A community is all the populations of different species in a habitat. They are interdependent: each species depends on others for food, shelter, pollination or seed dispersal. A stable community is one where species and environment are in balance, such as a tropical rainforest. Populations are limited by abiotic factors (light, temperature, water, pH, wind, carbon dioxide, soil minerals) and biotic factors (food, predators, pathogens, competition).
Ecologists sample populations rather than count every individual. A quadrat is a square frame placed at random to estimate population size: mean number per quadrat × (total area ÷ quadrat area). A transect is a line along which quadrats are placed to see how distribution changes, for example from shade into sun. Larger samples give more reliable estimates.
Worked example
Daisies average 4 per 1 m² quadrat in a 200 m² field. Estimate the population.
- Mean per quadrat = 4 per m².
- Field area = 200 m².
Answer:Estimate = 4 × 200 = 800 daisies.
Key words
- community
- all the populations in a habitat
- interdependence
- species depending on each other
- quadrat
- a square frame for sampling
- transect
- a line for sampling changes across a habitat
New questions every time.
NGSS-referenced: HS-LS2-1