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Q33 - Tracing Indirect Effects Through a Food Web

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

The scheme below shows a model of interactions between populations of an ecosystem. Capital letters indicate the populations. Two-ended arrows (↔) indicate whether there is a direct interaction between two populations; the interaction can be beneficial (+), harmful (−), or neutral (0) for each of the populations, marked at the corresponding end of the arrow.

Network diagram of seven boxes (A-G) connected by labeled double-headed arrows: A↔B (+ near A, − near B), A↔E (+ near A, − near E), B↔E (+ near B, − near E), C↔E (+ near C, − near E), C↔D (0 at both ends), D↔F (+ near D, − near F), D↔G (+ near D, − near G), F↔G (− at both ends). Fig.1.

Determine how the change in size of one population affects (both directly and indirectly) the size of other populations in the ecosystem.

  • A: Increase
  • B: Not change
  • C: Decrease
  • D: Cannot be predicted (i.e. the change has both increasing and decreasing effects on the other population)
Q33.1. A diagram shows populations A-G linked by two-ended interaction arrows, each end marked + (benefit), − (harm), or 0 (neutral) for that population. What effect does a DECREASE in population A have on population B?
Q33.2. What effect does a decrease in population A have on population D?
Q33.3. What effect does a decrease in population A have on population E?
Q33.4. What effect does an INCREASE in population D have on population G?
Q33.5. What effect does an increase in population D have on population C?
Q33.6. What effect does an increase in population D have on population A?
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key