Human Populations

Lesson Objectives

  • 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 such as agriculture and urbanization have impacted the planet.
  • Describe sustainable development.


  • carrying capacity
  • Green Revolution
  • Industrial Revolution
  • invasive species
  • limiting factor
  • over-consumption
  • overpopulation
  • pesticide
  • sustainable development


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


Biotic and abiotic factors determine the population size of a species in an ecosystem. What are some important biotic factors? Biotic factors include the amount of food that is available to that species and the number of organisms that also use that food source. What are some important abiotic factors? Space, water, and climate all help determine a species population.

When does a population grow? A population grows when the number of births is greater than the number of deaths. When does a population shrink? When deaths exceed births.

What causes a population to grow? For a population to grow there must be ample resources and no major problems. What causes a population to shrink? 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.

Carrying Capacity

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.

Limiting Factors

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: nutrient, space, and water availability are examples (Figure below). The size of a population is tied to its limiting factor.

In a desert such as this, what is the limiting factor on plant populations? What would make the population increase? What would make the population decrease?

What happens if a limiting factor increases a lot? Is it still a limiting factor? 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, some other ingredient will be limiting.

Species ordinarily produce more offspring than their habitat can support (Figure below). 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.

A frog in frog spawn. An animal produces many more offspring than will survive.

Human Population Growth

Human Population Numbers

Human population growth over the past 10,000 years has been tremendous (Figure below). The entire human population was estimated to be

  • 5 million in 8000 B.C.
  • 300 million in A.D. 1
  • 1 billion in 1802
  • 3 billion in 1961
  • 7 billion in 2011

As the human population continues to grow, different factors limit population in different parts of the world. What might be a limiting factor for human population in a particular location? Space, clean air, clean water, and food to feed everyone are limiting in some locations.

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

An interactive map of where human population growth has been over time:

Not only has the population increased, but the rate of population growth has increased (Figure below). Estimates are that the population will reach 7 billion in 2012, 13 years after reaching 6 billion.

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 that the growth rate is increasing has declined. Still, a recent estimate by the United Nations estimates that 10.1 billion people will be sharing this planet by the end of the century. The total added will be about 3 billion people, which is more than were even in existence as recently as 1960.

Earth’s Carrying Capacity for Humans

What is Earth’s carrying capacity for humans? Are humans now 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 (Figure below). This population was reached about 10,000 years ago. At the time, people lived together in small bands of hunters and gatherers. Typically men hunted and fished; women gathered nuts and vegetables.

In a hunter-gatherer society, people relied on the resources they could find where they lived.

Obviously, human populations have blown past this hypothetical carrying capacity. By using our brains, our erect posture, and our hands, we have been able to manipulate our environment in ways that no other species has ever done. What have been the important developments that have allowed population to grow?

About 10,000 years ago, we developed the ability to grow our own food. Farming increased the yield of food plants and allowed people to have food available year round. Animals were domesticated to provide meat. With agriculture, people could settle down, so that they no longer needed to carry all their possessions (Figure below). 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.

More advanced farming practices allowed a single farmer to grow food for many more people.

When advanced farming practices allowed farmers to grow more food than they needed for their families (Figure below), some people were then able to do other types of work, such as crafts or shop keeping.

Farming increasingly depended on machines. Rows of a single crop and heavy machinery are normal sights on modern-day farms.

The next major stage in the growth of the human population was the Industrial Revolution, which started in the late 1700s (Figure below). This major historical event marks when products were first mass produced and when fossil fuels were first widely used for power.

Early in the Industrial Revolution, large numbers of people who had been freed from food production were available to work in factories.

Every major advance in agriculture has 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.

The Green Revolution has allowed the addition of billions of people to the population in the past few decades. The Green Revolution has improved agricultural productivity by:

  • Improving crops by selecting for traits that promote productivity; recently genetically engineered crops have been introduced.
  • Increasing the use of artificial fertilizers and chemical pesticides. About 23 times more fertilizer and 50 times more pesticides are used around the world than were used just 50 years ago (Figure below).
  • Agricultural machinery: plowing, tilling, fertilizing, picking, and transporting are all done by machines. About 17% of the energy used each year in the United States is for agriculture.
  • Increasing access to water. Many farming regions depend on groundwater, which is not a renewable resource. Some regions will eventually run out of this water source. Currently about 70% of the world’s fresh water is used for agriculture.

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. The Green Revolution is credited for feeding 1 billion people that would not otherwise have been able to live.

What is the flip side of this? The flip side is that for the population to continue to grow, more advances in agriculture and an ever increasing supply of water 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, such as groundwater.

The question is, even though we have increased the carrying capacity of the planet, have we now exceeded it (Figure below)? Are humans on Earth experiencing overpopulation?

Manhattan is one of the most heavily populated regions in the world.

There is not yet an answer to that question, but there are many different opinions. 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. This has not happened, at least not yet. Some scientists think that the carrying capacity of the planet is about 1 billion people, not the almost 7 billion people we have today. The limiting factors have changed as our intelligence has allowed us to expand our population. Can we continue to do this indefinitely into the future?

Humans and the Environment

The Green Revolution has brought enormous impacts to the planet. Natural landscapes have been altered to create farmland and cities. Already, half of the ice-free lands have been converted to human uses (Figure below). 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.

Similar to the biomes map in the Climate chapter, this map shows human ecosystems. Much of the planet’s surface is populated and used by humans.

Modern agricultural practices produce a lot of pollution (Figure below). Some pesticides are toxic. Dead zones grow as fertilizers drain off farmland and introduce nutrients into lakes and coastal areas. 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 move between air, water, and land. As a result, no location or organism — not even polar bears in the remote Arctic — is free from pollution.

Pesticides are hazardous in large quantities and some are toxic in small quantities.

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 below).

The percentage of people in the world that live in abject poverty is decreasing somewhat globally, but increasing in some regions, such as 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 (Figure below). 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.

Since CO2 is a waste product from fossil fuel burning, CO2 emissions tell which countries are using the most fossil fuels, which means that the population has a high standard of living.

Many problems worldwide 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 because of 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 bird species on the island of Guam (Figure below).

An Australian Brown Tree Snake

A dynamic map of the spread of invasive zebra mussels is found here:

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

Back to the question: Have 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.
  • the environment is being damaged.

Seen this way: The answer 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.
  • Many rivers are already dammed as much as they can be.
  • Groundwater is being used far more rapidly than it is being replaced.
  • Fossil fuels and mineral resources are being used faster than they are being replaced.
  • Forests are being chopped down in developed and developing nations.
  • Wild fish are being overharvested.
  • The environment is certainly being damaged
  • Pollution is discussed in the coming chapters.
  • Temperatures are rising and the effects are being seen worldwide.
  • Humans have caused the rate of extinction of wild species to increase to about at least 100 times the normal extinction rate.

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 (Figure below). Diseases kill many of the world’s children before they reach five years of age.

Nearly half of the people living in Africa do not have access to clean water.

What if humans suddenly disappeared? Here is a very interesting view of Earth recovering from our presence:

Sustainable Development

A topic generating a great deal of discussion these days is sustainable development. The goals of sustainable development are to:

  • help people out of poverty.
  • protect the environment.
  • 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, because 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.

And what of the waste products of society? Will producing all that we need to keep the population growing result in a planet so polluted that the quality of life will be greatly diminished (Figure below)? Will warming temperatures cause problems for human populations? The only answer to all of these questions is, time will tell.

As China industrializes, its cities have become among the most polluted in the world.

Lesson Summary

  • 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, 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 to secure a long-term future for our species and the other species we share the planet with.

Review Questions

  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? Why do you say that?
  5. What role has agriculture played in human population and why?
  6. Discuss the good and bad points about the Green Revolution.
  7. In the United States, 17% of energy is used for agriculture. How is this possible, if plants photosynthesize with sunlight?
  8. 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?
  9. What evidence is there that humans are exceeding Earth’s carrying capacity for our species?
  10. What is sustainable development?

Further Reading / Supplemental Links

Points to Consider

  • 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?