An ecosystem includes interacting living, or biotic, components and non-living, or abiotic, conditions. A population contains members of one species in a stated place; a community contains interacting populations. Adaptations can be structural, physiological or behavioural. For example, a thick cuticle is structural, altered salt handling is physiological, and seeking shade is behavioural.
A quadrat count can be converted to density by dividing number by area. Mean sample density multiplied by habitat area gives an estimate when the samples represent the habitat. Frequency is the percentage of sampled quadrats in which a species occurs. A transect shows changes along a line. Zonation describes changes across space; stratification describes layers, such as a forest canopy and understorey.
Example 1
Four 1m2 quadrats contain 2,4,6 and 4 plants. Estimate abundance in a representative 100m2 patch.
Mean density =42+4+6+4=4 plants/m2. Estimated abundance =4×100=400 plants.
Example 2
A species occurs in 6 of 10 quadrats. Calculate frequency.
Frequency =106×100=60%. This records occurrence, not individual abundance.
Watch out: Frequency measures occurrence. A species can occupy few quadrats but have many individuals within them.