An explain answer has three parts: a claim, the evidence and the reason that links them. Most learners give the claim and skip the reason.
This lesson is part of biodiversity in the SPM Biology subject guide. It pairs well with comparing major organism groups.
What are the three levels of biodiversity?
| Level | Meaning | Example |
|---|---|---|
| Genetic | Variety of genes in a species | Different rice varieties |
| Species | Variety of species in an area | Many kinds of trees in a forest |
| Ecosystem | Variety of habitats | Mangrove, rainforest, coral reef |
Match the level to the evidence in the question before you write.
Worked example: two farms, one outbreak
Original data. Two farms of equal size were hit by the same pest outbreak.
| Farm | Rice grown | Yield lost |
|---|---|---|
| Farm X | One variety only | 70% |
| Farm Y | Five different varieties | 20% |
Question: Explain how this shows the importance of genetic diversity.
Claim: Genetic diversity reduces the damage from a pest outbreak.
Evidence: Farm X, with one variety, lost 70% of its yield, but Farm Y, with five varieties, lost 20%. That is 50 percentage points less loss on Farm Y.
Reason: Plants of one variety have the same genes, so if the pest can harm one plant it can harm all of them. Farm Y had varieties with different genes, and some of them resisted the pest. The survivors kept the yield higher.
A common mistake: the claim with no reason
| Answer | Status |
|---|---|
| Diversity is important because it protects crops. | Weak: a claim only |
| Farm Y lost 20% while Farm X lost 70%, because different varieties have different genes and some resist the pest. | Strong: evidence and reason |
The word “because” is a useful check. If your answer has no “because” followed by a biological idea, add the reason.
Applying the frame to species diversity
Take an original forest case: a plot with many tree species keeps its birds when one tree species dies of disease, but a plot with one tree species loses them all.
- Claim: species diversity supports stable food supplies.
- Evidence: the mixed plot kept its bird numbers, while the single-species plot lost them.
- Reason: birds in the mixed plot could feed on other trees, so a loss of one species did not remove their food.
Use the claim evidence and explanation revision desk to check your frames on other cases.
Check yourself
In a coral reef, reef A has 40 fish species and reef B has 8. A disease wipes out one fish species in each. Reef A loses 2% of its fish, reef B loses 25%. Write a claim, evidence and reason.
Answer
Claim: species diversity makes an ecosystem more stable when one species is lost.
Evidence: Reef A, with 40 species, lost 2% of its fish, but Reef B, with 8 species, lost 25%.
Reason: in reef A the lost species is a small part of the food web, and the other species still fill its role. In reef B, the lost species supports a larger share of the web, so the loss spreads.
What to study next
Test the frame with the biodiversity practice set.
If you want a teacher to mark your evidence and reason steps, see online one-to-one Biology tuition.