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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 Chapter 35. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What term do ecologists use to describe the interaction that benefits both species?
  2. How would extinction of the yucca plant affect the yucca moth?
  3. What are the three groups of consumers that exploit other species for food?
  4. Why are the spines on cacti that have been grazed longer than on the plants that have not?
  5. Why are many of the frogs found in the tropics so brightly colored?
  6. What behavioral strategy do pigeons use to reduce goshawk predation?
  7. Why was the American chestnut virtually defenseless against the fungus that causes chestnut blight?
  8. Under what circumstances will character displacement occur?

A Guide to the Reading

When exploring the content in Chapter 35 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.


Interactions among organisms affect individuals, populations, communities, and ecosystems. Ecologists classify these interactions by whether the interaction is beneficial or harmful to each of the interacting species. The key to understanding this classification is understanding that ecologists use the terms beneficial and harmful in the context of reproductive success. Any change that improves an organism’s chances of mating or extends its lifespan would be considered beneficial. The death of an organism before it reproduces is, of course, the ultimate harm. Many varieties of mutualism occur. One of the most common is the bacteria that inhabit the digestive systems of animals and assist in the digestive process. As you read about other examples of mutualism, such as seed dispersal and pollination, evaluate each for the principles discussed above. Several misconceptions about mutualism are common. You might think, for example, that the relationship benefits both species equally, but this is generally not correct. It is common for each participant to experience some cost because of the relationship. Mutualism simply occurs when the benefits are greater than the costs. Some mutualistic relationships have become surprisingly specialized. In the most extreme examples, each participant is entirely dependent on the other, and neither can use an otherwise environmentally suitable habitat unless the other is present. As a result, mutualisms can influence the distribution and abundance of species. Similarly, should one species become locally or globally extinct, the other species will experience the same fate.

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

  • In Section 35.1, There are many types of mutualisms
  • In Section 35.1, Mutualists are in it for themselves
  • In Section 35.1, Mutualisms can determine the distribution and abundance of species


Exploitation includes those interactions where one species benefits at the expense of another. Virtually all consumers, defined as organisms that are unable to produce their own food, are dependent on exploitation, making it one of the most common interactions in nature. Even those consumers that appear to be feeding on organic debris will be consuming the bacteria that first colonized the item. Natural selection has equipped the majority of prey species with a variety of defenses that work to reduce the harmful effects of exploitation. Induced defenses are those whose activity is stimulated by an attack. Plants often produce toxic chemicals only after grazing begins. Consider the elaborate response of the human immune system discussed in Chapter 28 to infection by pathogenic bacteria or parasites. Prey defenses create an opportunity for changes within the consumer. For those species with long-term associations, many cycles of adaptation may have taken place, resulting in some unusual specializations. Consider the rough-skinned newt. Its skin glands produce a toxin so potent it is considered one of the most poisonous organisms on Earth. Only one predator has evolved the ability to overcome this toxin, the common garter snake. High toxicity is a personally marginal defense if the individual must die for it to be useful. The majority of toxic prey species advertise their toxicity through warning colorations. Bright, bold, or otherwise, highly visible patterns serve to communicate to potential consumers that the prey item will be distasteful or harmful. A variety of additional strategies have evolved within prey populations that do not involve toxicity. Consider the herding, flocking, or schooling behavior of some animals, birds, and small fish. Despite the fact that a group is more easily located than a single individual, the chances of any individual surviving an attack is much greater when within the group. The development of defensive and offensive strategies by natural selection is slow, often requiring dozens or hundreds of generations. The introduction of prey or consumer species into an ecosystem is often accompanied by extensive disruption. You read earlier about the introduction of the zebra mussel, a prey species growing without the control exerted by its natural predators. This chapter describes the effect of chestnut blight, a fungal consumer introduced to a prey population, the American chestnut, which lacked any natural defenses. As has been true in many other similar situations, the American chestnut is now virtually extinct where its distribution overlaps that of the fungus.

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

  • Section 35.2, Exploitation
  • Figure 35.8, Safety in Numbers


Competition is considered to be an interaction where both participants are harmed. There are two main types of competition. In interference competition one species is prevented from using a resource by the actions of the competitor species. The decline of coyotes following the reintroduction of wolves in Yellowstone National Park provides a good example. Although not eating the same foods, wolves are highly territorial. Their territories left few spaces where coyotes could hunt without the risk of attack from wolves. Although any given wolf territory contains an abundance of unused food items suitable for the coyote, the action of the wolves interferes with the coyote feeding, and the population has declined. In exploitation competition the resource is used by both species, and its use by one competitor prevents its use by the other. Competition for space is one of the most common examples of exploitation competition. On rocky intertidal shorelines throughout the world competition occurs for attachment sites. Ecologists had long wondered why so many intertidal organisms lived in horizontal bands with clear boundaries between them. Research has shown that the lower limit of the distribution of one species is almost always the result of a biological interaction with the species below, typically the better competitor.

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

  • In Section 35.3, Competition can limit the distribution and abundance of species
  • Figure 35.10, What Keeps Them Apart?

Tying it all together

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


  • Chapter 3 – in Section 3.5, Some fungi live in beneficial associations with other species
  • Chapter 31 – in Section 31.4, Roots absorb nutrients from the soil through cells and along walls


  • Chapter 16 – in Section 16.1, Natural selection and genetic drift are important aspects of evolution
  • Chapter 28 – Section 28.4, Our Third Line of Defense: The Immune System
  • Chapter 37 – Section 37.2, Energy Capture in Ecosystems


  • Chapter 34 – Section 34.4, Limits to Population Growth
  • Chapter 36 – in Section 36.1, Keystone species have profound effects on communities

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