Distribution, niches, sampling and succession

What you’ll learn

• Explain how biotic and abiotic factors affect distribution.
• Choose appropriate quadrat and transect sampling.
• Describe succession through changes made by organisms.

Understand the idea

A population comprises members of one species in a defined area; a community includes its populations of different species. A habitat is the place an organism lives, while an ecosystem includes the community and its physical surroundings. A niche includes an organism’s resource use, environmental requirements and interactions. Light, water and temperature are abiotic influences; predation, competition and disease are biotic influences.

Random quadrats reduce selection bias when estimating abundance across an area. A line or belt transect samples systematic change along an environmental gradient. Record percentage cover, frequency or counts as appropriate, use enough independent samples, and measure associated abiotic factors consistently. Correlation between an organism and a factor does not by itself show causation: both could respond to another variable.

In primary succession, pioneers colonise a surface initially lacking developed soil. Their activity and decomposition add organic matter and alter water retention, nutrient availability and shelter, enabling later species to establish. Competition and environmental modification change the community over time. A relatively stable climax community can develop under a given climate, but disturbance or management can redirect succession; recovery where soil remains is secondary succession.

Key formulae

$$\text{mean density}=\frac{\text{total individuals counted}}{\text{total area sampled}}$$

$$\text{estimated population}=\text{mean density}\times\text{habitat area}$$

Worked example

Eight randomly positioned $0.50\,\mathrm{m^2}$ quadrats contain a total of $60$ daisies. Estimate the number in a uniform $200\,\mathrm{m^2}$ lawn.

1. Total sampled area is $8\times0.50=4.0\,\mathrm{m^2}$.
2. Mean density is $60/4.0=15\,\mathrm{m^{-2}}$.
3. Scale to the whole area: $15\times200=3000$.

Answer: Approximately 3000 daisies, provided the random samples adequately represent the lawn.