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

  1. How many days would be required for an E. coli population, growing exponentially, to produce as many individuals as there are atoms in the universe?
  2. Under what circumstances can exponential growth be observed?
  3. What pattern of growth is seen when essential resources become limited?
  4. What happened to the reindeer population of Saint Paul Island?
  5. How are the population densities of the snowshoe hare and the Canada lynx related?
  6. What types of information would biologists need to be able to predict the impact of logging on spotted owl populations?
  7. How many humans can the Earth support?
  8. In what ways have humans altered their carrying capacity during the past 10,000 years?
  9. How many acres are required to support the average citizen of the United States?

A Guide to the Reading

When exploring the content in Chapter 34 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.

What Are Populations?

A population is a group of individuals of the same species living and interacting in a common geographic location. As a result of additions to the population through births and immigration and losses from death and emigration, population size has the potential to change with time. Biologists often find it convenient to focus on the biological factors affecting populations. Although the movements of individuals may be locally important, they have no net effect on the total number of individuals; one individual joining a population simply means that it has left another.

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

  • Section 34.1, What Are Populations?
  • Section 34.2 Changes in Population Size

Patterns of Change

The key to understanding how populations change is to recognize the importance of resources, understand that they have a limited availability, and realize that the individuals of any population require many different resources, each being more or less common. Even when food is abundant, for example, if the appropriate cover required by juveniles is limited, most will be found and taken by predators, and the population will grow slowly or not at all. Populations display a variety of growth patterns in nature, but two patterns are observed repeatedly. Both are best understood as they relate to resource availability. During exponential growth a constant proportion of individuals are added during each unit of time. Such growth occurs only when the population is too small to appreciably impact the quantity of resources provided by the environment. When represented graphically, the number of individuals plotted against time produces a curve that resembles the letter J. Most populations, however, do not maintain a consistent growth rate over time. As resources become limited, the growth rate declines. This type of population growth can be represented by an S-shaped curve. Such populations may appear to be growing exponentially at first, but eventually they stabilize. Biologists use the term carrying capacity to describe this level. It is important to understand that these patterns of growth do not represent an either-or situation. Many populations can grow exponentially when their numbers are small, but later convert to an S-shaped pattern as resources become less common.  Population growth models can be easily constructed using spreadsheets (see Related Activities), but such representations are often oversimplified. A constant population size implies a constant replenishment of resources. This may happen; a good example is the regrowth of prairie grasses during the spring from their dormant root systems. Recall, however, that organisms can substantially influence the physical environment. For example, too many hooves seeking a limited amount of grass can result in soil erosion, fewer surviving plants, and less grass for grazing at a later time. Most populations experience variations in carrying capacity, resulting either from direct influences from the population itself or, more commonly, from climatic factors. In the last chapter you read about the El Niño event. Although not described in these terms in Chapter 33, you probably realize now that it’s the carrying capacity of the open ocean biome that changes when ocean circulation patterns change. When population densities are high, virtually all interactions between population members become more stressful. Competition for food, space, and shelter from predators and weather intensify. Less food, or sunlight, makes an individual more susceptible to disease, and the high number of individuals improves the chances that a disease will quickly spread within the population. When the impact of any factor causes the density of the population to change, that factor can be described as density-dependent. Alternatively, some factors are considered density-independent. One way to distinguish between density-dependence and density-independence is to consider the likelihood of an event relative to population size. Consider predation; when competition for food is low, prey animals can feed in relative safety. When population densities are high, the safe food is quickly eaten, leaving only the more exposed food sources available. Venturing out into the open to nibble a blade of grass may be an animal’s last meal.

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

  • In Section 34.4, Growth is limited by essential resources and other environmental factors
  • In Section 34.4, Some growth-limiting factors depend on population density; others do not
  • Figure 34.11, Same Species, Different Outcomes

The Human Population

Not surprisingly, one of the most important issues facing population ecologists is the future of human growth. Understanding population growth has provided ecologists with some important predictive abilities, and many wonder if these are applicable to the human population. The key to understanding the connection to human growth, and the principles discussed in this chapter is recognizing that the human carrying capacity has not been static. Technological advances have repeatedly increased the human carrying capacity. Advances in medicine have extended life expectancy and lowered infant mortality. Advances in agriculture have stabilized the food supply in many developed regions. Yet, while the human growth curve continues to maintain its J-shape, poverty, starvation, disease, and other indicators of density-dependence are becoming all too common. Will new technologies extend the carrying capacity even further, or do the declining life styles of so many of the world’s citizens represent the future of humanity?

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

  • Section 34.6, Human Population Growth: Surpassing the Limits?
  • Figure 34.12, Rapid Growth of the Human Population

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:

What Are Populations?

  • Chapter 38 – Section 38.1, Land and Water Transformation

Patterns of Change

  • Chapter 35 – Section 35.2, Exploitation
  • Chapter 35 – Section 35.3, Competition
  • Chapter 36 – Section 36.2, Communities Change over Time

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