Carrying Capacity


Carrying Capacity Definition

Carrying capacity is the number of organisms that an ecosystem can sustainably support.

An ecosystem’s carrying capacity for a particular species may be influenced by factors as its ability to regenerate the food, water, atmosphere, and other necessities that the organisms need to survive.

Ecosystems cannot exceed their carrying capacity for a long period of time. In situations where the population density of a given species exceeds the ecosystem’s carrying capacity, the species will deplete its own source of food, water, or other necessities, and will soon begin die off. The species population can grow until it reaches that line, at which point resources will not be sufficient to allow it to continue to grow over the long-term:

Carrying Capacity

An example of a situation in which the carrying capacity of an environment is exceeded is the example of deer in North America.

After the widespread elimination of wolves – the natural predator of North American deer – the deer reproduced until their need for food plant species exceeded the environment’s ability to regenerate their food plants. In many areas, this resulted in large numbers of deer starving until the deer population was severely reduced.

Humans have managed to increase the Earth’s carrying capacity for human life using technological inventions such as agriculture and genetically modified crops, which have a greater ability to produce food in some environments than natural crops.

Still, some scientists fear that humans may exceed the Earth’s carrying capacity for humans, and encourage the use of contraception to decrease birth rates in order to prevent human populations from exhausting their sources of food and other vital resources.

Examples of Carrying Capacity

Example #1: The Daisyworld Model

The hypothetical “Daisyworld” model is a model developed by scientists to study how organisms chance their environment, and how ecosystems self-regulate.

In the original “Daisyworld” mathematical simulation, there were only two types of life forms: black daisies, which increase the environment’s temperature by absorbing heat from the Sun (this is a real property of black materials), and white daisies, which decrease the environment’s temperature by reflecting the Sun’s heat (this is also a real effect of white-colored materials).

Each species of daisies had to live in a proper balance with the other species. If the white daisies overpopulated, the world would become too cold. Daisies of both types would begin to die off, and the world would start to regain its pre-daisy equilibrium. The same held true for black daisies: if they overpopulated, the world would become warmer and warmer until the daisies began to die off again.

Real-life ecosystems are much more complicated than this, of course.

Each organism has many needs, and how well the environment can meet those needs might depend on what other organisms it shares the environment with. Such is the case for…

Example #2: The North American Deer

Before Europeans colonized North America, one of its main forest herbivores were deer. Ordinarily existing in small groups, populations of deer were kept in check by wolves, the top predator of these forest ecosystems.

Deer, being fairly large as North American herbivores go, were capable of eating leaves off of trees and shrubs, as well as low-growing plants like flowers and grass. And they required a lot of leaves to keep them going, as members of different species of deer could weigh anywhere from 50 to 1,500 pounds!

But when European settlers severely depleted the population of wolves, who they found to be a danger to human children and livestock, an unexpected consequence resulted: deer began to multiply out of control, until they exceeded the carrying capacity of their environment.

As a result, deer began to starve. Plants species also began to suffer, some even being threatened with extinction as the starving deer ate all the green plants they could find.

When humans realized what was happening – and it began to affect their own food sources, after wild deer began to invade gardens and farms looking for crops to eat – they began to give nature a helping hand in reducing the deer population.

In modern times, some areas “cull” deer – a practice where deer are systematically hunted, not just for meat or sport, but to prevent deer starvation and damage to plants. Other areas have even begun to re-introduce wolves – humans’ old enemy – and have seen healthier ecosystems, gardens, and crops as a result.

The story of the North American wolves and deer has acted as a cautionary tale for people considering making changes of any kind to their natural environment, which might have unintended consequences.

Example #3: Humans

Humans have become one of the world’s only global species my mastering technology. Time and time again, the human species has overcome a factor, such as availability of food or presence of natural predators, that limited our population.

The first major human population explosion happened after the invention of agriculture, in which humans learned that we could grow large numbers of our most nutritious food plants by saving seeds to plant in the ground, and making sure those seeds got enough water, were protected from competition from weeds and from being eaten by other animals, etc..

When agriculture was invented, the human population skyrocketed – scientists think that without agriculture, between 1 million and 15 million humans were able to live on Earth at a time. Today, there are about 1 million humans in the city of Chicago alone!

By the Middle Ages, when well-organized agriculture had emerged on every continent, there were about 450 million – or about half a billion – humans on earth.

A new revolution in Earth’s capacity to carry humans began in the 18th and 19th centuries, when humans began to apply advanced and automated technology to agriculture. The use of inventions such as the mechanical corn picker and crop rotation – a way of growing different crops in sequence that enriches the soil and leads to higher yields – allowed humans to produce even more food. As a result, the world population tripled from about half a billion to 1.5 billion people.

In the twentieth century, a third revolution occurred when humans began to learn how to rewrite the genomes of the plants, using viruses to insert new genes into seeds directly instead of relying on selective breeding and random mutation to increase crop yields. The result was another drastic increase in the Earth’s ability to produce food for humans.

During the 20th century, Earth’s human population more than quadrupled, from 1.5 billion to 6.1 billion. We’ve come a long way from the pre-agricultural days!

But some scientists worry that we may be well on our way to exceeding the Earth’s carrying capacity – or that we may have already done so.

Scientists point out that the practices currently used to grow genetically modified crops may not be sustainable, as they often require the use of chemicals that can harm other parts of our ecosystems.

Scientists point to the rapid decline of bee populations – which are necessary to pollinate some of our crops, and which many scientists believe are being killed by pesticides we use to protect those same crops – as evidence that our current food production practices may not be sustainable for much longer.

The proliferation of poisonous algae, which can poison our water supplies and which feeds on the same fertilizer we use to feed our crops, is another worrisome sign that we may be exceeding our carrying capacity, and may begin to cause problems for ourselves if our population continues to grow.

Some believe that humans will continue to invent newer, better ways to grow food and to make water safe to drink. Many scientists agree that this is true: however, they caution that we should not simply continue to grow our population and assume that advances in technology will be able to take care of everyone.

Instead, scientists urge us to consciously plan our birth rates and population growth to avoid humanitarian crises.

Related Biology Terms

  • Climate change – A process by which the Earth’s weather changes significantly over time. Earth’s climate can be changed when organisms living on Earth alter its atmospheric chemistry, as humans have done by releasing large amounts of greenhouse gases through their activities.
  • Ecosystem – A community of living things which interact with each other and their environment. Organisms in ecosystems compete with each other, but they also often rely on each other for long-term survival.
  • Mass extinction – An event in which many species die off, often because environmental change occurred faster than they could adapt. Historical causes of mass extinction include asteroid impacts and climate change.

Test Your Knowledge

1. Which of the following is NOT true of carrying capacity?
A. An ecosystem’s carrying capacity depends on an interplay of many variables, such as weather, soil type, what food species are available, and how quickly they are able to regenerate.
B. When an organism exceeds the environment’s carrying capacity, it is instantly corrected.
C. Ecosystems are stable over time. Their carrying capacity does not change.
D. B and C.

Answer to Question #1

2. Which of the following is an example of carrying capacity being reached?
A. The peppered moth changing its camouflage colors in response to human pollution.
B. Bacteria in a petri dish multiplying until they run out of nutrients.
C. A leopard seal hunting a penguin for prey.
D. Deer multiplying until they deplete their food source and begin to starve.
E. B and D.

Answer to Question #2

3. Which of the following is NOT a reason why scientists are concerned that humans may be pushing the limits of Earth’s carrying capacity?
A. Bee colonies dying off due to pesticide use.
B. Poisonous algae proliferating and excreting toxins into lakes and rivers.
C. Water sources that humans rely on in desert environments becoming depleted.
D. All of the above.

Answer to Question #3

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