton Burrows (Devon), Studland (Dorset), or Formby (Lancashire), for example.

The Stages of Dune Succession

The Key Principle of Succession

Each stage creates the conditions that allow the next stage. Pioneer species tolerate extreme conditions but are eventually outcompeted by species that arrive later, because those later species cannot survive in the original bare-sand conditions but thrive in the modified environment that pioneers have created. The ecosystem becomes progressively more complex — more soil, more species, more stable abiotic conditions — until it reaches the climax community for that climate.

🌳 Small-Scale Case Study: Epping Forest Oak Woodland

Epping Forest is a 2,400+ hectare ancient oak woodland on the Essex/London border — one of England's oldest Royal Forests. It is the standard OCR B and AQA example for a small-scale UK ecosystem demonstrating interdependence.

Biotic Components

Abiotic Components

Interdependence: What Happens if the Oak Trees Are Removed?