Q18 — Ant Invasion, Visibility, and a Lion-Zebra-Buffalo Trophic Cascade

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

Symbiosis and trophic interactions are essential to maintain ecosystems. In this question, you will analyze the complex ecological relationship between whistling thorn trees, acacia ants (which receive nutrition and shelter from whistling thorn trees), megaherbivores (elephants), predators (lions), and their prey (zebras and buffalos) in an African savanna. You know that between 2000 and 2020, big-headed ants, an invasive ant species exterminating acacia ants, increased in number in the region.

Panel A: a scatter plot of visibility with/without megaherbivores, in invaded vs. uninvaded areas. Panel B: a logistic curve of the probability of a zebra kill against visibility. Panel C: two trend lines of the annual proportion of zebra vs. buffalo kills against whistling-thorn tree coverage area. Figure 1. A. Results showing how big-headed ant invasion affects visibility with (Y) or without (N) megaherbivores. The higher the number of whistling thorn trees, the lower the visibility. B. Results depicting how visibility affects zebra kill occurrence by lions. C. The proportion of prey (zebra or buffalo) killed by lions as a function of areal coverage of whistling thorn trees.

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

A. Based on the given data, the symbiosis between acacia ants and whistling-thorn trees is commensalism (only ants benefit from the interaction).

B. Zebras are more vulnerable to lion predation in big-headed ant-invaded areas.

C. Between 2000 and 2020, lions are expected to shift their prey from buffalos to zebras.

D. The study suggests that lions prefer ambushing zebras to chasing them for long distances.

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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF