Biology HL

1. 1.

   Explain how warming that reduces sea-ice cover can amplify further warming through an albedo feedback. Distinguish this feedback from the initial anthropogenic increase in greenhouse gases and from a stabilizing negative feedback.

   [4 marks] · no calculator

   Worked explanation

   1. Track the direction of change: less reflection means greater absorption, not necessarily more incoming sunlight.
   2. Close the loop by connecting warmer conditions back to further ice loss.
   3. Positive describes reinforcement, not a beneficial environmental outcome or a separate primary emission source.

   Common mistake: Positive feedback does not mean good feedback; it means a response amplifies the initial deviation.

   Marking points

   • Ice reflects a relatively high fraction of incoming solar radiation.
   • Replacing ice with darker water increases absorbed solar energy.
   • Additional warming promotes more ice loss, reinforcing the original change: positive feedback.
   • Anthropogenic greenhouse-gas increases can initiate warming; a negative feedback would oppose rather than reinforce the change.

   Examiner tip: Positive feedback does not mean good feedback; it means a response amplifies the initial deviation.

2. 2.

   In a hypothetical forest, peak caterpillar abundance shifts from day 140 to day 128, but peak nestling food demand shifts from day 142 to day 138. Calculate the gap between the two peaks before and after the shift, and explain a possible consequence for birds whose young depend heavily on caterpillars. Suggest why the two populations might respond differently to changing seasonal conditions and state one limitation of predicting bird population change from peak dates alone.

   [4 marks] · no calculator

   Worked explanation

   1. Compare consumer demand with food supply within each period rather than only comparing the shifts of one species.
   2. Demand now peaks later relative to the food pulse, increasing the temporal mismatch.
   3. Timing differences support a risk mechanism, but peak width, diet flexibility and other conditions affect realized outcomes.

   Common mistake: Both events moving earlier does not preserve synchrony when they move by different amounts.

   Marking points

   • The original peak gap is 142 - 140 = 2 days.
   • The new gap is 138 - 128 = 10 days, an increase of 8 days.
   • Less overlap between food availability and demand may reduce feeding success, nestling growth or survival.
   • Different temperature/photoperiod responses can shift linked populations differently; consequences depend on peak breadth and alternative foods, so these dates alone do not prove population decline.

   Examiner tip: Both events moving earlier does not preserve synchrony when they move by different amounts.

3. 3.

   Compare how increased seawater temperature and increased dissolved carbon dioxide can threaten a coral reef. Distinguish bleaching from reduced calcification and explain one food-web consequence if living coral cover declines.

   [4 marks] · no calculator

   Worked explanation

   1. Keep thermal symbiosis disruption distinct from the carbonate-chemistry mechanism.
   2. Bleached coral may be alive but stressed; recovery depends on conditions and duration.
   3. Loss of habitat-forming organisms can propagate effects beyond corals to the wider community.

   Common mistake: Ocean acidification does not mean seawater must become pH below 7, and bleaching is not simply acid dissolving colour from coral.

   Marking points

   • Thermal stress can cause loss of symbiotic algae or their pigments, producing bleaching.
   • Loss of the symbionts reduces an important photosynthetic carbon/energy supply to corals.
   • Dissolved carbon dioxide acidifies seawater and reduces carbonate availability, hindering calcium carbonate skeleton formation.
   • Reduced living coral/habitat can reduce shelter or food for associated species and alter reef food webs; bleaching is not necessarily immediate coral death.

   Examiner tip: Ocean acidification does not mean seawater must become pH below 7, and bleaching is not simply acid dissolving colour from coral.

4. 4.

   In a hypothetical region, snow and ice retreat, exposing darker land. Explain the ice-albedo feedback and distinguish a positive feedback from the claim that every region must warm at the same rate.

   [3 marks] · no calculator

   Worked explanation

   1. Track reflected versus absorbed incoming energy, not a supposed increase in solar output.
   2. Close the causal loop: warming leads to ice loss, which can increase absorbed energy and further warming.
   3. The sign of a feedback describes its direction relative to the initial disturbance, not whether its effects are beneficial or geographically uniform.

   Common mistake: Positive feedback means reinforcing, not beneficial; the mechanism is not that melting ice creates extra sunlight.

   Marking points

   • Exposed darker surfaces reflect a smaller fraction of incoming short-wave radiation than snow/ice.
   • Greater absorption can increase warming and promote further snow/ice loss, reinforcing the initial change.
   • Positive describes amplification of a change; local conditions and other feedbacks mean it does not imply identical warming rates everywhere.

   Examiner tip: Positive feedback means reinforcing, not beneficial; the mechanism is not that melting ice creates extra sunlight.

5. 5.

   Initially, a hypothetical caterpillar food peak coincides with peak food demand from an insect-eating bird's chicks. Over a decade, the food peak moves ten days earlier but peak chick demand moves only two days earlier. Explain the new timing mismatch and propose a measurement needed before concluding that this change caused lower breeding success.

   [3 marks] · no calculator

   Worked explanation

   1. Compare changes in the two timings rather than comparing either date with a calendar alone.
   2. An eight-day relative shift indicates changed synchrony, but the ecological effect depends on the widths and positions of the demand and supply periods.
   3. A causal conclusion requires reproductive data and alternative explanations, not timing trends alone.

   Common mistake: A timing mismatch can affect breeding success, but two shifting dates alone do not prove a population decline or its cause.

   Marking points

   • The relative shift is eight days, so the food peak now occurs eight days before peak chick demand.
   • Reduced overlap between food availability and chick demand can limit food supply during the demand period.
   • Measure actual overlap/food supply and breeding success across comparable years or sites, considering other influences such as weather and predation.

   Examiner tip: A timing mismatch can affect breeding success, but two shifting dates alone do not prove a population decline or its cause.

Original, unofficial practice questions from SubjectScout — not official past-paper questions or complete syllabus coverage. Added English teaching explanations are AI-authored; independent teacher review is pending.