High School Earth Science/Human Populations

Improvements in agriculture, sanitation, and medical care have enabled the human population to grow enormously in the last few 100 years. As the population grows, consumption, waste, and the overuse of resources also grow. People are beginning to discuss and carry out sustainable development that decreases the impact humans have on the planet.

Lesson ObjectivesEdit

  • Describe how changes in a limiting factor can alter the carrying capacity of a habitat.
  • Discuss how humans have increased the carrying capacity of Earth for our species and how we may have exceeded it.
  • Discuss how human activities like agriculture and urbanization have impacted the planet.
  • Describe what sustainable development is.


The population size of a species depends on the biotic and abiotic factors present in that ecosystem. Biotic factors include the amount of food that is available to that species and the number of organisms that use that species as food. For life to thrive, a specific amount of abiotic factors are necessary. For example, too little water may cause land plants or animals to become dehydrated. Too much water, however, may cause drowning.

A population grows when the number of births is greater than the number of deaths. It shrinks, if deaths exceed births. For a population to grow, there must be ample resources and no major problems. A population can shrink either because of biotic or abiotic limits. An increase in predators, the emergence of a new disease, or the loss of habitat are just three possible problems that will decrease a population. A population may also shrink if it grows too large for the resources required to support it.

When the number of births equals the number of deaths, the population is at its carrying capacity for that habitat. In a population at its carrying capacity, there are as many organisms of that species as the habitat can support. The carrying capacity depends on biotic and abiotic factors. If these factors improve, the carrying capacity increases. If the factors become less plentiful, the carrying capacity drops. If resources are being used faster than they are being replenished, then the species has exceeded its carrying capacity. If this occurs, the population will then decrease in size.

Every stable population has one or more factors that limit its growth. A limiting factor determines the carrying capacity for a species. A limiting factor can be any biotic or abiotic factor: a nutrient, space, and water availability are examples. The size of a population is tied to its limiting factor. If the limiting factor decreases, the population decreases. If the limiting factor increases, the population increases. If a limiting factor increases a lot, another factor will most likely become the new limiting factor.

This may be a bit confusing so let's look at an example of limiting factors. Say you want to make as many chocolate chip cookies as you can with the ingredients you have on hand. It turns out that you have plenty of flour and other ingredients, but only two eggs. You can make only one batch of cookies, because eggs are the limiting factor. But then your neighbor comes over with a dozen eggs. Now you have enough eggs for seven batches of cookies, and enough other ingredients but only two pounds of butter. You can make four batches of cookies, with butter as the limiting factor. If you get more butter, something else will be limiting.

Species ordinarily produce more offspring than their habitat can support. If conditions improve, more young survive and the population grows. If conditions worsen, or if too many young are born, there is competition between individuals. As in any competition, there are some winners and some losers. Those individuals that survive to fill the available spots in the niche are those that are the most fit for their habitat.

Human Population GrowthEdit

Human population growth over the past 10,000 years has been tremendous (Figure 18.21). The human population was about 5 million in 8000 B.C., 300 million in A.D. 1, 1 billion in 1802, 3 billion in 1961, and 6.7 billion in 2008. As the human population continues to grow, different factors may emerge limiting human population in different parts of the world. Space may be a limiting factor or having enough clean air, clean water or food to feed everyone are concerns we are already facing.

Figure 18.21: Human population from 10,000 BC through 2000 AD showing the exponential increase in human population that has occurred in the last few centuries.

Not only has the population increased, but the rate of growth has also increased (Figure 18.22). It took all of human history for the population to reach the first 1 billion people, in around 1802. The second billion was added 125 years later, in 1927. It took 33 years for there to be 3 billion people in 1960, and only 15 years for there to be 4 billion people in 1975. Another billion was added by 1987, just twelve years later, and it took only another twelve years for the population to reach 6 billion people in 1999. Estimates are that the population will reach 7 billion in 2012, 13 years after reaching 6 billion.

Figure 18.22: The amount of time between the addition of each one billion people to the planet’s population including speculation about the future.

Although population continues to grow rapidly, the rate of growth has declined. Still, it is likely that there will be between 9 and 10 billion people sharing this planet by the middle of the century. The total added will be about 2.5 billion people, which is more than were even in existence as recently as 1950.

With so many more people on the planet than ever before, we must ask whether humans now are exceeding Earth's carrying capacity for our species. Many anthropologists say that the carrying capacity of humans on the planet without agriculture is about 10 million. This population was reached about 10,000 years ago. At the time, people lived together in small bands of hunters and gatherers. Commonly women gathered nuts and vegetables and men hunted animals and fished. People within a band shared their resources. Although they had trading networks with outside groups, trading was limited by what could reasonably be carried. For the most part, people relied on the resources that they could find where they lived.

As you can see, human populations have blown past this hypothetical carrying capacity. By using our brains, our erect posture, and our hands, we have been able to do things that no other species has ever done. About 10,000 years ago, we developed the ability to grow our own food. Farming allowed us to grow the plants we wanted to eat and to have food available year-round. We domesticated animals to have meat when we wanted. With agriculture, people could settle down, so that they no longer needed to carry all their possessions. They could develop better farming practices and store food for when it was difficult to grow. Agriculture allowed people to settle in towns and cities. Early farmers could grow only enough food for their families, with perhaps a bit extra to sell, barter, or trade. More advanced farming practices allowed a single farmer to grow food for many more people. Being freed from having to gather or grow food allowed people to do other types of work.

The next major stage in the growth of the human population was the Industrial Revolution, which started in the late 1700s. Increased efficiency in farming freed up large numbers of people available to work in factories. This major historical event marks when products were first mass produced and when fossil fuels were first widely used for power.

Every major advance in agriculture allowed global population to increase. Irrigation, the ability to clear large swaths of land for farming efficiently, and the development of farm machines powered by fossil fuels allowed people to grow more food and transport it to where it was needed. Currently about 70% of the world's fresh water is used for agriculture.

The biggest advance in agriculture in recent decades is called the Green Revolution. It is this advance that has allowed the population to grow so rapidly. The first focus of the Green Revolution was to improve crops. A tremendous increase in the use of artificial fertilizers, nutrients that help plants to grow and chemical pesticides, chemicals that kill pests followed. About 23 times more fertilizer and 50 times more pesticides are used around the world than just 50 years ago. Most agricultural work is now done by machines: plowing, tilling, fertilizing, picking, and transporting (Figure 18.23). About 17% of the energy used each year in the US is for agriculture.

Figure 18.23: Rows of a single crop and heavy machinery are normal sights for modern day farms.

The Green Revolution has increased the productivity of farms immensely. A century ago, a single farmer produced enough food for 2.5 people, but now a farmer can feed more than 130 people. Due to this increased productivity, the Green Revolution is credited for feeding 1 billion people that would not otherwise have been able to live.

The flip side of this is that for the population to continue to grow, more advances in agriculture will be needed. We've increased the carrying capacity for humans by our genius: growing crops, trading for needed materials, and designing ways to exploit resources that are difficult to get at, like groundwater. The question is, even though we have increased the carrying capacity of the planet, have we now exceeded it? Are humans on Earth experiencing overpopulation?

There are many different opinions about human population growth. In the eighteenth century, Thomas Malthus predicted that human population would continue to grow until we had exhausted our resources. At that point, humans would become victims of famine, disease or war. Some scientists think that the carrying capacity of the planet is around 1 billion people, not the almost 7 billion people we have today. How did we get to where we are today? Many of our limiting factors have changed as we have used our intelligence and technology to expand our resources. Can we continue to do this into the future? Do we now have more people and more impacts on our environment than the Earth can handle?

Humans and the EnvironmentEdit

Figure 18.24: Pesticides are hazardous in large quantities and some are toxic in small quantities.

Along with the increases in food that have come from the Green Revolution have come enormous impacts on the planet. More food has allowed the human population to explode. Natural landscapes have been altered to create farmland and cities. Already, half of the ice free lands have been converted to human uses. Estimates are that by 2030, that number will be more than 70%. Forests and other landscapes have been cleared for farming or urban areas. Rivers have been dammed and the water is transported by canals for irrigation and domestic uses. Ecologically sensitive areas have been altered: wetlands are now drained and coastlines are developed.

Modern agricultural practices produce a lot of pollution (Figure 18.24). Some pesticides are toxic. Fertilizers drain off farmland and introduce nutrients into lakes and coastal areas, causing fish to die. Farm machines and vehicles used to transport crops produce air pollutants. Pollutants enter the air, water, or are spilled onto the land. Moreover, many types of pollution easily moves between air, water, and land. As a result, no location or organism—not even polar bears in the remote Arctic—is free from pollution.

The increased numbers of people have other impacts on the planet. Humans do not just need food. They also need clean water, secure shelter, and a safe place for their wastes. These needs are met to different degrees in different nations and among different socioeconomic classes of people. For example, about 1.2 billion of the world's people do not have enough clean water for drinking and washing each day (Figure 18.25).

Figure 18.25: The percentage of people in the world that live in abject poverty is decreasing somewhat globally, but increasing in some regions, like Sub-Saharan Africa.

A large percentage of people expect much more than to have their basic needs met. For about one-quarter of people, there is an abundance of food, plenty of water, and a secure home. Comfortable temperatures are made possible by heating and cooling systems, rapid transportation is available by motor vehicles or a well developed public transportation system, instant communication takes place by phones and email, and many other luxuries are available that were not even dreamed of only a few decades ago. All of these need resources to produce and fossil fuels to power. Their production, use and disposal all produce wastes. Many people refer to the abundance of luxury items in these people's lives as over-consumption. People in developed nations use 32 times more resources than people in the developing countries of the world.

There are many problems worldwide that result from overpopulation and over-consumption. One such problem is the advance of farms and cities into wild lands, which diminishes the habitat of many organisms. In addition, water also must be transported for irrigation and domestic uses. This means building dams on rivers or drilling wells to pump groundwater. Large numbers of people living together need effective sanitation systems. Many developing countries do not have the resources to provide all of their citizens with clean water. It is not uncommon for some of these children to die of diseases related to poor sanitation. Improving sanitation in many different areas—sewers, landfills, and safe food handling—are important to prevent disease from spreading.

Wildlife is threatened by fishing, hunting and trading as population increases. Besides losing their habitat as land is transformed, organisms are threatened by hunting and fishing as human population grows. Hunting is highly regulated in developed nations, but many developing nations are losing many native animals due to hunting. Wild fish are being caught at too high a rate and many ocean fish stocks are in peril.

Humans also cause problems with ecosystems when they introduce species that do not belong in a habitat. Invasive species are sometimes introduced purposefully, but often they arrive by accident like rats on a ship. Invasive species often have major impacts in their new environments. A sad example is the Australian Brown Tree Snake that has wiped out 9 of the 13 native species on the island of Guam (Figure 18.26).

Figure 18.26: An Australian Brown Tree Snake perched on a post in Guam.

Pollution is a by-product of agriculture, urbanization, and the production and consumption of goods. Global warming is the result of fossil fuel burning.

Let's return to the question of whether humans have exceeded Earth's carrying capacity for our species. Carrying capacity is exceeded if resources are being used faster than they are being replenished. It is also exceeded if the environment is being damaged.

The answer to our original question therefore appears to be yes. Many resources are being used far in excess of the rate at which they are being replaced. The best farmland is already in use and more marginal lands are being developed. Most rivers in the developed nations and many in developing nations are already dammed. Groundwater is being used far more rapidly than it is being replaced. The same is true for fossil fuels and many mineral resources. Forests are being chopped down; and wild fish are being overharvested. Human have caused the rate of extinction of wild species to increase to about at least 100 times the normal extinction rate.

In addition, the stability of the environment decreases as landscapes are transformed, organisms die out and the planet becomes polluted. Although many more people are alive in the world than ever before, many of these people do not have secure lives. Many people in the world live in poverty, with barely enough to eat. They often do not have safe water for drinking and bathing. Diseases kill many of the world's children before they reach five years of age.

Sustainable DevelopmentEdit

A topic generating a great deal of discussion these days is sustainable development. This is development that attempts to help people out of poverty, while protecting the environment, without using natural resources faster than they can be replaced. It is development that allows people to use resources no faster than the rate at which they are regenerated.

One of the most important steps to achieving a more sustainable future is to reduce human population growth. This has been happening in recent years. Studies have shown that the birth rate decreases as women become educated. Educated women tend to have fewer and healthier children.

Science can be an important part of sustainable development. When scientists understand how Earth's natural systems work, they can recognize how people are impacting them. Scientists can work to develop technologies that can be used to solve problems wisely. An example of a practice that can aid sustainable development is fish farming, as long as it is done in environmentally sound ways. Engineers can develop cleaner energy sources to reduce pollution and greenhouse gas emissions.

Citizens can change their behavior to reduce the impact they have on the planet by demanding products that are produced sustainably. When forests are logged, new trees should be planted. Mining should be done so that the landscape is not destroyed. People can consume less and think more about the impacts of what they do consume.

Lesson SummaryEdit

  • Populations of organisms are kept to a habitat's carrying capacity by factors that limit their growth.
  • By developing agriculture and other technologies, the human population has grown well past any natural population limits.
  • Many people on Earth live in poverty, without enough food or clean water or shelter.
  • Overpopulation and over-consumption are causing resources to be overused and much pollution to be generated.
  • Society must choose development that is more sustainable, in order to secure a long term future for our species and the other species that we share the planet with.

Review QuestionsEdit

  1. If phosphorous is limiting to a species in an ecosystem and the amount of phosphorous is increased, what will happen to the population of that species? What will happen to the carrying capacity?
  2. Name some factors that could cause a population to increase. Try to include as many types of factors as possible.
  3. In terms of numbers of births and deaths, explain in detail why you think human population is growing so tremendously?
  4. If all people on Earth were allowed only to replace themselves (that is, each person could only have one child or each couple two children), what would happen to the

planet's population in the next decade? Would it decrease, increase, or remain exactly the same as it is now?

  1. What role has agriculture played in human population and why?
  2. Discuss the good and bad points about the Green Revolution.
  3. In the United States, 17% of energy is used for agriculture. How is this possible, if plants photosynthesize with sunlight?
  4. What is more threatening to the future of the planet: overpopulation or over-consumption? How does an increase in the standard of living for people living in poverty affect the planet?
  5. What evidence is there that humans are exceeding Earth's carrying capacity for our species?
  6. What is sustainable development?


carrying capacity
The number of individuals of a given species a particular environment can support.
Green Revolution
Changes in the way food is produced since World War II that have resulted in enormous increases in production.
invasive species
A species of organism that spreads in an area where it is not native, and negatively impacts the native vegetation. People often introduce invasive species either

purposefully or by accident.

limiting factor
The one factor that limits the population of a region. The limiting factor can be a nutrient, water, space, or any other biotic or abiotic factor that species need.
Resource use that is unsustainable in the long term; obtaining many more products than people need.
When the population of an area exceeds its carrying capacity or when long-term harm is done to resource availability or the environment.
A chemical that kills a certain pest that would otherwise eat or harm plants that humans want to grow.
sustainable development
Economic development that helps people out of poverty, use resources at a rate at which they can be replaced, and protects the environment.

Points to ConsiderEdit

  • How much impact on the planet does an infant born in the United States have during its lifetime, compared with one born in Senegal?
  • How does consuming less impact global warming?
  • Can ordinary people really make a difference in changing society toward more sustainable living?

The Carbon Cycle · Human Actions and the Land