A food web question has two parts that students often run together. First you predict from the feeding links. Then you test the prediction against data, and decide how strongly the data supports it.
This lesson is part of biological mechanisms in mixed tasks. The survey figures below are invented for teaching.
How do I read the food web?
Take this teaching web.
Grass is eaten by grasshoppers and rabbits. Frogs eat grasshoppers.
Snakes eat frogs and rabbits. Eagles eat snakes and rabbits. Arrows point from the organism eaten to the organism that eats it.
Grass is the producer. Grasshoppers and rabbits are primary consumers. Frogs and snakes can be secondary consumers, and eagles are higher consumers.
Worked example (invented data)
Disease reduces the rabbit population in a grassland. A survey compares one year before and one year after, using the same quadrat method in the same area.
| Measurement | Before | After |
|---|---|---|
| Rabbits (count) | 40 | 12 |
| Grass cover (%) | 55 | 78 |
| Grasshoppers (count) | 120 | 150 |
| Snakes (count) | 8 | 5 |
Step 1, predict: fewer rabbits means less grass eaten, so grass should rise. Less competition for grass may let grasshoppers rise. Snakes lose one food source, so they may fall.
Step 2, calculate: rabbits (12 − 40) ÷ 40 × 100 = −70%. Grasshoppers (150 − 120) ÷ 120 × 100 = +25%. Snakes (5 − 8) ÷ 8 × 100 = −37.5%. Grass cover rose by 78 − 55 = 23 percentage points.
Step 3, compare to the prediction: all three predictions match the direction of the data.
Step 4, write a measured conclusion: “The data is consistent with the loss of rabbits leading to more grass and more grasshoppers and fewer snakes. However, this is one survey in one place, so other causes such as rainfall cannot be ruled out. Repeating the survey at more sites would strengthen the conclusion.”
The mistake to watch for
Mistaken answer: “Snakes fell because they had less to eat, so the disease killed the snakes’ food.”
Snakes also eat frogs, and the frog count is not in the table. The answer treats a possible cause as proven and ignores the second food source. The correction is to say “may have contributed” and to name what extra data would help, such as a frog count.
Check yourself
1. Rabbits fall from 60 to 45. Find the percentage change.
Show answer
(45 − 60) ÷ 60 × 100 = −15 ÷ 60 × 100 = −25%. Check: 25% of 60 is 15.
2. In the teaching web, what would you predict happens to frogs if grasshoppers fall? Give a reason and one reason the prediction might not hold.
Show answer
Frogs may fall, because their food supply is smaller. It might not hold if frogs have another food source not shown, or if fewer snakes are eating frogs at the same time.
3. A survey shows grass cover rising from 55% to 78%. Is this a rise of 23% or 23 percentage points? Explain.
Show answer
It is 23 percentage points, the plain difference between two percentages. The relative rise is 23 ÷ 55 × 100 ≈ 41.8%, which is a different quantity.
Where this leads next
Next, check the depth of an inherited-trait explanation against 0653 scope. The scientific investigation critic can help you judge which extra measurements would make your food-web conclusion stronger, and the mixed practice set has more of these.
If you want a teacher to listen to how you phrase conclusions, online one-to-one Combined Science tuition offers that kind of feedback.