Food-web and land-cover data answer different questions. A food web shows who eats what and how energy moves between levels, and a land-cover table shows how much area each type of cover occupies and how that changes over time. Both ask you to state a pattern, calculate a change and comment on what it means.
This lesson builds on linking climate, vegetation and soil, because land cover is the visible result of that chain.
How do you read each kind of data?
For energy in a food web:
- Identify the producers (plants), then the primary consumers (eat plants), then secondary consumers (eat primary consumers).
- Note the energy at each level.
- Work out the share passed on: energy at the next level ÷ energy at this level × 100.
- Comment on why less is passed on.
For land cover in a table:
- State the starting and ending values with units and dates.
- Find the change: end − start.
- Find the percentage change: change ÷ start × 100.
- Say what the change might mean, then say what the table cannot tell you.
Worked example
All figures are invented for practice.
Part A, energy. A grassland food chain holds 10,000 kJ in producers, 1,000 kJ in primary consumers and 100 kJ in secondary consumers.
Producers to primary: 1,000 ÷ 10,000 × 100 = 10%. Primary to secondary: 100 ÷ 1,000 × 100 = 10%. Check: 10% of 10,000 is 1,000, and 10% of 1,000 is 100.
Comment: only about a tenth is passed on at each step, because energy is used in movement and life processes and some material is not eaten. This limits the number of feeding levels a food chain can support.
Part B, land cover. In a region of 20,000 km², forest covered 5,000 km² in 1990 and 4,000 km² in 2020.
Change: 4,000 − 5,000 = −1,000 km². Percentage change: −1,000 ÷ 5,000 × 100 = −20%.
Share of the whole region: in 1990, 5,000 ÷ 20,000 = 25%. In 2020, 4,000 ÷ 20,000 = 20%. The share fell by 25 − 20 = 5 percentage points.
Comment: forest lost one fifth of its area over thirty years. The table does not say why, so any cause would need other evidence.
The mistake to watch for
A common slip is to write “forest fell by 5%” in Part B.
Mistaken answer: the forest share fell by 5%.
The 5 is a change in percentage points of the whole region. The forest itself fell by 20%.
The correction is to ask which total the percentage uses. Percentage change uses the starting value of the same item. Percentage points compare two shares of the same whole.
Check yourself
1. A food chain holds 8,000 kJ in producers. If 10% passes to each level, how much reaches the secondary consumers?
Show answer
Primary: 10% of 8,000 = 800 kJ. Secondary: 10% of 800 = 80 kJ.
2. Wetland covered 600 km² in the first year and 540 km² in the last year. Give the percentage change.
Show answer
Change = 540 − 600 = −60 km². Percentage change = −60 ÷ 600 × 100 = −10%. Check: 10% of 600 is 60.
3. Suggest one thing a land-cover table cannot tell you.
Show answer
It cannot show why the change happened, or where inside the region it happened. Both need extra evidence such as maps or records.
Where this leads next
Next, use these figures inside an explanation in explaining a disturbance with a causal chain. The food-web and energy-flow explorer lets you change energy values, and the statistics and distribution explorer checks percentage work.
If calculation is secure but commentary stays short, individual feedback helps a great deal. See how this works in Geography tuition with a one-to-one teacher.