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You are reading an older version of this FlexBook® textbook: Human Biology - Ecology Go to the latest version.

How do humans affect other species?

How do you and other humans fit into the environment? Are humans separate from the environment? Do they compete with other species for resources? Let's find out!

“You must teach your children that the ground beneath their feet is the ashes of our grandfathers. So that they will respect the land, tell your children that the earth is rich with the lives of our kin. Teach your children what we have taught our children-that the earth is our mother. Whatever befalls the earth, befalls the sons of the earth.”

Chief Seattle, as translated by Dr. Henry Smith

quoted in The Earth Speaks

How do you think ecologists use the term population? Do they mean the people in your town? Do they mean the people in the world-what about all the frogs in a pond or the trees in a watershed? Actually, if you answered yes to all of those examples, you are correct! A population is all of the organisms of a certain type living in a certain area at a certain time. Most often, people think only of human populations. But ecologists may study the populations of any type of organism.

Let's talk about the population of people living in Pellston, which is a small town in Michigan. The size of the human population in the town can do only three things: It can go up. It can go down. Or it can remain the same. What are ways to increase the number of people in Pellston? People living in Pellston can have children or people from elsewhere can move to the town. Moving into an area is called immigration (ih-muh-GRAY-shun). What are some ways to decrease the number of people in Pellston? People can die or people can move away. Moving away from an area is called emigration (eh-muh-GRAY-shun). Those four occurrences-birth, death, immigration, and emigration-cause the size of any population anywhere to change.

Did You Know?

According to the World Bank, half of the population in developing countries will be under 15 years of age by the year 2000. That means that more than 600 million jobs will be needed in those countries in the next few years.

It is important to remember that population size means only the number of individual organisms. For example, the number of individuals who actually live in Pellston is constantly changing. People are moving there and people are leaving. Babies are being born and people are dying. However, the number of people in Pellston seems to stay around 530. That is the population size listed on the sign posted at the outskirts of town.

What is the population size of your class? Your school? Your town?

In many ways, Pellston is like other places in the world because its population can change. However, Pellston is not typical because its population remains pretty stable. A population is stable when the number of births and the number of people immigrating in is balanced by the number of deaths and the number of people emigrating out. Most human populations are growing. A population grows if more people are born or immigrate into the area than die or emigrate out. Some human populations are shrinking. Populations shrink if more people die or emigrate from the area than are born or immigrate into the area.

Now let's look at a human population that is a bit bigger than Pellston's-the world's population. In 1997, the world's human population was over 5.9 billion. But it hasn't always been that large. Look at the graph in Figure 9.1 on page 57.

The graph shows the approximate growth of the world's population since 8000 B.C. Notice how the line wobbles up and down but basically stays flat from 8000 B.C. until after 1000 A.D. After 1000 A.D., the population starts to increase dramatically. The graph shows that it took thousands of years for the human population to reach 1 billion, another 130 years to reach 2 billion, another 30 years to reach 3 billion, another 15 years to reach 4 billion, and another 12 years to reach 5 billion! This pattern is alarming because it shows that it is taking less and less time for humans to add another billion people to the world's population.

What Do You Think?

How many people do you think the world needs? Did we have enough in 1900? In 1950? Do we still need more people? Explain your reasoning.

Figure 9.1 This graph shows how the world human population has grown. The years shown in brackets along the right side indicate when the population reached the numbers along the vertical axis.

What is happening with the world's human population as shown in Figure 9.1? You may remember that populations grow only if the birthrate increases, the death rate decreases, or more people immigrate into the population. Immigration and emigration are not involved when studying the world's human population. That's pretty obvious since people aren't moving away from Earth. So what occurrences are affecting population increase in our world? More babies are being born and fewer people are dying.

Ecologists who keep track of human populations are called demographers (dem-OG-rah-fers). Demographers tell us that the most dramatic change in the world population has been the result of fewer people dying. Advances in medicine, sanitation, and food production have allowed people to live longer and to have children who also live longer and eventually have more children.

Mexico is one example of a country whose population size has been greatly affected by these changes. Approximately 40 people in 1,000 died each year around 1920. This number shows the mortality rate, or death rate. By 1990, the mortality rate had greatly decreased. Only 6 people in 1,000 died each year. So, on average, 34 more people out of 1,000 are now surviving, living, and having children every year. Contrast this change with what has happened with the birthrate. The birthrate is the number of children born each year. In 1920, approximately 40 babies were born per 1000 people. In 1990, approximately 30 babies per 1,000 people were born.

Figure 9.2 Birth-and Death Rates in Mexico, 1895-1990.

The mortality rate and birthrate combine to affect population growth. For example, approximately the same number of people were being born as were dying in Mexico around 1915. That kept the population stable. But many more people are being born than are dying now. As a result, the population is growing.

Did You Know?

Today's physicians effectively treat diseases that once killed hundreds of thousands of people. Furthermore, fewer children are dying right after birth. For example, in the past a woman in Kenya who gave birth to eight children could expect only four to survive beyond infancy. Today a woman in a similar situation would probably see most of her children survive to have children of their own.

One way of thinking about population growth is to calculate the doubling time of a population. The doubling time is the amount of time it takes for a population to double in size. Most developed countries such as the United States and most European countries are growing slowly and have long doubling times. For example, demographers predict that the population of the United States will double in about 89 years. Most developing countries such as many countries in Africa, Asia, and South America are growing more quickly than developed countries. So developing countries have short doubling times. The average doubling time for many developing countries is approximately 34 years. That may seem like a long time to you. But 34 years really isn't a long time. Just think: In 34 years a developing country will need twice as many houses, twice as much energy, and twice as much food as it does now. This growth is bound to cause some problems. Imagine what would happen if the number of people in your neighborhood doubled. Where would you put the extra apartments, houses, roads, grocery stores, schools, and everything else people need to live?

Activity 9-1: Brush Rabbit Boom

Introduction

Brush rabbits live throughout the state of California. In order to survive, brush rabbits need dense, brushy areas for their nests. They also need grasslands and meadows for grazing in the spring and summer. In the fall and winter, they need leaves, twigs, buds, and bark. If the rabbits have the food, water, shelter, and space they need, they will keep reproducing. They can produce about three litters per year, with three or four bunnies per litter. Their predators-bobcats and coyotes-keep the population in check. When all of these factors are in balance, the brush rabbit population remains relatively stable. In this activity you investigate how changes in resources can affect the brush rabbit population.

Materials

  • Pencils
  • Data Sheet: Graph A
  • Data Sheet: Graph B
  • Resource
  • Activity Report
  • Graph paper (optional)

Procedure

Step 1 Using the information given below, complete the Data Sheet for Graph A.

Population Growth Information for Graph A

In one habitat, the brush rabbit population has been growing steadily. The rabbits have enough food and water. They also have enough space and brush cover for their nests. While the bobcat population has remained stable, the population of one of their predators-the coyote-has been decreasing over the past five years. Farmers have been trapping coyotes to protect their chickens, and some of the coyotes have been killed.

The following assumptions give you numbers for the brush rabbit and coyote populations for the last five years. Use this information to fill out the Data Sheet for Graph A.

Assumptions

  • The estimated brush rabbit population was 250 rabbits five years ago when the coyote population was stable.
  • Over the past five years the estimated rabbit population has increased by 25 rabbits per year over the original population.
  • None of the rabbits left the area.
  • There were no shortages of essential resources that limited the population.

Step 2 Using the information given below, complete the Data Sheet for Graph B.

Population Growth Information for Graph B

Use the same brush rabbit habitat described above. But assume that the coyote population was reduced to zero and that hunters have killed all but three bobcats. This drastic change in the predator population has resulted in the brush rabbit population doubling each year for five years, starting with year one.

The following assumptions give you numerical information for the brush rabbit and coyote population for the last five years. Use this information to fill out the Data Sheet for Graph B.

Assumptions

  • The brush rabbit population doubled every year from year one to year five.
  • The number of brush rabbits five years ago was 250.
  • None of the rabbits left the area.
  • There were no shortages of essential resources that limited the population.

Step 3 Use your completed graphs to answer the questions on the Activity Report.

Did You Know?

People living in the United States consume more resources than people living in most other countries do. We also produce more waste by processing all those resources.

Population Growth and Resource Use

The number of people is just one part of the story of human population growth. The other part is how people use the resources that are available to them. For example, as a person living in the United States, during your lifetime you will probably consume;

  • a number of calories equal to that contained in 100,000 hamburgers, 2 million French fries, 50,000 chocolate shakes, 50,000 apples, and 4,000 gumdrops,
  • 500,000 gallons (2,272,500 liters) of water just to shower
  • 1 ton of soap (0.9 metric ton)
  • enough energy to drive a car around the world 1,500 times
  • 150 trees for wood and paper
  • tons of metals, cloth, plastics, and glass

A typical person living in a developing country would use far less of all of these products.

It can be hard to compare the resources used by people in different countries. For example, you may eat hamburgers or other meat several times a week, while a student in India may never eat meat. Ecologists have tackled this problem by trying to reduce all comparisons to the amount of energy used by people in different parts of the world.

One way to measure energy is to measure the number of calories in food or fuel. Another unit used to measure the amount of energy in something is a gigajoule (GIG-uh-jool). A joule is equal to 0.24 calories, and giga means ‘one billion’. Ecologists have calculated that the average person living in the United States uses about 300 gigajoules of energy each year to do everything from heating their homes, to growing the food they eat, to traveling. The average person living in Mexico uses about 50 gigajoules each year. The average person living in Nigeria uses less than 10 gigajoules of energy per year.

How many calories are there in 300 gigajoules?

People in developed countries such as the United States use about 10 times as much energy per person as people in many developing countries. For example, farmers in developing nations may use only human muscle power and a few domestic animals to work their fields. In contrast, farmers in developed countries use machines and products that consume a lot of energy such as tractors, loaders, combines, fertilizers, and pesticides.

The difference in energy use shows how differently the various people around the world use resources. One person in the United States uses about 6 times as many resources in a year as one person in Mexico, and about 100 times as many resources as one person in Nigeria. In other words, you can raise one child in the United States or 100 children in Nigeria with the same amount of resources.

Suppose you wanted to reduce the amount of resources that humans consumed in an area.

  • Explain one strategy to keep the number of people in the area from growing.
  • Explain one strategy to reduce the amount of resources that each person used.

Which countries would you advise to use each strategy?

Figure 9.3 Compare the average energy use per person in these six countries.

These energy comparisons show that it doesn't matter only how many people live in an area. It also matters how they use their resources. Ecologists estimate that humans are using about 40 percent of the world's terrestrial net primary productivity. Terrestrial net primary productivity is a name for all of the sun's energy that is trapped and stored by all of the plants on the Earth's land surfaces during photosynthesis. Remember how all energy can be ultimately traced back to the sun?

Population growth and the use of resources are the keys to many environmental problems we have. More people use more and more resources. This increasing use is depleting our supply of important resources. And the more resources we use, the more waste and pollution we are dumping into the environment.

Suppose that the world's population doubles in 40 years, and energy consumption rates remain the same. What percentage of the world's net primary productivity will be used by humans in 40 years? What percentage will be left over for all of the other living things on Earth?

Review Questions

  1. What is a population? What are four ways in which its size may change?
  2. Suppose the doubling time for a population of algae in a pond is 24 hours. Also suppose the pond is now at capacity for algae. How long ago was the pond only half full of algae? How long ago was the pond one fourth full of algae?
  3. Is the size of the world's human population growing or shrinking? Is it doing this at a constant rate?
  4. Do 100 people living in developing countries such as Nigeria have the same impact on the environment as 100 people living in the United States? Explain your answer.

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