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What Affects Population Size?

A population is a group of organisms belonging to a single species that lives in a given area. Imagine that each of the three different colors of paper clips that you have in an envelope represents a different species of organisms.

Working with a partner and using the envelopes of paper clips that you both have, answer the following questions.

Section 5-1

Interest Grabber

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1. How many populations of organisms do you have in your envelope? In your partner’s envelope?

2. Choose one of the populations in one of the envelopes. How might this population grow in size?

3. How might this same population decrease in size?

Section 5-1

Interest Grabber continued

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5–1 How Populations Grow A. Characteristics of Populations B. Population Growth C. Exponential Growth D. Logistic Growth

Section 5-1

Section Outline

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How Populations Grow

• Sea Otters are an important part of a food chain off of America’s Pacific Northwest coast. • Sea Otters eat Sea Urchins which, in turn, eat kelp. • Almost a century ago, sea otters were almost eliminated by hunting.

• How would this affect their food chain?

• Sea urchin population grew rapidly. • The kelp forest was almost eaten to destruction. • As a result, Sea otters were placed on the endangered species list which prevented hunting them. • The populations went back to normal.

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Characteristics of Populations

• Four important characteristics of a population are its geographic distribution, density, growth rate, and population’s age structure. • Geographic distribution, or range, describes the area inhabited by a population. It could be as small as a drop of water or as large as the Pacific Ocean. • Population Density is the number of individuals per unit area. • PD = # Individuals

Unit area

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Population Growth

• Four factors can affect population size: the number of births, the number of deaths, and the number of individuals that enter or leave the population. • Birth rate greater than the death rate….population increases. • Death rate greater than the birth rate….population decreases. • Immigration…organisms entering population…population increases. • Emigration…organisms leaving population…population decreases.

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Exponential Growth

• Under ideal conditions (abundant space and food and protected from predators and disease) population size will increase extremely fast. • This is called exponential growth. • J- Shape curve.

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Logistic Growth

• Obviously, in the real world, exponential growth can not occur. • As resources become less available, the growth rate slows down or stops. • This type of growth is called Logistic growth. • S-type curve. • The largest number of individuals that an environment can support is called the carrying capacity.

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Population Growth can be

represented by characterized by characterized by represented by

which cause a

Exponential growth

Logistic growth

Falling growth rate

S-shaped curve

Limits on growth

No limits on growth

J-shaped curve

Constant growth rate

Unlimited resources

Section 5-1

Concept Map

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Num

ber o

f Yea

st C

ells

Time (hours)

Carrying capacity

Section 5-1

Figure 5-4 Logistic Growth of Yeast Population

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Name That Resource!

A situation that causes the growth of a population to decrease is called a limiting factor. Some limiting factors depend on the size of the population. Other limiting factors affect all populations in similar ways, regardless of the population size.

Section 5-2

Interest Grabber

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1. Imagine a small island that has a population of five rabbits. How might each of the following factors affect the rabbit population? a. climate b. food supply c. predation

2. Now imagine another small island that has a population of 500 rabbits. How would the same factors affect this population?

3 Which of the factors depend on population size? Which factors do not depend on population size?

Section 5-2

Interest Grabber continued

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5–2 Limits to Growth A. Limiting Factors B. Density-Dependent Factors

1. Competition 2. Predation 3. Parasitism and Disease

C. Density-Independent Factors

Section 5-2

Section Outline

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Limiting Factors

• Factors that cause a population growth to decrease.

What is the panda’s limiting factor?

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Density-Dependent Factors

• Limiting factors that depend on the size of the population. • Examples of D-D factors:

–  Competition: Organisms fight for resources to survive. •  Food, water, space, sunlight, and other essentials

–  Predation: The size of a population can depend on the level of predation (animals that hunt and kill another animal for food).

–  Parasitism: Parasites take nutrients from their hosts possibly causing disease and death.

–  Disease: Causes organisms to weaken and die.

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Predator vs. Prey

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Density-Independent Factors

• D-I factors affect all population regardless of population size.

–  Unusual weather, natural disasters, seasonal cycles, and human activities.

• What are some examples??

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Growth of Aphids

Exponential growth

Steady population size

Peak population size

Rapid decline

Steady population size

Section 5-2

A Density-Dependent Limiting Factor

4

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60 50 40 30 20 10 0 1955 1960 1965 1970 1975 1980 1985 1990 1995

2000 1600 1200 800 400

0

2400

Moose Wolves

Section 5-2

Figure 5-7 Wolf and Moose Populations on Isle Royale

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How Fast Are We Growing?

Until about 500 years ago, the world’s human population remained fairly stable. Then, as advances in medicine, agriculture, and technology occurred, the human population began growing very rapidly. Today, the world’s human population is greater than 6 billion people, and it continues to grow, but at a slower rate.

Section 5-3

Interest Grabber

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1. The human population is increasing by about 1.4 percent each year. Assume that the population is 6 billion (6,000,000,000). How large will the population be in one year?

2. If the human population continues to grow at a rate of 1.4 percent per year, the population would double in size (to 12 billion people) in only 51 years! What effect might this increase in population have on the environment and on other people?

Section 5-3

Interest Grabber continued

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5–3 Human Population Growth A. Historical Overview B. Patterns of Population Growth

1. The Demographic Transition 2. Age Structure

C. Future Population Growth

Section 5-3

Section Outline

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Historical Overview

• For most of history, human population growth has been slow thanks to limiting factors such as scarce food, incurable disease, and poor living conditions. • More than half the children would not grow up to see adulthood. • About 500 years ago, the human population began to grow more rapidly. • Improvements in agriculture, industry, sanitation, medicine, and health care dramatically reduced the death rate and longevity increased.

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Agriculture begins

Plowing and irrigation

Bubonic plague

Industrial Revolution begins

Section 5-3

Human Population Growth

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Patterns of Population Growth

• Human population has been experiencing exponential growth for the last several hundred years. • How long is this going to last???? • The study of human population is called demography. • Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.

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The Demographic Transition

• A hypothesis that countries have completed a dramatic change in birth and death rates. • United States, Japan, and much of Europe have reached the demographic transition. • Countries such as China and India still have high birth rates.

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Age Structure

• Scientists can predict population growth by using age-structure diagrams. • They break populations down by age and gender. • Each bar represents 5 year groups with males on the left of center and females to the right.

• Rwanda is predicted to double in 30 years!!

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U.S. Population Rwandan Population Males Males Females Females

Section 5-3

Figure 5-13 Age Distribution

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Future Population Growth

• Will the human population continue to grow??

• Scientists predict the population will reach 9 billion by 2050.

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Links from the authors on the gray wolf Interactive test

For links on populations, go to www.SciLinks.org and enter the Web Code as follows: cbn-2051. For links on population growth, go to www.SciLinks.org and enter the Web Code as follows: cbn-2053.

Go Online

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Interest Grabber Answers

1. How many populations of organisms do you have in your envelope? In your partner’s envelope? Each envelope represents a community of organisms, and each color of paper clip in each envelope represents a single population. Therefore, each student should have three populations represented.

2. Choose one of the populations in one of the envelopes. How might this population grow in size? The population (some paper clips) could reproduce, or some could move from the partner’s envelope into the other envelope (immigration).

3. How might this same population decrease in size? The population (some paper clips) could be thrown away (death), or some could move from the partner’s envelope into the other envelope (emigration).

Interest Grabber Answers

1. Imagine a small island that has a population of five rabbits. How might each of the following factors affect the rabbit population? a. climate b. food supply c. predation All of these factors could limit this population. The food supply would have to dwindle substantially in order for it to affect only five rabbits. One predator could wipe out the entire population of five rabbits.

2. Now imagine another small island that has a population of 500 rabbits. How would the same factors affect this population? Predation and the availability of food will have more of an effect on this population than the smaller one because there are more rabbits to be preyed upon and to compete for the food.

3. Which of the factors depend on population size? Which factors do not depend on population size? The factors that depend on population size include the competition for food and predation. The factor that does not depend on population size is climate.

Interest Grabber Answers

1. The human population is increasing by about 1.4 percent each year. Assume that the population is 6 billion (6,000,000,000). How large will the population be in one year? 6,000,000,000 X 0.014 = 84,000,000 people will be added in one year, so the population would be 6.08 billion in one year.

2. If the human population continues to grow at a rate of 1.4 percent per year, the population would double in size (to 12 billion people) in only 51 years! What effect might this increase in population have on the environment and on other people? Students might suggest that this increase in the population would increase the need for roads, medicines, food, and all necessary resources. In addition, people would be living in more crowded areas, and pollution would increase because there would be more vehicles on the roads.

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