Biology · Ecology and Evolution

Predator–Prey Dynamics

When one population changes, another often responds.

Predator and prey populations are connected through feeding relationships. Their populations can rise and fall in repeating patterns because changes in one population affect the other.

🌱Food
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🐇Prey
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🦊Predator
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What Is Predator–Prey Dynamics?

Predator–prey dynamics describe the changes that occur in predator and prey populations as they interact over time.

Predators depend on prey for energy. Prey are affected by predation. As prey become more abundant, predators may have more food available. As predator populations increase, predation pressure can cause prey populations to decrease.

Big idea: Predator and prey populations can influence one another, creating population cycles over time.

Read the Data

Reading a Predator–Prey Graph

A classic predator–prey graph shows the populations of both organisms over time. The populations often rise and fall in cycles, with the predator population following the prey population.

Predator–Prey Population Cycle
Illustrative model showing the typical relationship between the two populations
Population Size Time Prey Predator
The exact shape and timing vary between ecosystems. The important pattern is that predator population changes generally follow changes in prey population.

Notice that the peaks are not perfectly aligned. The predator population lags behind the prey population.

The Cycle

The Predator–Prey Cycle

1
Prey increase. More food and favorable conditions can allow the prey population to grow.
2
Predators increase. More prey provides more food, allowing predator survival and reproduction to increase.
3
Prey decrease. A larger predator population increases predation pressure.
4
Predators decrease. With fewer prey available, predators have less food and their population can decline.
5
The cycle can begin again. Reduced predation may allow the prey population to recover.
A Critical Concept

Why Do Predator Populations Lag Behind?

Predators do not instantly respond to a change in prey abundance. They need time to find food, grow, reproduce, and increase their population.

This creates a time lag. The prey population may already be declining by the time the predator population reaches its peak.

When reading the graph, look for the predator peak occurring after the prey peak.

Real Ecosystems

Factors That Change the Cycle

Real ecosystems are more complicated than a simple predator–prey model. Many factors can influence population sizes.

Food Availability

Changes in the food available to prey can affect prey population growth.

Competition

Other organisms may compete with prey or predators for limited resources.

Disease

Disease can reduce populations independently of predation.

Environmental Change

Temperature, drought, habitat changes, and other conditions can alter population dynamics.

Population Ecology

Carrying Capacity and Prey

A prey population cannot increase indefinitely. Resources such as food, water, shelter, and space can limit population growth.

The carrying capacity is the largest population size that an environment can support over time under particular conditions. Predation is one factor that can influence population size, along with many other limiting factors.

Watch Out

Common Misconceptions

“Predator and prey populations peak at exactly the same time.”
Predator populations typically lag behind prey populations.
“More prey always means more predators immediately.”
Predator populations respond over time because reproduction and population growth take time.
“Predators are the only factor controlling prey populations.”
Food, disease, competition, habitat, climate, and other factors also affect populations.
“A predator–prey graph represents every ecosystem exactly.”
The graph is a model. Real population dynamics can be more complicated.
Test-Day Connections

If You See This on a Test, Think This

“Prey population increases.”
Predators may increase after a time lag.
“Predator population increases.”
Predation pressure on prey may increase.
“Prey population decreases.”
Predators may later decrease because less food is available.
“Why are the peaks offset?”
Predator populations respond with a time lag.
“What else can affect the populations?”
Resources, competition, disease, habitat, climate, and other factors.
Check Your Understanding

Quick Practice

1. What is predator–prey dynamics?
Answer: The changes in predator and prey populations as they interact over time.
2. What usually happens after the prey population increases?
Answer: The predator population may increase after a time lag because more prey are available.
3. Why does the predator population eventually decrease?
Answer: As prey become less abundant, predators have less food available.
4. What does “time lag” mean in a predator–prey graph?
Answer: The predator population changes after the prey population changes rather than at exactly the same time.
5. Name two factors besides predation that can affect population size.
Answer: Examples include food availability, competition, disease, habitat, and environmental conditions.
Remember These

Key Takeaways

  • Predators depend on prey for energy.
  • Predator and prey populations can cycle over time.
  • Predator population changes generally lag behind prey population changes.
  • More prey can provide more food for predators.
  • More predators can increase predation pressure on prey.
  • Real ecosystems are influenced by many factors besides predation.
  • Predator–prey graphs are models of population dynamics, not exact predictions of every ecosystem.
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