Question 99·Hard·Central Ideas and Details
Subterranean fungal filaments, known as mycorrhizal networks, have long been portrayed as selfless conduits that allow trees to share water and nutrients with their neighbors, earning them the popular nickname “the wood-wide web.” Yet recent ecological studies reveal a more calculating system: fungi expend energy to transport resources only when they receive carbon compounds from trees in return, and trees allocate those compounds preferentially to fungal partners that deliver the greatest nutritional payoff. According to evolutionary biologist Toby Kiers, these findings suggest that cooperation in the soil is sustained less by generosity than by continuous negotiation of costs and benefits.
Which choice best expresses the central idea of the text?
For central idea questions, briefly summarize the passage in your own words, paying special attention to contrasts signaled by words like “yet,” “however,” or “but.” Then eliminate choices that (a) are too narrow (only one detail), (b) make the relationship one-sided when the passage presents reciprocal exchange, or (c) miss the passage’s key shift from an older view to a newer interpretation. Choose the option that best matches the passage’s overall message, not just a single sentence.
Hints
Look for contrast words
Focus on how the word "Yet" changes the direction of the passage. What idea comes before it, and what different idea comes after it?
Summarize each sentence in a few words
Try to restate each sentence in 5–7 simple words. Then ask: What do these short summaries have in common, and how do they show a shift in understanding?
Watch for choices that are too narrow or one-sided
The passage describes how both fungi and trees respond to costs and benefits. Be wary of choices that focus on only one side’s behavior or that portray the relationship as mostly one-sided.
Think about how cooperation is maintained
According to the last sentence, what keeps cooperation going in the soil—pure generosity, or something like negotiation and exchange? Look for the answer choice that reflects that idea.
Step-by-step Explanation
Identify what the passage is mainly about
Read the whole passage and ask: What is the author trying to explain overall?
The passage is about mycorrhizal networks (underground fungal filaments connected to trees) and how scientists now understand their behavior. It focuses on the nature of cooperation between trees and fungi, not on a single isolated detail.
Notice the contrast between old and new views
Pay close attention to the first sentence and the word "Yet" at the start of the second sentence.
- First sentence: mycorrhizal networks have been portrayed as "selfless conduits" that let trees share resources, earning the nickname "the wood-wide web." This is the old, altruistic picture.
- Second sentence begins with "Yet", signaling a contrast. It introduces "a more calculating system" where fungi act only when they get carbon in return, and trees favor fungi that give the best nutritional payoff.
So the central idea must include both the earlier altruistic portrayal and the newer, more strategic, give-and-take understanding.
Clarify how cooperation is described now
Look closely at the description of how the system works in the newer view:
- Fungi "expend energy" to move resources only when they receive carbon compounds from trees.
- Trees "allocate" those compounds "preferentially" to fungi that give "the greatest nutritional payoff."
- Kiers says cooperation is kept up "less by generosity than by continuous negotiation of costs and benefits."
This language describes a mutual trade based on self-interest and ongoing negotiation, not a one-sided relationship.
Eliminate choices that are too narrow or distort the relationship
Now compare each answer to the passage:
- Eliminate choices that focus on only one side (only fungi’s behavior or only trees’ behavior) without capturing the passage’s broader point about reciprocal negotiation.
- Eliminate choices that turn the interaction into mostly one-sided extraction when the passage emphasizes exchange and both parties responding to costs and benefits.
The remaining choice should (1) show the shift from a selfless to a more strategic view and (2) describe a mutually beneficial, self-interested trade between trees and fungi.
Select the answer that matches the contrast and the mutual, self-interested trade
The only option that correctly captures both the change in understanding (from altruistic aid to something else) and the idea of mutually beneficial trade governed by self-interest and negotiation is:
Once considered altruistic systems of underground aid, mycorrhizal networks are now understood to function through mutually beneficial trade governed by self-interest.