Species diversity in the Amazon River basin

Paragraph 1

The old belief that climatic stability accounts for the high level of species diversity in the Amazon River basin of South America emerged, strangely enough, from observations of the deep sea. Sanders discovered high diversity among the mud-dwelling animals of the deep ocean. He argued that such diversity could be attributed to the absence of significant fluctuations in climate and physical conditions, without which the extinction of species should be rare. In the course of time new species would continue to evolve, and so the rate of speciation would be greater than the rate of extinction, resulting in the accumulation of great diversity. Sanders argued that the Amazon tropical rain forest is analogous to the deep sea: because the rain forest has a stable climate, extinction should be rare. Evidence that some species of rain-forest trees have persisted for some 30 million years in the Amazon basin, added to the absence of winter and glaciation, supports this view.

Paragraph 2

Recently however, several observations have cast doubt on the validity of the stability hypothesis and suggest that the climate of the Amazon basin has fluctuated significantly in the past. Haffer noted that different species of birds inhabit different corners of the basin in spite of the fact that essentially unbroken green forest spreads from the western edge to the eastern edge of the region. This pattern presented a puzzle to biologists studying the distributions of plants and animals: why would different species inhabit different parts of the forest if the habitat in which they lived had a stable climate?

Paragraph 3

Haffer proposed a compelling explanation for the distribution of species. Observing that species found on high ground are different from those on low ground and knowing that in the Amazon lowlands are drier than uplands, he proposed that during the ice ages the Amazon lowlands became a near-desert arid plain; meanwhile, the more elevated regions became islands of moisture and hence served as refuges for the fauna and flora of the rain forest. Populations that were once continuous diverged and became permanently separated. Haffer’s hypothesis appears to explain the distribution of species as well as the unusual species diversity. The ice-age refuges would have protected existing species from extinction. But the periodic geographic isolation of related populations (there have been an estimated 13 ice ages to date) would have facilitated the development of new species as existing species on the lowlands adapted to changing climates.

Paragraph 4

Although no conclusive proof has yet been found to support Haffer’s hypothesis, it has led other researchers to gauge the effects of climatic changes, such as storms and flooding, on species diversity in the Amazon basin. Their research suggests that climatic disturbances help account for the splendid diversity of the Amazon rain forest.

Topic and Scope:

Species diversity in the Amazon River basin; specifically, two

contrasting hypotheses about what type of climate conditions contributed to the diversity.

Purpose and Main Idea:

Author wants to highlight evidence supporting a new view that climatic disturbance — not climatic stability, as stressed by the old hypothesis — has been the driving force behind the rich variety of species in the Amazon basin.

Paragraph Structure:

Para 1, introductory in purpose, describes the old belief (Sanders’ hypothesis) that species diversity resulted from a stable climate. Note the Keyword “however” at the beginning of Para 2: For the next Para ¶s, the direction of the argument changes. The focus now is on the new belief that it was climatic instability that led to the diversity. More specifically, Para 2 describes a puzzling observation about bird species that inspired the new view. Para 3 describes Haffer’s explanation of (his new hypothesis about) that puzzle; his idea is that differing and changing climates in upland and lowland areas could have served to isolate animal populations and lead to species diversity. The basic idea in Para 4 is that Haffer’s hypothesis, though unproven, has been influential — it has prompted new research that adds support to his view.

The Big Picture:

  • Like most science passages, this one is thick with details. The key ideas here (two contrasting hypotheses) shouldn’t be hard to see, but the amount of detail makes things tougher to navigate. A savvy test-taker might have chosen to move on to the next passage and save this one for last.
  • Note the simple, “classic” structure of this passage: The author starts by citing the old, outdated view (¶ 1), and then moves to a lengthier discussion of the new view (¶s 2-4). We’ve seen this “old view/new view” format time and again in CAT reading passages; being aware of it makes reading a tougher argument (forming a roadmap of the key ideas) that much easier. And having the roadmap under your belt, of course, scores points! 
  • As you read a tougher science passage, remember that most questions (including Inference and Explicit Text questions) will typically hinge on key points. In other words, you’re rewarded for not getting bogged down in the fuzzy details. A strategic focus on the key ideas — the big idea and paragraph topics — always gains the most points.  • While pushing your way through a tough passage, don’t stop to dope out details. Here, for instance, you could have moved confidently into ¶ 3 without being 100% clear on the ‘bird puzzle’ in ¶ 2. You can always go back to specific details and understand them in greater depth, as needed.    

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