1. 1
   In 2016, sea surface temperatures over the northern Great Barrier Reef stayed 1–2 °C above the normal summer maximum for about eight weeks. Surveys found that most shallow-water corals turned white, and about 30% of them had died by the end of the year. Over the same reef, the average pH of surface sea water has fallen from about 8.2 before the industrial revolution to about 8.1 today, as atmospheric carbon dioxide has risen.

   (a)

   How does the rise in atmospheric carbon dioxide threaten reef-building corals?

   [1 mark]

   • ACarbon dioxide raises the temperature of the water directly, dissolving coral skeletons
   • BDissolved carbon dioxide forms carbonic acid, lowering pH and suppressing calcification
   • CCarbon dioxide is toxic to the symbiotic algae in coral tissue
   • DDissolved carbon dioxide raises pH so calcium carbonate precipitates in the water instead of in corals

   (b)

   What caused the corals to turn white in 2016?

   [1 mark]

   • AThe corals expelled their symbiotic algae in response to the high water temperature
   • BThe lower pH dissolved the coloured pigments in the coral tissue
   • CThe coral polyps died and were replaced by white calcium carbonate from the sea
   • DReduced nutrient upwelling starved the corals of plankton

   (c)

   Outline why the loss of corals can cause the collapse of a reef ecosystem.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In an oak woodland in the Netherlands, mean spring temperature rose by 1.2 °C between 1985 and 2005. Over this period the date of peak biomass of caterpillars, which feed on newly opened oak leaves, became 15 days earlier, but the mean egg-laying date of great tits became only 4 days earlier. Great tit parents feed their nestlings mainly on these caterpillars, and demand is greatest about 10 days after hatching. In West Greenland, the onset of spring growth of Arctic mouse-ear chickweed, a food plant of reindeer, has also become earlier as temperatures have risen, while the date on which migrating reindeer arrive at their calving grounds has not changed.

   (a)

   Which explanation of the data from the oak woodland is best supported?

   [1 mark]

   • ABoth caterpillars and great tits time their breeding by photoperiod
   • BBoth species respond equally to temperature
   • CCaterpillar development is cued mainly by temperature, whereas great tit egg-laying is cued largely by photoperiod
   • DGreat tits time egg-laying directly from the number of caterpillars they see

   (b)

   Which best explains the change in West Greenland?

   [1 mark]

   • AReindeer migration is cued by temperature and chickweed growth by photoperiod
   • BWarming has stopped chickweed growing in West Greenland
   • CReindeer now arrive before the chickweed has started to grow
   • DChickweed growth is cued by temperature, while reindeer migration is cued by photoperiod, so reindeer now arrive after the plants' peak

   (c)

   Explain the likely effect of the change in timing on the breeding success of great tits in the oak woodland.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Tawny owls in southern Finland occur as two colour variants, a grey morph and a brown morph. Plumage colour is strongly inherited. Between the 1960s and 2010 the proportion of brown owls in the population rose from about 12% to about 50%, over a period in which mean winter snow depth decreased. In winters with deep snow cover, annual survival of adults was 76% for grey owls and 60% for brown owls. In winters with little snow, survival was 74% for grey owls and 73% for brown owls.

   (a)

   Calculate how much lower the survival of brown owls was than that of grey owls in winters with deep snow, as a percentage of grey owl survival, and deduce what the data show about the fitness of the two morphs.

   [3 marks]

   (b)

   Explain how climate change could have caused the change in the proportion of brown owls, and evaluate whether the data show that the population has evolved.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Norway spruce was planted widely as a monoculture across lowland central Europe, far outside its natural mountain range. The spruce bark beetle (Ips typographus) breeds under the bark of spruce trees, and its larvae kill trees by destroying the tissue beneath the bark. In this region the beetle used to complete one generation per year. After a series of hot, dry summers from 2018, two generations per year became normal, with a third in the warmest years. Drought-stressed trees produce less resin, which normally defends them against beetles. In one region, the volume of spruce killed by beetles rose from about 0.5 million cubic metres per year before 2015 to over 10 million cubic metres in 2019. Forest managers must now decide whether to clear dead stands and replant them with spruce, or to let the land regenerate naturally with native broadleaf trees such as beech and oak.

   (a)

   Explain the increase in the number of beetle generations per year and its likely consequences for the carbon balance of the forest.

   [6 marks]

   (b)

   Discuss whether the managers should replant with spruce or allow natural regeneration with native broadleaf trees, as approaches to carbon sequestration.

   [6 marks]

   Total for question 4: 12 marks

End of questions