Population size and ecosystems: factors limiting population size; sampling techniques; succession; the flow of energy through trophic levels; and the carbon and nitrogen cycles.

What this dot point is asking

Eduqas wants you to explain the factors limiting population size, describe sampling techniques, explain succession, describe energy flow through trophic levels, and outline the carbon and nitrogen cycles. This topic links the energy theme to whole ecosystems.

Factors limiting population size

A population grows until limited by its environment, settling around the carrying capacity:

• Density-dependent factors act more strongly as the population becomes denser: competition (for food, space, light), predation and disease.
• Density-independent factors affect the population regardless of its size, such as temperature, drought or natural disasters.

Sampling techniques

To estimate abundance and distribution:

• Quadrats (frames of known area) for plants and slow-moving organisms; place them randomly (random coordinates) to avoid bias, count individuals or estimate percentage cover, find the mean, then scale up by the total area.
• Transects (a line or belt across the habitat) to study how a community changes along an environmental gradient, for example up a shore.
• Mark-release-recapture for motile animals: capture, mark and release a sample, then recapture later. The Lincoln index estimates population as $\dfrac{\text{first marked} \times \text{second total}}{\text{number recaptured that were marked}}$.

Succession

Energy flow through trophic levels

Energy enters as light, fixed by producers (photosynthesis), and passes to consumers and decomposers. At each trophic level, much energy is lost (in respiration as heat, in waste and in uneaten parts), so only about 10 percent passes on. This is why food chains rarely exceed four or five levels and why pyramids of energy narrow upwards. Net primary productivity is the energy stored by producers after their own respiration.

The carbon and nitrogen cycles

Worked sampling calculation

Examples in context

Example 1. Legumes and crop rotation. Farmers grow legumes (with nitrogen-fixing bacteria in their root nodules) to enrich soil nitrogen naturally, reducing fertiliser use, a direct application of the nitrogen cycle.

Example 2. Why top predators are rare. Because only about 10 percent of energy passes between trophic levels, there is little energy left to support large numbers of top carnivores, which is why they are few and food chains are short.

Try this

Q1. Distinguish between a density-dependent and a density-independent limiting factor, with an example of each. [2 marks]

• Cue. Density-dependent acts more strongly as density rises (for example disease, competition); density-independent acts regardless of density (for example temperature, flood).

Q2. Explain why food chains rarely have more than four or five trophic levels. [2 marks]

• Cue. Energy is lost (in respiration, waste and uneaten parts) at each level, so only about 10 percent passes on; eventually too little remains to support another level.

Q3. Name the bacteria that convert ammonium to nitrate in the soil. [1 mark]

• Cue. Nitrifying bacteria.