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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Interlude A. These questions will help guide your exploration and assist you in identifying the key concepts presented in this interlude.

  1. Why do scientists believe that humans are responsible for the new mass extinction?
  2. Which kingdom holds almost half of the 1.5 million known species?
  3. What is the significance of the “barking deer” discovery?
  4. What earned the Xerces Blue butterfly its dubious fame?
  5. Which organism has people in Guam fearful of the toilets they use?
  6. How many pounds of solid waste are produced by the average college student each year?
  7. Why was Chicken Little (“…the sky is falling!”) right about his prophetic warning?
  8. What is an ecotron?
  9. Why does the destruction of redwood forests contribute to the current water shortage in northern California?
  10. Where do the anti-cancer drugs Taxol and Vinblastine come from?

A Guide to the Reading

When exploring the content in Interlude A for the first time, the following concepts typically give students the most difficulty.  For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas. 

Number of Species on Earth

While scientists have been hard at work classifying and naming the 1.5 million known species on earth, the total number of species living on earth can only be estimated.  Because there is no direct way to count all of them, most estimates for the total number of species range anywhere from 3 to 30 million.  These estimates are obtained in a number of different ways, such as the “fogging” technique employed by Terry Erwin as described in the text.  It is important to understand that regardless of the technique used, these estimates all depend on a set of assumptions, which, of course, could always turn out to be incorrect.  Therefore, the total number of species present on earth will always remain a controversial topic in science.

For more information on this concept, be sure to focus on:

  • In Interlude A, Scientists use indirect methods to estimate total species numbers

Mass Extinction

As discussed in the text, several mass extinction events have occurred throughout the history of the earth.  Many scientists believe that we are currently undergoing another mass extinction event driven by the presence of humans and the explosion in the human population.  While it is difficult to determine the exact extinction rate occurring around the globe, scientists have documented that upwards of 20 percent of the freshwater fish and bird species that existed 2000 years ago are now extinct.  The primary difference between this mass extinction event and those that occurred previously is the influence of the human element. 

For more information on this concept, be sure to focus on:

  • In Interlude A, The current mass extinction probably began with early humans
  • In Interlude A, Scientists have made estimates of species losses occurring today

Competition Amongst Species

One of the major threats to biodiversity discussed in the text involves the introduction of foreign species to an ecosystem.  This phenomenon occurs almost exclusively through the action of humans, either purposely or by accident.  When such a foreign species is particularly well suited to an environment, it may thrive and begin to compete with existing species within the locale.  In this case, the foreign species may actually deprive native species of resources, resulting in the extinction of the native species.  The text describes several examples along with the impact such a loss of biodiversity has on an ecosystem.

For more information on this concept, be sure to focus on:

  • In Interlude A, Introduced foreign species can wipe out native species and
  • Figure A.8, The Threat of Nonnative Species

The Effects of Mutation

The text describes the alarming threat posed by changes in the climate to biodiversity.  One group that appears to be particularly affected by this threat are the amphibians.  While many factors appear to be involved in the disappearance of amphibian species worldwide, one factor warrants particular attention: ultraviolet radiation.  Due to the reduction in the amount of rain in the mountainous northwest United States, developing frog and toad eggs receive greater exposure to ultraviolet light.  The text indicates this amplified exposure results in increased vulnerability to disease and infection.  Why is this?  It turns out that ultraviolet light is a powerful mutagen – an agent capable of altering the genetic code of life, DNA.  When such “mutations” occur, genes vital to the survival of the organism may be rendered useless, resulting in a weakened individual.  The concept of mutation will be covered in more detail in future chapters.

For more information on this concept, be sure to focus on:

  • In Interlude A, Some threats are difficult to identify or define

Why is Biodiversity Important?

While the loss of a single spider or frog species may seem insignificant in the grand scheme of things, it is important to understand the importance of biodiversity on the health of the ecosystems on our planet.  The text describes research conducted by scientists which supports the notion that the greater the diversity of species present in an ecosystem, the heartier it is.  This can be calculated experimentally using measures such as “productivity” (biomass produced).  In addition, the biodiversity present in an ecosystem can help reduce invasion by foreign species and the potentially devastating competition that may result (see “Competition Amongst Species” above).  Furthermore, humans depend on a wide variety of goods and services provided by the vast number of species in the biosphere.  We are still discovering new and potentially beneficial products obtained from plant and animal species alike.  Wouldn’t it be a shame if the extinction of a particular plant species occurred prior to the discovery of its beneficial use as a new anti-cancer drug?

For more information on this concept, be sure to focus on:

  • In Interlude A, Biodiversity can improve the function of ecosystems
  • In Interlude A, Biodiversity provides people with goods and services and 
  • Figure A.13, Biodiversity and Your Health

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and may also be revisited and discussed in greater detail in subsequent chapters, including:


  • Chapter 17 – in Section 17.4, Mutation: The Random Production of New Alleles

Adaptation to Environment

  • Chapter 18 – in Section 18.2, Adaptation Does Not Craft Perfect Organisms
  • Chapter 18 – in Section 18.4, Speciation: Generating Biodiversity

Mass Extinction

  • Chapter 19 – in Section 19.4, Mass Extinctions: Worldwide Losses of Species


  • Chapter 19 – in Section 19.7, An Overview of the Evolutionary History of Life

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