Ecology
Biology · GCSE · The School
Competition and interdependence
Organisms compete for what is limited: plants for light, water, space and mineral ions; animals for food, territory and mates. They also depend on each other — for pollination, seed dispersal and food. Remove one species and the effects run through the web, which is why a single extinction is rarely a single loss.
The cycles that keep it running
CARBON: photosynthesis removes CO2 from the air; respiration, decomposition and combustion return it. WATER: evaporation, condensation, precipitation, and transpiration from plants. Decomposers are the step people forget — without them, nutrients stay locked in dead material and the cycle stops.
Biodiversity and human effect
BIODIVERSITY is the variety of species in an ecosystem, and a varied ecosystem is more stable because it does not depend on any one species. Human activity reduces it through habitat destruction, pollution, and land taken for farming and building. It can be protected: breeding programmes, protected habitats, hedgerow and field-margin schemes, and reducing deforestation.
You cannot count every daisy in a field, so throw a 1 m² quadrat at random ten times and count within it: 4, 7, 2, 9, 5, 6, 3, 8, 5, 1. Mean = 50 ÷ 10 = 5 per m². The field is 2,000 m², so an estimate of 10,000 daisies. Now notice why the throws must be RANDOM: placing the quadrat where the daisies look thickest would have given a mean of 9 and an estimate nearly double the truth.
Reading a food web for consequences
Exam questions rarely ask you to label a food web; they ask what happens when one organism is removed, and the mark is in following the effect more than one step. If a predator is removed, its prey increases FIRST, then the prey's own food source decreases, and then the prey population falls again because it has eaten itself out. That second bounce is where the marks are. Always trace at least two steps, and say which direction each population moves and why, rather than stating that the web is disrupted, which tells a marker nothing.
Answering with data, not opinion
Ecology questions usually supply a table or a graph and expect you to use it. Quote the actual figures with their units, and quote the comparison rather than one number: species richness fell from 14 to 6 over the twenty years, a drop of just over half. Then give the biological reason. Candidates lose marks by describing the trend in words only, or by explaining a mechanism without ever touching the data they were given. Where the data does not support a conclusion, say so — recognising that a sample of one field cannot support a national claim is itself a creditable point.