Question 1: Explain Merit

Kauri snails, or pūpū-rangi (Paryphanta spp.), are northern giant land snails that evolved in New Zealand before two nocturnal predators, possums and hedgehogs, were introduced. They live in fertile soil full of earthworms and can live 20 years or more. They are most active at night and spend the day under leaf litter or plants. Their droppings show they eat mainly earthworms and some small snails, whose shells supply calcium. They mate mostly from April to July, apparently set off by weather such as rain, and lay about six eggs three times a year, in a hole they dig or under leaf litter at the foot of a tree. Hedgehogs (Erinaceus europaeus) are also out at night and eat kauri snails; where it is cold they may hibernate for some months of the year, once the ground falls below 11 °C. Brushtail possums (Trichosurus vulpecula), also nocturnal, eat kauri snails too, spotting the slow snails from up in the trees.

Explain the advantage and the disadvantage of night activity for the snail, and how laying three times a year, hibernation and the possum’s nocturnal behaviour each support survival.

The 2024 paper, page 2 ↗ Marking schedule, page 2 ↗

Work it out

Try it on paper first. Stuck? Take one hint at a time.

1 The describe answer named the advantages of night activity for the snail. Why do they help it survive?
A snail loses water through its moist skin and mucus, so the humid night lets it move and feed without drying out, and its earthworm prey is at the surface then. More food and less water loss mean a better chance of surviving.
2 Why is it also a disadvantage?
Possums and hedgehogs arrived after the snail evolved, and both are nocturnal mammals. Out at night, the slow, large snail meets predators it has no defence against, so more snails are eaten and fewer breed: its reproductive success falls.
3 How does hibernation raise the hedgehog’s chance of surviving?
When the ground is below 11 °C its food is scarce and keeping warm costs a lot of energy. Hibernating cuts its metabolic rate, so it lives off stored fat until conditions improve instead of starving.
4 How does laying three times a year raise the snail’s reproductive success?
It can lay when conditions suit the eggs and young, and each clutch is a separate chance: if one is eaten or fails, others may still hatch. Over a year, and a life of 20 years or more, more young survive.
5 How does being nocturnal help the possum?
Its activity is in step with its prey’s: it is out when the snails are moving and easiest to find, so it catches more of them.

Answer Night activity helps the snail survive because the humid night lets it feed without drying out while its earthworm prey is at the surface. But possums and hedgehogs, nocturnal mammals introduced after it evolved, hunt then too, so the slow snail is easily caught and its reproductive success falls. Hibernation helps the hedgehog survive the cold: below 11 °C food is scarce and keeping warm is costly, so it lowers its metabolic rate and lives on stored fat. Laying three times a year lets the snail lay when conditions suit, and spreads the risk across clutches, so more young survive over its long life. Being nocturnal synchronises the possum with its prey, so it is out when the snails are moving and catches more of them.

What the marker wanted

Achieved The describe points: the two relationships, the snail’s advantage and disadvantage of being nocturnal, hibernation, laying more than once, and what nocturnal means.
Merit Explains: night activity giving the snail more food or less drying out, linked to survival; night activity exposing it to nocturnal mammal predators, linked to lower reproductive success; hibernation helping the hedgehog through harsh conditions on stored reserves; how laying several times a year raises reproductive success; and the possum’s activity matching its prey’s. Three make M6.

One answer, N0 to E8. Merit points sit above the Achieved ones: two give M5, three M6.

In our words, from NZQA’s 2024 marking schedule.

Common mistake

Naming an advantage or disadvantage without saying why it is one. NZQA’s 2024 report says that is what held many Question One answers at Achieved.

How did it go?

Got it moves it further out. Review brings it back tomorrow.