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← Theoretical A

Q14 — Latitudinal Diversity Gradient: Three Hypotheses

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

The latitudinal diversity gradient describes a common pattern in the species richness seen in most groups of organisms whereby there are fewer species in the temperate regions than in the tropics. Species richness is lowest at the poles and highest on the equator. The ubiquity of this pattern suggests the possibility of a single general explanation but our understanding of this pattern is incomplete. The current latitudinal zones arose approximately 45 million years ago, after a period in which the Earth was primarily tropical. The following three hypotheses (Figure 1A-C) could possibly explain this gradient:

Three line graphs of species richness vs. time for tropics (solid) and temperate (dashed) regions. Graph A: temperate richness plateaus at a lower carrying capacity than tropics. Graph B: temperate richness rises more slowly (lower diversification rate) than tropics, both curves starting together. Graph C: temperate richness starts diversifying later (delayed starting point) but at the same rate, ending up lower. Figure 1. A difference in carrying capacity. B difference in diversification rate. C difference in the starting point of diversification. Note that in each graph only one factor is changed.

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

A. The higher variance in species age in the tropics compared to temperate regions is an argument supporting the hypothesis C.
B. The reappearance of the diversity gradient even after the global ecosystem catastrophe would argue against the hypothesis C.
C. Hypothesis B is supported if greater seasonal variation would increase the diversification rate.
D. The fact that most temperate taxa can be found in the tropics, but not vice versa, supports the hypothesis A.
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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)