Ecologists estimate population sizes by sampling rather than counting everything. A QUADRAT is a square frame placed on the ground; you count the organisms inside it.
Quadrats must be placed RANDOMLY (e.g. using random coordinates) to avoid bias and give a representative, valid sample. To estimate a total population: mean number per quadrat ÷ quadrat area = density; density × total area = estimated population.
A transect is used when you expect a gradual change — for example, plant species from a hedge into open field. Place quadrats at regular intervals along the line and record what's present.
High biodiversity makes ecosystems more stable and resilient. Human activities reduce it: deforestation, intensive farming (monocultures, hedgerow removal, pesticides), pollution, and habitat destruction from building.
Positive human actions can protect biodiversity: breeding programmes, protected areas and reserves, replanting hedgerows and forests, reducing deforestation, and recycling to cut habitat loss from resource extraction.
Natural selection: variation exists in a population; individuals with advantageous features are more likely to survive, reproduce and pass on those features; over generations the population changes.
Antibiotic-resistant bacteria are a modern example. In a population of bacteria there is variation; some carry a resistance allele (often from a random mutation). When an antibiotic is used, non-resistant bacteria die, but resistant ones survive, reproduce, and pass on resistance — so the resistant strain spreads.
This is why doctors avoid over-prescribing antibiotics and patients should complete the full course — to reduce the chance of resistant strains developing.