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← Theoretical A

Q15 — ENSO and Ecosystem Effects

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Ecology

Walker circulation is the model that describes how temperature differences across the Pacific Ocean can explain local pressure and precipitation: warm, moist air rises and dries out over the Western Pacific, flows east, becomes cold and dry and sinks over the Eastern Pacific, then returns west warming and moistening again.

Diagram of the Walker Circulation loop across the Pacific Ocean: warm moist air rising and drying over the Western Pacific, flowing east aloft, sinking as cold dry air over the Eastern Pacific, then returning west along the surface, warming and moistening again. Figure 1. Walker Circulation.

Alterations in Walker Circulation are called El Niño Southern Oscillation (ENSO). This oscillation has 3 phases: Neutral (normal Walker Circulation), El Niño (weak or reversed Walker Circulation), and La Niña (strong Walker Circulation). Here are some examples of the influence of ENSO on smaller ecosystems:

Panel A: world map marking three ecosystem locations (A: Great Salt Lake, USA; B: Australia; C: equatorial Pacific near 150°W). Panel B: line graph of Australian red kangaroo population size (thousands) from 1979-1988, peaking sharply around 1981 and rising again in 1988. Panel C: two satellite-style strips of the equatorial Pacific at 0°, C1 showing high productivity (yellow/red) and C2 showing low productivity (blue), plus bar charts of corixids and four grazer taxa in years X, Y, and Z at the Great Salt Lake. Figure 2. Top left: Global context of the smaller ecosystems mentioned. A, B, and C represent data from the corresponding ecosystems.

Location Ecosystem/Community Changes
A Great Salt Lake grazers (native species represented by 4 colors) and corixids (invasive freshwater predators) Ecosystem in years X, Y, and Z
B Australian red kangaroo population Size of kangaroo population (in thousands)
C Intertropical Convergence Zone - winds blowing the surface of the water cause upwelling of deep-nutrient-rich water Two consecutive years (C1 and C2); color represents productivity of algae ranging from low (blue) to high (red).

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. In neutral Walker Circulation, the Western Pacific has higher terrestrial ecosystem productivity than the Eastern Pacific.
B. Increase in the number of red kangaroos in 1988 was potentially caused by El Niño.
C. C1 shows pre-El Niño year and C2 is El Niño year.
D. Changes during year Y are probably caused by La Niña.
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Question reproduced from IBO 2024, Theoretical Exam Part A, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)