Limiting Factors

Essential Question

What controls how large a population can get?

Rabbits can reproduce quickly, yet rabbit populations do not fill the whole meadow. Something holds them back. In this lesson, you’ll learn what limits population size, how crowding changes the effect of some limits, and why carrying capacity can change. By the end, you should be able to use data to explain what limits a population.

Limiting Factors

• A limiting factor keeps a population from growing larger
• Resources: food, water, space, shelter
• Other limits: predators, disease, competition, harsh weather
• The scarcest resource sets the limit

A limiting factor is anything that keeps a population from growing larger. Some are resources that organisms need, such as food, water, space, and shelter. Others are pressures on the population, such as predators, disease, competition, and harsh weather. When a resource runs short, it limits the population even if the other resources are plentiful, so the scarcest resource often sets the limit. Together, limiting factors determine the carrying capacity (K), the largest population the environment can support over time.

Density-Dependent and Density-Independent Factors

• Density: how crowded a population is
• Density-dependent: effect grows with crowding
• Examples: competition, disease, predation
• Density-independent: effect does not depend on crowding
• Examples: wildfire, flood, drought, severe cold

Limiting factors fall into two groups. A density-dependent factor has a stronger effect when the population is more crowded. Disease spreads more easily among crowded animals, and crowded individuals compete harder for food. A density-independent factor affects the population no matter how crowded it is. A wildfire or a severe cold snap can harm a small population and a large population alike. Density-dependent factors tend to slow growth as a population nears its carrying capacity, which is why they help explain the S-shaped graph from the last lesson.

Stable and Changing Conditions

• In stable conditions, population size stays near K
• Numbers rise and fall a little around K
• A change in a limiting factor changes K
• The population then shifts toward the new K

While conditions stay stable, interactions among organisms and their environment keep a population’s size fairly steady. It rises a little when resources are plentiful and falls a little when they are scarce, staying near its carrying capacity. Carrying capacity is not fixed. If a limiting factor changes, such as a drought that reduces the food supply, the carrying capacity changes too, and the population shifts toward the new, lower level. If conditions change a lot or for a long time, the types and numbers of organisms in the ecosystem can change as well.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Fish in a Pond

A pond produces 12,000 kg of algae each year. Each fish in the pond needs 40 kg of algae per year, and food is the only limiting factor. Estimate the carrying capacity for the fish. Then find the new carrying capacity after a drought reduces the algae supply by 25%.

Show the solution

1. Carrying capacity: 12,000 kg of algae per year ÷ 40 kg per fish per year = 300 fish
2. Algae after the drought: 12,000 kg × 0.75 = 9,000 kg per year
3. New carrying capacity: 9,000 kg ÷ 40 kg per fish = 225 fish
4. Change: 300 − 225 = 75 fewer fish
5. Reasoning: a smaller food supply supports a smaller population, so K went down.
6. Answer: K = 300 fish; after the drought, K = 225 fish.

Example 2: Crowding and Reproduction

Researchers measured how many offspring each female produced in populations of different densities (sample data). Describe the pattern. How many offspring per female were produced at 40 individuals per hectare? By what percent did offspring per female fall between the lowest and highest density? Is crowding a density-dependent or density-independent factor here?

Show the solution

1. Pattern: as density increases, offspring per female decrease.
2. At 40 per hectare: 4.0 offspring per female.
3. Percent decrease: (6.0 − 1.0) ÷ 6.0 × 100 = 83%
4. Classify: density-dependent, because the effect on reproduction changes with how crowded the population is.
5. Reasoning: in a crowded population, individuals compete for limited resources, so fewer offspring are produced. This slows population growth.
6. Answer: Offspring per female decrease as density increases; 4.0 at 40 per hectare; 83% decrease; density-dependent.

Example 3: A Drought and Carrying Capacity

The graph shows the size of a deer population over 20 years (sample data). A drought began in year 11. Use the graph to write a claim, evidence, and reasoning statement about how the drought changed the population. Include mean population sizes for years 1 to 10 and years 11 to 20.

Show the solution

1. Mean for years 1 to 10: the ten values add to 4,000, so the mean is 4,000 ÷ 10 = 400 deer.
2. Mean for years 11 to 20: the ten values add to 2,400, so the mean is 2,400 ÷ 10 = 240 deer.
3. Percent change: (400 − 240) ÷ 400 × 100 = 40% lower
4. Claim: the drought lowered the carrying capacity, so the population fell to a smaller size.
5. Evidence: before the drought the population stayed between 380 and 420 deer, with a mean of 400. After the drought began, it dropped to 290 in year 11 and the mean for years 11 to 20 was 240, which is 40% lower.
6. Reasoning: in stable conditions, the population stays near K. A drought reduces food and water, which are limiting factors, so the environment supports fewer deer and the population shifts to a new, lower level.
7. Answer: The drought lowered K; the mean fell from 400 to 240 deer, a 40% decrease.

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Practice

Sort limiting factors into density-dependent and density-independent, estimate carrying capacity from a resource supply, and use data to explain what limits a population. Try it on your own first, then check your work with the answer key.

• Limiting Factors Practice Worksheet (PDF)
• Limiting Factors Practice Answer Key (PDF)

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Lesson Slides

• 2.4 Limiting Factors Slides (PDF)

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Big Ideas

• A limiting factor keeps a population from growing larger
• Density-dependent factors get stronger as a population gets crowded
• Density-independent factors act regardless of crowding
• Carrying capacity depends on the supply of limiting resources
• If a limiting factor changes, the population shifts toward a new carrying capacity

Back to the essential question: resources and other limiting factors control how large a population can get. Density-dependent factors, such as competition and disease, become stronger as a population gets crowded, which holds the population near its carrying capacity. If conditions change, the carrying capacity changes and the population shifts to match. Next, you’ll compare how different species reproduce.

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