Food chains, predator–prey cycles and sampling

An ecosystem is made of feeding relationships, and you need to know how to describe, draw and investigate them.

Producers and consumers

• Producers are organisms that make their own food, almost always green plants (or algae) using photosynthesis. Producers are the start of every food chain.
• Primary consumers eat producers (herbivores).
• Secondary consumers eat primary consumers (often small carnivores).
• Tertiary consumers eat secondary consumers (often top predators).

Food chains and food webs

A food chain shows energy flow as arrows from prey to predator:

grass → rabbit → fox

A food web shows multiple interconnected food chains.

Energy flows in only one direction along the chain — from producer to top predator. Most energy is lost at each step (B7.8 covers this in detail).

Predator–prey cycles

In a stable community, predator and prey populations are linked in a cycle:

1. Plenty of prey → predator population rises.
2. More predators → prey population falls.
3. Less prey → predator population falls (less food).
4. Fewer predators → prey population recovers.

If you plot the two populations on the same axes, the predator graph is similar in shape to the prey graph but lags slightly behind. Famous examples include lynx/snowshoe hare in Canada (data from Hudson's Bay Company fur records) and lions/zebra on African plains.

Required practical: sampling using quadrats

A quadrat is a small frame (often 0.5 m × 0.5 m) placed on the ground to sample organisms.

Method:

1. Mark out the area to be studied (e.g. a 10 m × 10 m field).
2. Use random sampling — generate random coordinates so each quadrat position is unbiased.
3. Place the quadrat and count the number of each species (or estimate % cover for plants).
4. Repeat at least 10 quadrats; calculate the mean number per quadrat.
5. Estimate the total population = mean × (total area / quadrat area).

Example: mean = 6 dandelions per 0.5 m × 0.5 m quadrat (= 0.25 m²). Total area = 100 m². Estimated total = 6 × (100 / 0.25) = 2400 dandelions.

Transects — sampling along a gradient

A transect is a line across an area along which quadrats are placed at fixed intervals (e.g. every 1 m). Useful when you want to see how species change along a gradient (e.g. from high-tide mark up a beach, or from path edge into woodland).

Belt transect — quadrat placed at each interval along the line, recording species present.

Avoiding bias

• Random sampling prevents you choosing "interesting" patches.
• Repeats average out unusual patches.
• Same quadrat size for comparison.
• Identify species correctly with a key.

Links

Builds on B7.1 (community structure). Leads to B7.3 (cycles of materials), B7.5 (biodiversity), and B7.8 (efficiency of energy transfer).