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

  1. In what way is the digestive system of a deer like a temperate coniferous forest?
  2. How does a food web differ from a food chain?
  3. What actions by the common Pacific seastar, Pisaster ochraceus, cause ecologists to consider it a keystone species?
  4. What distinguishes a climax community?
  5. What feature, if intact, permits secondary succession to begin following a disturbance?
  6. Queensland, Australia is located near the equator; what was its location 1 billion years ago?
  7. In what ways does eutrophication alter the appearance and habitat potential of a lake?
  8. What happened after the comb jelly was accidentally introduced to the Black Sea?
  9. Which New Mexico habitat will recover most quickly, a grassland used for cattle grazing or one used to test nuclear bombs?

A Guide to the Reading

When exploring the content in Chapter 36 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.

The Effects of Species Interactions on Communities

Communities are composed of populations of different species in a particular environment. That environment may be as large as a forest or as small as the digestive system of a deer. Communities are described and understood not by their size, but rather, on the basis of the interactions that occur between those species. The interactions within a community are complex and subject to the effects of many factors, both natural and human-caused. The most thoroughly studied aspect of communities is the pathway of energy and nutrient distribution. Recall that all organisms can be classified as producers or consumers. Consumers obtain their energy and nutrients by eating all or portions of other organisms or their remains. Only one type of producer is recognized. However, ecologists distinguish between consumers on the basis of the type of food they eat. Primary consumers obtain the majority of their energy directly from producers. Secondary consumers feed primarily on primary consumers, while tertiary consumers feed primarily on secondary consumers. It is important to understand that these terms reflect concepts that help us understand nature, rather than mandatory categories. For example, when a deer grazes on woodland shrubs, it consumes not only the leaf but also bacteria and fungal spores. Although small in comparison to the energy in the leaf, the bacteria and fungi, which are both consumers, represent a portion of the deer’s energy budget. As you read further about communities, remember that nature is much more complex and intriguing than our simple descriptions sometimes suggest. Communities are often disproportionately affected by one or a few species called keystone species. If removed, the entire composition of the community may change. The experiments with seastars described in the text illustrate this concept. Once seastars were removed from the community, mussels were able to out-compete virtually every other attached intertidal organism. Such results were not entirely predictable. Seastars were neither a dominant nor abundant community member.  The loss of even a single species through natural or human-influenced activities can have disastrous effects on community structure.

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

  • In Section 36.1, Food webs consist of multiple food chains
  • Figure 36.3, A Food Web
  • In Section 36.1, Keystone species have profound effects on communities
  • Figure 36.4, A Keystone Species

Communities Change over Time

All communities change over time. One source of change involves biological interactions and is called succession. In succession, a series of communities occupies a site in a somewhat orderly and predictable fashion. In Chapter 33 you learned that living organisms can alter the physical environment. An existing plant community exerts powerful effects on the structure of the soil. Its roots penetrate the soil; its leaves and roots may decompose and add organic material, and the shade it creates may reduce evaporation and raise moisture levels. With time, these improvements may help competitors displace some species, and the composition of the community will change. Ecologists distinguish between primary succession, which usually begins on a newly created habitat, and secondary succession, which occurs after a disturbance removes the existing community but leaves the soil and the roots and seeds it contains intact. Change in communities may be unrelated to biological processes. You also learned in Chapter 33 that climate is one of the most influential factors on community and biome development. The Earth’s climate regularly cycles through long scale periods of cooling and warming. We are presently in a relatively warm interglacial period. Should the climate cycle as it has for the past 400,000 years, we can anticipate another cooling trend and ice age in the near future. Even if climatic cycles were to cease, habitats would continue to experience slow climatic change because of continental drift (Chapter 19). The slow movement of the continental land masses eventually moves them to different latitudes and the climates associated with them. A slow northward drift has moved North America during the past 135 million years from a semitropical location to its current temperate and arctic position.

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

  • In Section 36.2, Succession establishes new communities and replaces disturbed communities Figure 36.5, Succession
  • In Section 36.2, Communities change as climate changes
  • Figure 36.7, Climate Change, Community Change

Recovering from Disturbance

One of the most important questions facing ecologists involves the potential for the recovery of a community after disturbance. Natural disturbances, such as floods, volcanic eruptions, fires, and windstorms, have always challenged communities. Not surprisingly, most communities are able to recover from such disruptions. More alarming, however, is the relatively sudden emergence of human-induced disturbance. Lacking the long-term evolutionary experience that characterizes the relationship between communities and natural disturbance, most communities are less resilient when exposed to human impacts. Fortunately, some disturbances are recoverable. The text describes the consequences of nutrient enrichment, a process called eutrophication, in aquatic biomes.   Once the source of the nutrient is identified and stopped, most aquatic systems recover. Introduced species represent a much more serious challenge. In some instances, the additional introduction of a consumer may curtail the ecological damage caused by the initial introduction. The text describes the control of both the comb jelly and prickly pear cactus in Chapter 34. Although successful in these two examples, introduced species do not typically result in happy endings. Many species have been driven to extinction by introduced species, and once gone, the community will never return to its original condition. Certain agricultural practices have the potential to degrade communities in ways that may also permanently alter them. Throughout the world, overgrazing is converting grasslands into deserts. Once the grassland soil structure is lost, it may take thousands of years to recover. If its initial existence depended on interactions with particular species, it may never become reestablished. Since humans depend on the continuous functioning of communities, these issues take on serious implications. A variety of moral, ethical, and practical considerations confront us as we consider our impact on the environment.

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

  • Section 36.3, Recovery from Disturbances
  • Figure 36.9, Changes in the Black Sea
  • Figure 36.11, Overgrazing Can Convert Grasslands into Deserts

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:

The Effects of Species Interactions on Communities

  • Chapter 35 – Section 35.2, Exploitation

Communities Change over Time

  • Chapter 33 – Section 33.4, Life on Land
  • Chapter 33 – Section 33.5, Life in Water
  • Chapter 38 – Section 38.3, Changes in the Global Nitrogen Cycle
  • Chapter 38 – Section 38.5, Global Warming

Recovering from Disturbance

  • Chapter 31 - Biology in the News, Dead Zone Spreads in Gulf of Mexico
  • Chapter 38 - Biology Matters, What’s the Size of Your Footprint?
  • Interlude G, Sources of Hope for the Future

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