Question

Difficulty: MediumDetermining Paragraph-Level Main Ideas

The following passage is adapted from an essay about desert ecology and plant survival strategies.

Paragraph 3 (lines 15–28)
In the arid expanse of the Sonoran Desert, seedling survival is a precarious venture, with young cacti frequently succumbing to intense solar radiation and foraging herbivores. However, a biological phenomenon known as 'nurse parenting' dramatically alters these odds. Mature desert trees, such as the foothill paloverde, serve as vital protectors for vulnerable saguaro seedlings. By establishing themselves beneath the dense canopy of a paloverde, saguaro seedlings are shielded from the scorching midday sun, which significantly reduces soil temperature and moisture loss. Furthermore, the physical barrier of the host tree's low-hanging branches deters large herbivores like jackrabbits from grazing on the tender young cacti. Over decades, as the saguaro grows and develops a robust root system, it eventually outcompetes its host tree for water and nutrients, sometimes leading to the paloverde's demise, but the initial phase of their relationship remains a crucial cornerstone of desert regeneration.

Which of the following statements best expresses the main idea of Paragraph 3?

  1. A
    Saguaro seedlings establish a mutually beneficial partnership that permanently enhances the growth of the host tree.
  2. B
    Desert plants employ a diverse array of evolutionary adaptations to survive in extremely arid environments.
  3. Mature trees facilitate desert regeneration by sheltering vulnerable seedlings from environmental stressors and predators.Answer
  4. D
    Young saguaro cacti are highly susceptible to being eaten by herbivores like jackrabbits.

Answer

Mature trees facilitate desert regeneration by sheltering vulnerable seedlings from environmental stressors and predators.
The correct answer accurately summarizes the main idea of Paragraph 3. The paragraph focuses on how mature desert trees (like the foothill paloverde) act as 'nurse plants' that protect vulnerable seedlings (like the saguaro) from intense heat, water loss, and herbivores, thereby facilitating survival and desert regeneration.

Step-by-Step Solution

1
Analyze the focus of Paragraph 3.
The paragraph introduces 'nurse parenting,' explaining how mature trees like the foothill paloverde shield saguaro seedlings from solar heat and herbivores.
Identifying the subject and central claim of the paragraph is the first step in determining its main idea.
2
Evaluate the details to see how they support the central claim.
The details (reduction of soil temperature, deterrence of jackrabbits) illustrate the specific ways mature trees protect the seedlings, and the conclusion notes this relationship is key to desert regeneration.
Distinguishing supporting details from the primary claim ensures that the main idea is broad enough to cover the paragraph's purpose but narrow enough to fit its specific scope.
3
Formulate a paraphrase of the paragraph's overall message and compare it with the options.
The overall message is that mature trees aid desert survival and regeneration by protecting young seedlings from environmental dangers and predators. This aligns with the correct option.
Paraphrasing prevents falling for options that copy keywords literally but distort the overall meaning.

Key Concept

Determining the main idea of a single paragraph within a passage requires distinguishing the paragraph's local thesis from supporting details and the global theme of the entire passage.
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