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Q44 - Tracing Detrital and Grazing Food-Web Pathways and Cesium Bioaccumulation Timing

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Ecology

Figure 1 shows a simplified food web in temperate forests. Spiders (generalist predators) consume herbivores (40%) and detritivores (60%). Passerine birds (generalist predators) consume herbivores (50%), spiders (30%), and detritivores (20%). Detrital insects spend their larval period in soil but move aboveground as winged adults, becoming prey for spiders. Conversion efficiency between adjacent trophic levels is assumed to be 10%. 137Cs entered forest soil after the Fukushima accident, is difficult for vascular plants (and thus herbivores) to absorb from roots years later, but fungi absorb and accumulate it, which is then consumed by detritivores. Figure 2 shows three patterns (A, B, C) of how 137Cs concentration (relative to 1 in 2011) changes over several years for three organisms.

Figure 1. Figure 1.

Figure 2. Figure 2.

Using the information and data, determine which of the statements are true or which are false.

Q44-1. The prey biomass of spiders consists of 40% herbivores and 60% detritivores, while that of passerine birds consists of 50% herbivores, 30% spiders, and 20% detritivores. What is the contribution of the pathway from the detrital food web to passerine birds, as a percentage of biomass of detritivores relative to the combined biomass of herbivores and detritivores reaching passerine birds? Answer as a 2-digit integer, cutting off after the decimal point.
Q44-2. Given the described 137Cs behavior (fungi accumulate it, vascular plants and their herbivores absorb little from soil), choose the most appropriate combination of organisms (grasshopper, spider, earthworm) matching graphs A, B, and C in Figure 2.
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Question reproduced from IBO 2020, Theoretical Exam 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)