Reindeer, Lichen and a Warming Tundra
On a tundra island, reindeer feed mainly on lichen in winter, pawing through snow to reach it. Warmer winters bring more frequent freeze–thaw cycles that encase the lichen in hard ice. Researchers record: lichen cover fell from 45 % to 20 % over 15 years; reindeer numbers fell from 1 200 to 700; the number of wolves, which hunt reindeer, fell from 40 to 15; and shrubs expanded into areas that were formerly lichen. A student argues: 'The wolves declined because the hunters shot them; the reindeer decline has nothing to do with the ice.'
1. Construct a food chain for this island and explain how ice-encrusted lichen could reduce reindeer numbers.
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Food chain: lichen (producer) → reindeer (herbivore) → wolf (predator).
Mechanism:
- Lichen is the limiting winter food - the reindeer cannot dig through hard ice, so less of the food is accessible even though some lichen remains.
- Reduced energy intake in winter leads to lower body condition, lower birth rates and higher mortality, especially of calves.
- The carrying capacity of the island for reindeer falls, so the population declines from 1 200 to 700.
Key point: the ice acts through the availability of food, not through direct harm to the animals - a classic bottom-up effect from a changing abiotic factor.
2. Explain the decline in wolves without assuming hunting, and state what additional evidence would be needed to test the student's claim.
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Reasoning chain:
- Wolves depend on reindeer for food; fewer reindeer means a lower food supply and a lower carrying capacity for predators.
- Predator numbers typically track prey with a lag and at a lower abundance, so a fall from 40 to 15 is consistent with a bottom-up effect.
Evidence needed to test the hunting hypothesis:
- Records of hunting/quota and of wolf mortality causes (shot vs. starvation/disease).
- Data on reindeer carcass availability and wolf body condition.
- Timing: if wolf decline follows the reindeer decline, predation is the more parsimonious explanation; if it precedes it, hunting or another factor is more likely.
Conclusion: both factors could act together; the data given do not support the student's confident claim, and correlation alone does not establish the cause.
3. The shrub expansion is a change in the community. Explain what this could do to the lichen and to the island's biodiversity, and why such changes can be hard to reverse.
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Reasoning chain:
- Taller shrubs shade low-growing lichen and intercept snow, changing light and temperature at ground level - lichen loses competitive advantage and cover falls further.
- Shrubs add woody biomass, so some species gain; but the specialists of open tundra (lichen, and animals dependent on it) lose habitat, so overall biodiversity of the tundra community declines even if total plant mass rises.
- Shrubs change the abiotic conditions (snow cover, soil temperature, permafrost), which further favours shrubs - a positive feedback.
Why hard to reverse: the new community is a stable state with its own feedbacks. Even if the climate cooled, established shrubs and altered soil would slow the return of lichen, and reindeer grazing - now reduced - no longer suppresses shrub growth. Restoration would require both climate and grazing pressure to change.