Carrying Capacity and Limiting Factors

Carrying Capacity and Limiting Factors

Middle School

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Learning Objectives

• Define biotic and abiotic factors and their impact on population size.

• Explain how limiting factors determine the carrying capacity of an ecosystem.

• Describe how populations change due to births, deaths, immigration, and emigration.

• Interpret population graphs to identify exponential and logistic growth.

Key Vocabulary

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Ecosystem

All the living and nonliving things that interact with each other in a specific environment.

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Limiting Factor

An environmental factor that causes the growth of a population to decrease or stop growing.

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Carrying Capacity

The greatest number of organisms that a habitat can successfully support over a long time.

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Immigration

The movement of new individuals into a population from other places, causing the population to grow.

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Emigration

The movement of individuals out of a population, which causes the population size to get smaller.

Biotic and Abiotic Factors

• An ecosystem includes all living and nonliving parts of an environment.

• Biotic factors are the living parts, like plants and animals.

• Abiotic factors are nonliving parts, like water, air, and sunlight.

Multiple Choice

Which of the following lists contains only abiotic factors that can affect a population?

• Wind, soil, and microorganisms

• Plants, animals, and fungi

• The number of predators and available food

• Temperature, water, and sunlight

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How Populations Change in Size

Births and Deaths

• A population grows when the birth rate is greater than the death rate.

• A population shrinks when the death rate is greater than the birth rate.

• The balance between births and deaths determines if a population’s size increases or decreases.

Moving In and Out

• Immigration is when individuals join a population, which causes the population to increase.

• Emigration is when individuals leave a population, which causes the population to decrease.

• The movement of individuals into or out of an area also affects a population’s size.

Multiple Choice

A population of squirrels in a park will shrink if...

• Immigration is higher than emigration.

• The death rate is greater than the birth rate.

• The birth rate is higher than the death rate.

• The birth rate is equal to the death rate.

What Are Limiting Factors?

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Biotic Factors

• ​Biotic limiting factors are the living parts of an ecosystem.

• ​​This includes competition for food and the number of predators.

• ​The spread of diseases and parasites also limits population growth.

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Abiotic Factors

• ​Abiotic factors are the nonliving parts of an ecosystem.

• ​​This includes the availability of water, space, and sunlight.

• ​Weather conditions and natural events like fires are also factors.

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Human Activities

• ​Human actions can act as powerful limiting factors for populations.

• ​​Building cities and roads reduces the available natural habitat.

• ​Damming rivers and cutting down forests also limits populations.

Multiple Choice

A sudden wildfire sweeps through a forest. This event is an example of what kind of limiting factor?

• A factor related to competition.

• An abiotic limiting factor.

• A biotic limiting factor.

• A population-dependent factor.

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Predator-Prey Relationship

• The predator-prey relationship creates a delicate balance between the two populations.

• ​When the prey population increases, the predator population also increases.

• As predators eat more prey, the prey population begins to decrease.

• With less available food, the predator population then also decreases.

Multiple Choice

In a predator-prey cycle, what is the direct cause for a decrease in the predator population?

• A decrease in competition.

• An increase in available habitat.

• An increase in the prey population.

• A decrease in the prey population.

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What Is Carrying Capacity?

• Carrying capacity is the largest population an environment can sustain over time.

• It is determined by limiting factors like available food, water, and space.

• If below capacity, the population increases as birth rates are higher.

• If above capacity, the population decreases as death rates are higher.

Multiple Choice

According to the concept of carrying capacity, at what point does a population's size become stable?

• When the birth rate equals the death rate.

• When the population is far below the carrying capacity.

• When the birth rate is much higher than the death rate.

• When the population grows far above the carrying capacity.

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Population Growth Graphs

• Exponential growth is shown by a J-shaped curve on a graph.

• This happens when a population has unlimited resources like food and space.

• Logistic growth is represented by an S-shaped curve on a graph.

• Growth stops when the population reaches the ecosystem's carrying capacity.

Multiple Choice

An 'S-shaped' curve on a population graph represents logistic growth. What does the flat top part of the 'S' represent?

• The point where the population has reached its carrying capacity.

• The beginning of exponential growth in a new environment.

• A period of rapid population growth with no limits.

• A sudden crash in the population due to a disaster.

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Common Misconceptions About Population Growth

Misconception	Correction
Carrying capacity is a fixed number that never changes.	It can change if environmental conditions change.
A population will only grow or only shrink.	Most populations rise and fall around the carrying capacity.
Exponential growth can continue forever.	Limiting factors will always slow a population's growth.
Only living (biotic) factors limit populations.	Nonliving (abiotic) factors like temperature also limit populations.

Multiple Choice

A forest ecosystem experiences a prolonged drought, significantly reducing the amount of available water and edible plants. How will this most likely affect the carrying capacity for a deer population in that forest?

• The deer will immigrate to another forest, leaving the carrying capacity unchanged.

• The carrying capacity will decrease because key abiotic and biotic factors have become scarce.

• The carrying capacity will increase because there will be fewer predators.

• The carrying capacity will remain the same, but the deer population will grow.

Multiple Choice

What is the key difference in the environmental conditions that lead to a J-shaped (exponential) growth curve versus an S-shaped (logistic) growth curve?

• J-shaped curves represent a stable population, while S-shaped curves represent a shrinking one.

• J-shaped curves happen when death rates are high, while S-shaped curves happen when birth rates are high.

• J-shaped curves are caused by high emigration, while S-shaped curves are caused by high immigration.

• J-shaped curves occur in environments with unlimited resources, while S-shaped curves occur when limiting factors restrict growth.

Multiple Choice

A new, fast-spreading disease is introduced into a dense wolf population. Based on your understanding of limiting factors, predict the most likely sequence of events for the wolf and moose (their primary prey) populations.

• The wolf population will decrease due to the disease, leading to a temporary increase in the moose population.

• The moose population will decrease first, causing the wolf population to decrease from starvation.

• The wolf population will increase due to the disease, causing the moose population to decrease.

• Both populations will increase because the disease is a natural biotic factor.

Multiple Choice

A new housing development is built, clearing a large section of a forest that was home to many bird species. Analyze how this human activity acts as a limiting factor and predict its most likely impact on the carrying capacity for those birds.

• It has no effect on the carrying capacity, only on the immediate bird population size.

• It reduces available habitat, food sources, and nesting sites, thus lowering the carrying capacity.

• It increases the carrying capacity by providing new nesting sites on houses.

• It increases food availability by attracting more insects, which raises the carrying capacity.

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Summary

• Biotic and abiotic factors limit population size based on resource availability.

• A population grows or shrinks due to births, deaths, and migration.

• Carrying capacity is the maximum population an ecosystem can support.

• Growth is shown by J-shaped curves, or S-shaped curves near carrying capacity.

Poll

On a scale of 1-4, how confident are you about predicting how a population will change based on limiting factors?

• 1

• 2

• 3

• 4