Ecosystems

Environmental management has its own precise vocabulary. Examiners reward exact definitions, so learn these word-perfect.

Term	Definition	Example
Population	All the individuals of one species living in the same area at the same time.	All the oak trees in a wood.
Community	All the populations of all the species living and interacting in the same area.	All the plants, animals and microbes in the wood.
Ecosystem	A community of organisms together with the non-living (abiotic) environment they interact with.	The wood: its organisms + soil, water, light and air.
Biome	A large-scale ecosystem with a characteristic climate and community, found across the world.	Tropical rainforest, desert, tundra, grassland.
Habitat	The place where an organism lives.	The bark of an oak tree; a rock pool.
Niche	The role an organism plays in its ecosystem — how it feeds, where it lives, and how it interacts with others.	A ladybird's niche: eating aphids on plant leaves.

See the scales as nested boxes. A single organism belongs to a population; populations together form a community; the community plus its physical surroundings make an ecosystem; and many similar ecosystems across the globe make a biome.

Habitat vs niche — the classic trap. A habitat is an address (where it lives); a niche is a job (what it does and how it lives). Two species can share a habitat but cannot share an identical niche for long — one will out-compete the other (the competitive exclusion principle).

Every ecosystem has two kinds of component.

Abiotic components (the non-living, physical/chemical environment):

• temperature, humidity, water, oxygen, salinity (saltiness), light and pH.

These set the conditions organisms must tolerate. For example, low oxygen in stagnant water limits which animals can survive; high salinity in an estuary excludes freshwater species.

Biotic components (the living organisms):

• Producers — organisms (mainly green plants and algae) that make their own food by photosynthesis.
• Consumers — organisms that eat other organisms (primary, secondary, tertiary — see the energy section).
• Decomposers — bacteria and fungi that break down dead organisms and waste, recycling nutrients.

How biotic factors affect number and diversity. The living organisms themselves shape how many individuals, and how many species, an ecosystem can support — through three key interactions:

• Competition — when organisms compete for the same limited resource (food, water, light, space, mates):
  • Intra-specific competition is within one species (e.g. two oak trees competing for light). It is often the most intense, because individuals need exactly the same resources.
  • Inter-specific competition is between different species (e.g. lions and hyenas competing for prey). It can reduce the population of the weaker competitor or exclude it altogether.
• Grazing — herbivores feeding on plants. Moderate grazing can increase plant diversity (by stopping one fast-growing species dominating); heavy grazing reduces it.
• Predation — predators eating prey. Predator and prey numbers rise and fall together in linked cycles: more prey → more predators → fewer prey → fewer predators, and so on.

Together these interactions keep populations in check and influence the biodiversity of the ecosystem.

These two processes power almost all life and drive the carbon cycle.

Photosynthesis is the process by which plants (and algae) synthesise glucose using carbon dioxide, water and light energy. The light energy is captured by the green pigment chlorophyll.

• Word equation: carbon dioxide + water → glucose + oxygen
• Chemical equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Glucose stores chemical energy and is used to build the plant's body — so photosynthesis converts light energy into the chemical energy that enters every food chain.

Limiting factors. The rate of photosynthesis is limited by whichever needed factor is in shortest supply. The three you must know are:

• light (no light → no photosynthesis; brighter light → faster, up to a point),
• carbon dioxide concentration (more CO₂ → faster, up to a point),
• water availability (a shortage closes the leaf pores and slows the process).

At any moment, increasing the factor that is currently limiting will speed photosynthesis up; increasing the others will not.

Aerobic respiration is the set of chemical reactions in cells that break down glucose to release energy, using oxygen and producing carbon dioxide and water. It happens in all living organisms, all the time.

• Word equation: glucose + oxygen → carbon dioxide + water
• Chemical equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Notice that respiration is essentially photosynthesis in reverse: one stores energy and removes CO₂ from the air; the other releases energy and returns CO₂ to the air. This balance is the heart of the carbon cycle.

A food chain shows how energy and nutrients pass from one organism to the next. The arrow means 'is eaten by' and always points in the direction of energy flow.

Feeding levels (trophic levels):

• Producer — makes its own food by photosynthesis (the first trophic level). Example: grass.
• Primary consumer — a herbivore that eats producers. Example: grasshopper.
• Secondary consumer — eats primary consumers. Example: frog.
• Tertiary consumer — eats secondary consumers. Example: snake.
• Decomposer — breaks down dead organisms and waste at every level, recycling nutrients.

A trophic level is simply a feeding level — the position an organism occupies in a food chain.

A food web joins many food chains together to show the real, interconnected feeding relationships in a community. One organism can be at different trophic levels in different chains.

Energy is lost at every step. Energy enters the chain when producers capture light. As it passes from level to level, most is lost and only a small fraction (often quoted as around 10%) is passed on. Energy is lost because organisms:

• use energy in respiration, which is released as heat,
• lose energy in waste (faeces and urine) and in uneaten parts (bones, roots),
• do not get eaten — energy in organisms that die goes to decomposers.

Why this matters. Because so much energy is lost at each transfer:

• food chains are usually short (rarely more than 4-5 links),
• there are fewer organisms, and less biomass, at higher levels (few top predators can be supported),
• it is more energy-efficient for people to eat lower down the chain (eating crops rather than animals fed on crops).

Carbon is constantly recycled between the atmosphere, living organisms, the oceans and the rocks. The Cambridge stages you must know are photosynthesis, respiration, feeding, decomposition, fossilisation and combustion.

How carbon moves:

• Photosynthesis removes CO₂ from the air and locks carbon into glucose and plant bodies (a carbon store). This happens on land (forests, grasslands) and in the oceans (phytoplankton) — ocean photosynthesis is a huge part of the cycle.
• Feeding passes carbon from plants to animals along food chains.
• Respiration (by plants, animals and decomposers) returns CO₂ to the atmosphere.
• Decomposition — decomposers break down dead organisms, releasing CO₂ through their respiration.
• Fossilisation — over millions of years, undecomposed remains are buried and compressed into fossil fuels (coal, oil, gas), a long-term carbon store.
• Combustion — burning fossil fuels (and wood) returns this stored carbon to the atmosphere as CO₂, often very quickly.

The natural balance — and how humans disturb it. For most of history, the CO₂ removed by photosynthesis roughly balanced the CO₂ returned by respiration, decomposition and natural combustion. Humans have upset this balance by:

• burning fossil fuels on a vast scale (combustion), releasing carbon stored over millions of years in just a few centuries,
• deforestation, which removes producers that would otherwise lock carbon away.

The result is rising atmospheric CO₂, the main driver of the enhanced greenhouse effect and climate change (Topic 8). Understanding the carbon cycle is therefore the foundation for understanding the biggest environmental-management challenge of all.