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Biology, 6/e
Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Population Ecology

Answers to Review Questions

Chapter 24 (p. 514)

1. The three types of dispersion are random spacing, even spacing, and clumping. The most frequent in nature is clumping because specific environmental conditions are neither randomly nor evenly distributed. Animals often congregate for various reasons, and the young of a species are likely to be near the parents.

2. Clumping is caused by lack of uniformity in the environment and by social interactions or the tendency for populations to occupy the center of their range.

3. Survivorship is the percentage of an original population that survives to a given age. The three types are I--large proportion of individuals reach their physiologically-determined maximum age and the greatest mortality is in the aged; II--mortality constant through all ages; and III--mortality especially high in the young stages and declines with age. Examples are as follows: I, humans; II, hydra; III, oysters.

4. Demography is the statistical study of populations. A life table charts the statistics of populations through time as to survival, progeny, mortality, etc. It gives a picture of the success of populations.

5. Although birds tend to have a regular number of eggs per clutch, failure of eggs to survive will cause an increase in the number of eggs the next year. Likewise, overproduction one year will cause a smaller clutch the next year.

6. The biotic potential of a population is the rate at which a given population will increase with no limits placed on it. The actual rate of population increase is the difference between birth rate and death rate per individuals per time. Two factors that affect it are emigration and immigration.

7. An exponential capacity for growth means that the rate of increase remains constant while the actual increase in numbers accelerates rapidly as the population size grows. This type of growth typically occurs when a species expands into a new habitat, such as the introduction of bacteria into a fresh medium.

8. Carrying capacity is the number of individuals that can be supported in a habitat indefinitely. This tends to be a dynamic measure, as environmental characteristics are also dynamic.

9. K-selected populations exist where selection favors those that compete well for and effectively utilize resources. They thrive when the population is near its carrying capacity. When population levels are far below their carrying capacity, selection will favor those that have a high reproductive rate. Such populations are termed r-selected.

10. The shape of a pyramid indicates the potential for future growth. The base of the pyramid represents future breeders. If the base is square, the population is stable. If the base is wide, the population is likely to grow. If the top is larger than the bottom, the population will decline.