GCSE Biology Ecosystems revision

A habitat is the place where an organism lives. All the organisms in that habitat form a community. Add the non-living surroundings — the soil, water, air and light — and their interactions with the community, and you have an ecosystem: organisms and environment working as one system.

A change in one factor ripples through the community: a drought shrinks plants, which shrinks the herbivores, which shrinks the carnivores — and populations reshuffle around whatever survives best.

Food chains trace energy from its source. Producers (green plants and algae) make glucose by photosynthesis. Primary consumers eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. Predators hunt other animals — their prey.

Predator and prey populations rise and fall in linked cycles — but the predator’s peak always lags behind the prey’s peak. More prey feeds more predators (prey up → predators up); too many predators crash the prey (prey down → predators starve down); fewer predators let prey recover (prey up again). Writing that four-beat cycle — including the lag — is the exam skill.

To estimate the population of an organism in a field, you do not count every individual. You drop a quadrat — a square frame — many times in random positions, count the organism in each, and take the mean per quadrat. Then multiply the mean by the number of quadrat-sized areas in the whole field to get the population estimate.

1. 1Randomise

   Use two random numbers as coordinates so sampling is not biased toward easy patches.

2. 2Count

   Count the target organism inside each quadrat; record every quadrat.

3. 3Mean

   Calculate the mean number per quadrat.

4. 4Scale

   Multiply the mean by (total field area ÷ quadrat area) to estimate the whole population.

A transect — a line tape across a habitat — samples how distribution changes along a gradient, for example plant cover from a shady wood edge to an open field. Random sampling estimates how many; transects show where and how it changes.

An adaptation is a feature that makes an organism better suited to survive and reproduce in its environment. Three kinds: structural (body shapes — a cactus’s spines, a polar bear’s thick fur), behavioural (actions — migration, nocturnal hunting) and functional (internal chemistry — desert animals producing very concentrated urine).

Extremophiles live where nothing else manages: bacteria in hot springs at temperatures that would cook most cells (their enzymes tolerate the heat), organisms in very salty water, or deep-sea vents under crushing pressure. Each extremophile’s biochemistry is an adaptation to the extreme — and a standing exam example.

Decomposers — bacteria and fungi — break down dead material and waste, returning nutrients to the soil for producers to reuse. Decay runs fastest when conditions suit the decomposers: warm (faster enzyme reactions), moist (reactions need water), and oxygen-rich (aerobic decay). Too cold, too dry or no oxygen — decay slows or stops (which is why freezing, drying and pickling preserve food).

In the decay required practical, fresh milk is made alkaline with sodium carbonate and phenolphthalein indicator (pink), then lipase is added and the tube is held at a chosen temperature. As the enzyme breaks down the fat in the milk, fatty acids form, the pH falls and the indicator turns colourless — timing that colour change measures the rate of decay. Repeating at different temperatures shows the effect of temperature on decay rate; the reagents are low-hazard, but wear eye protection when handling sodium carbonate and wash hands afterwards. Without oxygen, anaerobic decay produces biogas — mostly methane — which biogas generators harvest from waste as fuel.

The water cycle loops water through the system: the Sun evaporates water from seas and lakes; water vapour condenses into clouds; precipitation returns it to land and sea, feeding rivers back to the ocean. Organisms play a small part — transpiration, excretion — but the Sun drives the loop.

The carbon cycle moves carbon through life: photosynthesis pulls carbon dioxide into glucose; respiration by plants, animals and decomposers returns carbon dioxide; combustion of fossil fuels and wood releases carbon dioxide at once; and decay returns carbon to soil and air. The cycle matters because it recycles the carbon every organism needs — nothing is thrown away, only borrowed.

Biodiversity is the variety of organisms in an area — and human activity keeps reducing it. Land use paves and ploughs habitats; deforestation clears forests; peat bog destruction releases stored carbon and destroys specialist habitats; pollution poisons air, land and water. Protection works the other way: breeding programmes for endangered species, habitat protection and reserves, hedgerows instead of fences, and recycling rather than landfill.

(Biology only) Food security means having enough safe, nutritious food for a population. Threats: a rising human population, new pests and pathogens, environmental change (droughts, floods), the cost of agricultural inputs, and conflicts that disrupt supply. Responses include factory farming (more output per animal, at welfare cost), fertilisers to boost yields, GM crops engineered for resistance and nutrition, and eating less meat so crops feed people directly.