CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

 

17. Population Ecology

 

17.8. The Control of the Human Population—A Social Problem

 

Humans are different from most other organisms in a fundamental way: We are able to predict the outcome of a specific course of action. Current technology and medical knowledge are available to control the human population and improve the health and well-being of the people of the world. Why, then, does the human population continue to grow, resulting in human suffering and stressing the environment in which we live? Because we are social animals with freedom of choice, we frequently do not do what is considered best from an unemotional, unselfish, biological point of view. People make decisions based on historical, social, cultural, ethical, and personal considerations. In order to control the human population, individuals may need to set aside some of their wants and desires.

The biggest problems associated with the control of the human population are not biological problems; rather, they require the efforts of philosophers, theologians, politicians, and sociologists. As the population increases, so do political, social, and biological problems; individual freedom diminishes, intense competition for resources intensifies, and famine and starvation become more common. The knowledge and technology necessary to control the human population are available, but the will is not. What will eventually limit the size of our population? Will it be lack of resources, lack of energy, accumulated waste products, competition among ourselves, or rational planning of family size?

Studies of the changes in the population growth rates of various countries indicated that a major factor in determining the size of families is the educational status of women. Regardless of other cultural differences, as girls and women become educated, they have fewer children. Several reasons have been suggested for this trend. Higher levels of education enable women to get jobs with higher pay, which makes them less dependent on males for their support. Being able to read may lead to better comprehension of how methods of birth control work. Regardless of the reasons, increasing the educational levels of women has become a major technique used by rapidly growing countries that hope to control their populations.

 

17.8. CONCEPT REVIEW

20. How does human population growth differ from the population growth of other kinds of organisms?

21. What forces will ultimately lead to the control of human population growth?

 

Summary

A population is a group of organisms of the same species in a particular place at a particular time. Populations differ from one another in gene frequency, age distribution, sex ratio, population distribution, and population density. Organisms typically have a reproductive capacity that exceeds what is necessary to replace the parent organisms when they die. This inherent capacity to overreproduce causes a rapid increase in population size when a new area is colonized. A typical population growth curve consists of a lag phase, in which the population rises very slowly, followed by an exponential growth phase in which the population increases at an accelerating rate, followed by a leveling off of the population during the deceleration phase, which leads to a relatively constant population size in a stable equilibrium phase as the carrying capacity of the environment is reached. In some populations, a fifth phase, the death phase, occurs.

The carrying capacity is the maximum sustainable number of organisms an area can support. It is set by a variety of limiting factors. The availability of energy, the availability of raw materials, the accumulation of wastes, and interactions with other organisms are limiting factors. Because organisms are interrelated, population changes in one species sometimes affect the size of other populations. This is particularly true when one organism uses another as a source of food. Some limiting factors operate from outside the population and are known as extrinsic limiting factors; others are properties of the species itself and are called intrinsic limiting factors. Some limiting factors become more intense as the density of the population increases; these are known as density- dependent limiting factors. Limiting factors that are more accidental and unrelated to population density are called density-independent limiting factors.

Humans as a species have the same limits and influences that other organisms do. Our current problems of food production, energy needs, pollution, and habitat destruction are outcomes of uncontrolled population growth. However, humans can reason and predict, thus offering the possibility of population control through conscious population limitation.

 

Basic Review

1. The number of reproducing adults in a population compared with the number of juveniles is the

a. population density.

b. age distribution.

c. population distribution.

d. gene frequency.

2. The period of time when a population is growing rapidly is known as the _____.

3. Populations grow

a. because most species have a high reproductive capacity.

b. when birthrates are greater than death rates.

c. when there are high numbers of reproductive and juvenile individuals in the population.

d. All of the above are correct.

4. The maximum size of a population is set by limiting factors of the environment. (T/F)

5. A limiting factor that becomes more intense as the size of a population increases is known as a density- independent limiting factor. (T/F)

6. The carrying capacity

a. for the human population has been reached.

b. is determined by the limiting factors of the environment.

c. is the same for all organisms.

d. None of the above is correct.

7. When the size of a population is caused to stop growing because of competition among its members, there are _____ in action.

a. extrinsic limiting factors

b. density-independent limiting factors

c. intrinsic limiting factors

d. population distribution factors

8. The populations of all species eventually reach a stable equilibrium phase. (T/F)

9. Which one of the following populations would grow most rapidly?

a. a population of mice in which there were twice as many males as females

b. a population of mice that had reached its carrying capacity

c. a population of mice in which density-dependent limiting factors were acting strongly

d. a population that was in the lag phase

10. The human population has been increasing rapidly for the past 200 years because

a. humans have displaced other organisms.

b. humans have controlled many disease organisms.

c. humans have developed improvements in agriculture.

d. All of the above are correct.

11. Gene flow occurs when individuals _____ to new places.

12. Pollution can be considered to be a waste product. (T/F)

13. Which of the following is an extrinsic limiting factor?

a. the number of siblings in a bird nest

b. competition among individuals for food

c. rainstorms that kill many plant seedlings

d. None of the above is correct.

14. A K-strategist

a. lives a short time.

b. gives care to its young.

c. is always a tiny organism.

d. None of the above is correct.

15. The lag phase of a population growth curve results in

a. a reduction in the size of the population.

b. little increase in the size of the population.

c. a rapidly growing population.

d. None of the above is correct.

 

Answers

1. b 2. exponential growth phase 3. d 4. T 5. F 6. b 7. c 8. F 9. d 10. d 11. travel/migrate/move 12. T 13. c 14. b 15. B

 

Thinking Critically

Understanding Human Population Growth

Review figure 17.14; note that this population growth curve has very little in common with the population growth curve shown in figure 17.8. What factors have allowed the human population to grow so rapidly? What natural limiting factors will eventually bring our population under control? What is the ultimate carrying capacity of the world? What alternatives to the natural processes of population limitation could bring human population under control? Consider the following in your answers: reproduction, death, diseases, food supply, energy, farming practices, food distribution, cultural biases, and anything else you consider relevant.