To compare two changes fairly, list your starting assumptions first, then trace each change link by link. Using the same rules for both changes keeps the comparison fair.
This lesson is part of ecological evidence and population relationships. Building the web itself is covered in constructing food chains and webs.
What is the original web?
This paddy-field web is invented for practice.
- Rice plants are eaten by insects and by rats.
- Insects are eaten by frogs.
- Rats and frogs are eaten by snakes.
- Snakes are eaten by eagles.
What are the starting assumptions?
State these before predicting: all populations are stable at the start, only the links above exist, and we consider the short-term effect. Use the same three for both changes.
Worked example: two removals compared
Change 1: frogs are removed. Insects are no longer eaten by frogs, so the insect population increases. Snakes lose one food source and rely more on rats, so the rat population may decrease. More insects eat more rice.
Change 2: rats are removed. Rice loses one of its two eaters, so rice damage from rats stops. Snakes rely more on frogs, so the frog population may decrease, and this allows insects to increase.
| Effect | Frogs removed | Rats removed |
|---|---|---|
| Insects | Increase | Increase (through fewer frogs) |
| Rats | Decrease | Removed |
| Frogs | Removed | Decrease |
| Rice | More insect damage | Less rat damage, but more insects later |
The mistake that costs marks
The common error is to use different assumptions for each change. For example, assuming the snakes switch food in one case and starve in the other makes the comparison unfair.
A second error is skipping a link, such as jumping from “frogs removed” to “rice decreases” without naming the insects.
How do you write the comparison?
Name the direction of each change, the link, and one difference between the two. “Removing frogs increases insects directly, whereas removing rats increases insects only indirectly, through fewer frogs.” Notice the answer uses the same assumptions for both.
Check yourself
Using the same web and assumptions, predict the effect on rats if eagles are removed, and give the link.
Answer
Fewer eagles means fewer snakes are eaten, so the snake population increases. More snakes eat more rats and frogs, so the rat population decreases.
The link is eagle to snake to rat. Under the same short-term assumption, this is the immediate prediction.
What to study next
Go on to separating association from a demonstrated cause. The biological data interpretation trainer gives more practice.
If you would like a teacher to question your reasoning, see online one-to-one Biology tuition.