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Population Regulation

Have you ever wondered why Populations of organisms in nature do not continue to grow and expand endlessly? Perhaps you have wondered what factors prevent populations of organisms from growing exponentially. In the following article, we will discuss how Populations are regulated, which factors are involved, and how it relates to us as humans. 

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Population Regulation

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Have you ever wondered why Populations of organisms in nature do not continue to grow and expand endlessly? Perhaps you have wondered what factors prevent populations of organisms from growing exponentially. In the following article, we will discuss how Populations are regulated, which factors are involved, and how it relates to us as humans.

Definition of population regulation in ecology

Every living organism on Earth (including humans) has limits to how large its population grows. Infinite growth is not possible on a planet with finite resources and all populations will eventually be regulated. This article covers the mechanisms of population regulation. So what is the definition of population regulation in ecology?

Population regulation refers to the ecological processes (Biotic and Abiotic Factors) by which the growth of populations is limited due to the effects on birth and death rates.

The ecological factors that limit population growth are known as limiting factors. There are two different kinds of limiting factors - density-dependent and density-independent limiting factors. In addition, there are two kinds of population regulation - top-down regulation and bottom-up regulation.

Density-dependent limiting factors in population regulation

Density-dependent limiting factors impact a population’s per capita rate of growth based on the population’s density. These factors will generally cause the growth rate to drop as the population gets larger. Density-dependent limiting factors usually cause populations to reach a maximum level (called the population’s carrying capacity).

At this point, the population size will level off and usually, but not always, become stable. This is known as logistic growth. When a population's growth rate remains constant, no matter its size, it will continue to grow larger at an exponential rate. This is known as exponential growth.

This is very rare, and when it does occur, it will usually be quickly corrected by the density-dependent limiting factors in the environment.

Population Regulation The logistic growth model Study SmarterFigure 1: The logistic growth model. Source: Wiki Commons

Logistic growth occurs when density-dependent limiting factors cause population growth to gradually slow before reaching a maximum level at which growth will level off and become stable.

Exponential growth occurs when a population's growth rate remains constant, no matter the size, exceeding its carrying capacity.

Most density-dependent limiting factors are biotic. These factors can include intra - and interspecific competition, increased spreading of disease, and parasitism. In prey species, higher population densities may also result in higher predation rates. Individuals from a population that has reached carrying capacity may also wander out in search of new habitat that is not yet at capacity.

Biotic: Biotic factors are those that involve or are produced by living organisms.

Abiotic: Abiotic factors are those that do not involve and are not produced by living organisms.

By the early 1970s, saltwater crocodile (Crocodylus porosus) populations in Australia’s Northern Territory were nearing extinction, with only a few thousand individuals remaining. Thanks to protective efforts, over the following decades, the population recovered to the point that most rivers are believed to have reached carrying capacity, with crocodile populations nearly reaching pre-exploitation levels, leveling off, and becoming stable.

In this case, the density-dependent limiting factors include competition (e.g., finite prey availability and territoriality) and habitat limitations (e.g., breeding habitat and climatic restrictions), which prevent the crocodile population from continued expansion. In many of these rivers, this has resulted in smaller, less dominant males wandering out into the ocean and into suboptimal areas in search of new habitats, oftentimes bringing them into conflict with humans.

Population cycles

Populations experiencing density-dependent limiting factors often experience instability at carrying capacity, even without the effects of density-independent limiting factors. These populations may experience cycles of growth followed by a reduction in size in oscillating patterns called cyclical oscillations. Under specific circumstances, usually involving multiple species, these oscillations are driven by density-dependent limiting factors such as predation and resource abundance.

Density-independent factors in population regulation

Density-independent factors impact the per capita population growth rate regardless of the population’s density. Since these factors do not depend upon the population’s size, their impact does not amount to the “correction” that density-dependent factors bring to a population. In other words, density-independent factors can be potentially catastrophic to smaller populations, particularly populations of a species with a limited geographic range.

Density-independent factors can be abiotic, and perhaps the best example of a density-independent factor would be a natural disaster, such as a forest fire. A natural disaster may kill a significant fraction of the population living in the area, regardless of how large that population was, to begin with.

If the population is limited to only a small area, a single natural disaster could even push a species to extinction.

For example, black bear (Ursus americanus) populations are known to be affected by wildfires by way of decreased cub survival.

Top-down population regulation

Top-down population regulation refers to situations where species at higher trophic levels (e.g., apex predators at the top of the food chain) control the populations of species at lower trophic levels (e.g., prey). Due to this, it is also called "predator-controlled" regulation. Typically, the population size and density of the apex predator at the top of the food chain is much lower than that of its prey, which is usually quite abundant.

Occasionally this may not be the case, as is seen with the American alligator (Alligator mississippiensis) and caiman species, which are apex predators and are often very abundant.

For example, mountain lions (Puma concolor) may control mule deer (Odocoileus hemionus) populations, but the mule deer may control the populations of certain plant species.

Bottom-up population regulation

Bottom-up population regulation is dependent on the resources of an ecosystem. Since all of the higher trophic levels are dependent on the continued presence of those below them, when those resources at the lower level are diminished or absent, all trophic levels are affected.

For example, if vegetation experiences a mass die-off, this may result in a decline in the mule deer population due to starvation. This, in turn, may also result in a reduction in the mountain lion population due to a lack of prey.

Population regulation in humans

Fifty years ago, in 1972, the human population consisted of around 3.9 billion people. Today that number has grown to over 7.9 billion.

Thus, the human population has more than doubled, growing more in the last half-century than in the entirety of human existence (at least 200,000 years).

This exponential growth is largely due to better technology, food availability, medicine, and more, which have allowed humans to artificially increase their carrying capacity.

However, this exponential cannot persist indefinitely, as the methods used to increase our carrying capacity are being outpaced by many density-dependent limiting factors.

For humans, these limiting factors include widespread resource depletion (food, water, gas), climate change, increased spread of disease, and pollution.

Population Regulation Human population growth graph Study SmarterFigure 3: Human population growth since 1800, including future high and low projections. Source: Wiki Commons

Indeed, the consequences of these density-dependent factors have always been present, but their impact will continue to be amplified as the population further exceeds the natural carrying capacity. Density-independent factors also affect human populations, with some notable recent examples including earthquakes, hurricanes, and tsunamis that cause large-scale damage to infrastructure and high mortality. To counteract this growth and mitigate consequences, artificial methods of human population regulation have been proposed, including increased access to contraception, family planning, and increased education.

Population Regulation - Key takeaways

  • Population regulation refers to the ecological processes (Biotic and Abiotic Factors) by which the growth of populations is limited, due to the effects on birth and death rates.
  • There are two different kinds of limiting factors- density-dependent and density-independent limiting factors. Density-dependent limiting factors impact a population’s per capita rate of growth based on the population’s density. Density-independent factors impact the per capita population growth rate regardless of the population’s density.
  • There are two kinds of population regulation: top-down and bottom-up. Top-down population regulation refers to situations where species at higher trophic levels control the populations of species at lower trophic levels. Bottom-up population regulation is dependent on the resources of an ecosystem.
  • Populations may experience cycles of growth followed by a reduction in size in oscillating patterns, called cyclical oscillations.

Frequently Asked Questions about Population Regulation

Top-down population regulation refers to situations where species at higher trophic levels (e.g., apex predators at the top of the food chain) control the populations of species at lower trophic levels (e.g., prey). Due to this, it is also called "predator-controlled" regulation. Bottom-up population regulation is dependent on the resources of an ecosystem. Since all of the higher trophic levels are dependent on the continued presence of those below them, when those resources at the lower level are diminished or absent, all trophic levels are affected.

There are two kinds of population regulation - top-down regulation and bottom-up regulation. Top-down population regulation refers to situations where species at higher trophic levels (e.g., apex predators at the top of the food chain) control the populations of species at lower trophic levels (e.g., prey). Due to this, it is also called "predator-controlled" regulation. Bottom-up population regulation is dependent on the resources of an ecosystem. Since all of the higher trophic levels are dependent on the continued presence of those below them, when those resources at the lower level are diminished or absent, all trophic levels are affected. For humans, density-independent factors also affect human populations, with some notable recent examples including earthquakes, hurricanes, and tsunamis that cause large-scale damage to infrastructure and high mortality. To counteract this growth and mitigate consequences, artificial methods of human population regulation have been proposed, including increased access to contraception, family planning, and increased education.

Population regulation refers to the ecological processes (biotic and abiotic factors) by which the growth of populations is limited due to the effects on birth and death rates.

There are two kinds of population regulation - top-down regulation and bottom-up regulation. Top-down population regulation refers to situations where species at higher trophic levels (e.g., apex predators at the top of the food chain) control the populations of species at lower trophic levels (e.g., prey). Due to this, it is also called "predator-controlled" regulation. Bottom-up population regulation is dependent on the resources of an ecosystem. Since all of the higher trophic levels are dependent on the continued presence of those below them, when those resources at the lower level are diminished or absent, all trophic levels are affected.

Density-dependent limiting factors impact a population’s per capita rate of growth based on the population’s density. These factors will generally cause the growth rate to drop as the population gets larger. Density-dependent limiting factors usually cause populations to reach a maximum level (called the population’s carrying capacity). Density-independent factors impact the per capita population growth rate regardless of the population’s density. Since these factors do not depend upon the population’s size, their impact does not amount to the “correction” that density-dependent factors bring to a population. In other words, density-independent factors can be potentially catastrophic to smaller populations, particularly populations of a species with a limited geographic range.



Final Population Regulation Quiz

Population Regulation Quiz - Teste dein Wissen

Question

True or False: Every living organism on Earth has limits to its population size.

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Answer

True

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Question

The ecological factors that limit population growth are known as...

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Answer

Limiting factors

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Question

The two types of limiting factors are...

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Answer

Density-dependent and density-independent

Show question

Question

Density-dependent limiting factors...

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Answer

Impact a population’s per capita rate of growth dependent on the population’s density.

Show question

Question

Which are examples of density-dependent limiting factors?

Show answer

Answer

Limitations on the food supply within an ecosystem.

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Question

What is logistic growth?

Show answer

Answer

When density-dependent limiting factors cause population growth to gradually slow before reaching a maximum level at which growth will level off and become stable.

Show question

Question

What is exponential growth?

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Answer

When a population's growth rate remains constant, no matter the size, exceeding its carrying capacity.

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Question

Density-independent limiting factors... 

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Answer

Impact a population’s per capita rate of growth independent of the population’s density.

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Top-down population regulation refers to...


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Answer

Situations where species at higher trophic levels control the populations of species at lower trophic levels.

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Question

The cycles of growth followed by reductions in size are called...

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Answer

Cyclical oscillations

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Question

True or False: The human population has a carrying capacity.

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Answer

True

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Question

The human population has _____ in the last half century, which is more than in it has in the previous _____ years.

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Answer

more than doubled; at least 200,000

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What are some of the density-dependent limiting factors affecting the human population?

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Climate change

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Question

If vegetation experiences a mass die-off, this may result in a decline in the mule deer population due to starvation. This, in turn, may also result in a reduction in the mountain lion population due to a lack of prey. This is known as...


Show answer

Answer

Bottom-up population regulation

Show question

Question

Mountain lions may control mule deer populations, but the mule deer may control the populations of certain plant species. This is known as...

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Answer

Top-down population regulation

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Question

True or False: Logistic population growth is rare in nature.

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Answer

False

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Which is true about logistic population growth?

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Answer

The population's growth rate slows as it approaches carrying capacity.

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What are the two types of population growth?

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Logistic

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What are the two types of limiting factors?

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Density-dependent

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What is an example of a mammal species that has experienced exponential growth in its population?

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Humans

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Exponential growth is most often seen when?

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In experimental settings with bacteria

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The most common type of population growth is- 

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Logistic

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A population's largest size, dictated by resource limitations and other limiting factors, is its- 

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Carrying capacity

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Which are examples of density-dependent limiting factors?

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Competition with another species

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Virtually all naturally occurring populations experience...

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Logistic population growth

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Logistic population growth produces a __________ curve.

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S-shaped

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What density-dependent limiting factors prevent American alligator range expansion and further population growth?

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Competition with another crocodilian species

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What is one density-independent limiting factor that prevents American alligator range expansion and further population growth?

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Colder climates

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Logistic population growth occurs when a population's per capita growth rate _________ as its size ________.

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decreases; increases

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What is the equation for logistic population growth?

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Answer

(K-N/K)N

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Limiting factors are referred to as conditions or resources within an environment that _____ population growth. 

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restricts 

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True or false: population growth is the change in size of a population over a certain period of time. 


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True

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The carrying capacity is the  _____ number of individuals of a given species that an environment can support. 

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maximum 

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The ______ of a system is limited by limiting factors. 

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Carrying capacity

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____ factors are nonliving factors in an ecosystem such as temperature, sunlight, nutrients, water, pH, salinity and humidity.


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Abiotic 

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_____ factors are living factors such as competition for resources, predation, and disease. 


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Biotic 

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_________  are mostly biotic factors whose effects in population size depend on population density. 

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Density-dependent factors

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________ competition is the competition for limited resources between individuals of the same species. 

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Intraspecific  

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________ competition is the competition for limited resources between individuals of different species. 

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Interspecific

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Diseases and parasitism are both examples of _____________.

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Density-dependent limiting factors

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The competitive exclusion principle states that: 

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no two species can occupy the same niche

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_____ limiting factors are usually abiotic factors that limit a population size regardless of population density.

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Density-independent 

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True or false: The effects of weather change in the population of aphid insects is an example of density-dependent limiting factor.

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Answer

False

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Test your knowledge with multiple choice flashcards

True or False: Every living organism on Earth has limits to its population size.

The ecological factors that limit population growth are known as...

The two types of limiting factors are...

Next

Flashcards in Population Regulation43

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True or False: Every living organism on Earth has limits to its population size.

True

The ecological factors that limit population growth are known as...

Limiting factors

The two types of limiting factors are...

Density-dependent and density-independent

Density-dependent limiting factors...

Impact a population’s per capita rate of growth dependent on the population’s density.

Which are examples of density-dependent limiting factors?

Limitations on the food supply within an ecosystem.

What is logistic growth?

When density-dependent limiting factors cause population growth to gradually slow before reaching a maximum level at which growth will level off and become stable.

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