Fig. 3.1 depicts a red squirrel, $Sciurus\ vulgaris$. This species is native to the British Isles, so it has been present there for at least 10 000 years. In the 1800s, a related but slightly larger species, the grey squirrel, $Sciurus\ carolinensis$, was brought in from North America.
A record of the interaction between red squirrels and grey squirrels includes the following points:
- Grey squirrels, which occupy a similar niche to red squirrels, spread quickly after being introduced.
- Grey squirrels carry a virus that does not affect them, but that kills red squirrels.
- Red squirrels died out in many parts of the British Isles.
- More recently, some areas have shown the opposite pattern. Grey squirrel numbers have fallen. Red squirrels have returned to their former habitat.
- This has occurred where safeguarding a native predator species, the pine marten, $Martes\ martes$, has led to an increase in the number of these predators.
- Examination of pine marten faeces shows that they catch and eat far more grey squirrels than red squirrels.
(a)[2]
Use the information given to identify why red squirrels became extinct in many parts of the British Isles.
(b)[3]
Describe how the degree of molecular similarity between the two species, $S.\ vulgaris$ and $S.\ carolinensis$, can be investigated.
(c)[3]
Explain how the different impact of pine marten predation on red and grey squirrel populations is linked to natural selection and the separate evolution of each squirrel species.
Worked solution & mark scheme
This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Competition between different species” …