Question 117·Hard·Central Ideas and Details
To find out whether insect larvae speed up the breakdown of fallen leaves through their movement alone or through the microbes they introduce while feeding, Hui Zhang et al. filled sealed laboratory microcosms with equal masses of oak leaf litter. In “movement-only” microcosms, larvae were allowed to crawl over the leaves for two hours and were then removed before feeding began. In “whole-larva” microcosms, larvae remained for two weeks, during which they fed on the leaves and released gut microbes. After four weeks, the two treatments showed no significant difference in decomposition rate. Zhang et al. therefore concluded that larval movement—not microbial inoculation—was responsible for the accelerated decomposition observed in previous field studies.
Based on the passage, which choice best describes the primary purpose of including the “movement-only” microcosms in the experiment?
For questions about the purpose of an experimental condition, identify the experiment’s central comparison and list what each condition includes or excludes. Then choose the option that states the condition’s role in isolating a variable (creating a baseline/control) and enabling the study’s conclusion, while rejecting choices that shift the purpose to an untested variable or an unstated measurement.
Hints
Restate the experimental question
Look at the first sentence: what two possible causes of faster leaf breakdown are the researchers trying to separate?
Compare the two treatments carefully
What does the “movement-only” treatment include that the “whole-larva” treatment also has, and what does it not include that the “whole-larva” treatment does?
Think about experimental design roles
In many experiments, one group acts as a comparison or control. How does the “movement-only” group help the researchers interpret the results of the “whole-larva” group?
Connect purpose to conclusion
The researchers conclude that movement—not microbes—caused accelerated decomposition. How did having a “movement-only” group make that conclusion possible?
Step-by-step Explanation
Identify the main research question
The first sentence states the goal: to find out whether larvae speed up leaf breakdown through their movement alone or through the microbes they introduce while feeding. So the entire experiment is designed to separate the effect of movement from the effect of gut microbes.
Understand the two types of microcosms
The passage defines two treatments:
- “Movement-only”: larvae crawl on the leaves for two hours, then are removed before feeding begins.
- “Whole-larva”: larvae stay for two weeks, feeding and releasing gut microbes onto the leaves.
The key difference is that only the second treatment includes sustained feeding and gut-microbe release; the first is designed to capture movement without feeding-related microbial inoculation.
Figure out the logical role of the movement-only setup
Because the movement-only microcosms include larval movement but exclude feeding and gut-microbe release, they provide a comparison point: if decomposition is similar in movement-only and whole-larva microcosms, then adding gut microbes via feeding did not change the rate. That makes the movement-only group a baseline/control for testing the extra effect of gut microbes.
Match this role to the answer choice
The primary purpose of the “movement-only” microcosms was to establish a baseline (control condition) to test whether larval gut microbes, beyond larval movement, influenced decomposition rates. That idea is captured by the choice: “They established a baseline to test whether larval gut microbes, rather than larval movement, influenced decomposition rates.”