Biology · 9th Grade · Ecology and Global Systems · Weeks 28-36

Community Interactions: Competition and Predation

Studying the dynamics of competition, predation, and herbivory within ecological communities.

• Common Core State Standards
• HS-LS2-6
• HS-LS2-2

About This Topic

Within ecological communities, organisms interact through competition, predation, and herbivory in ways that continuously shape population sizes and community structure. In 9th grade biology, students examine how the competitive exclusion principle predicts that two species occupying identical ecological niches cannot coexist indefinitely, with one eventually outcompeting the other. In practice, resource partitioning and character displacement allow similar species to coexist by dividing available resources across time, space, or food type. This content aligns with HS-LS2-6 and HS-LS2-2.

Predation drives some of the most visible evolutionary dynamics in nature. Prey species evolve defensive adaptations such as camouflage, toxins, and speed, while predators evolve corresponding offensive traits. This coevolutionary arms race produces the remarkable diversity of adaptive strategies observed across communities. Distinguishing between interspecific competition (between species) and intraspecific competition (within a species) helps students understand which competitive pressures are most intense and why they drive natural selection.

Active learning brings competition and predation to life through simulations and case studies. When students analyze predator-prey cycle data from real US ecosystems or compete for limited resources in a chip simulation, they develop an intuitive grasp of how these interactions stabilize or destabilize community dynamics.

Key Questions

1. Explain how the competitive exclusion principle shapes community diversity.
2. Analyze how defensive adaptations in prey influence predator evolution.
3. Differentiate between interspecific and intraspecific competition.

Learning Objectives

• Analyze data sets to compare the population dynamics of predator and prey species in a specific US ecosystem.
• Explain the mechanisms by which resource partitioning allows for the coexistence of competing species.
• Differentiate between interspecific and intraspecific competition, providing examples of each.
• Evaluate the impact of specific prey defensive adaptations on predator success and evolution.
• Predict the outcome of competition between two species given their resource needs and niche overlap.

Before You Start

Introduction to Ecosystems and Biotic Factors

Why: Students need a foundational understanding of what an ecosystem is and the role of living organisms within it before studying their interactions.

Population Growth and Limiting Factors

Why: Understanding how populations grow and are limited by resources is essential for grasping the concept of competition.

Key Vocabulary

Competitive Exclusion Principle	The ecological rule stating that two species competing for the exact same limited resources cannot coexist indefinitely; one species will eventually outcompete the other.
Niche Partitioning	The process by which competing species use the environment differently in a way that helps them to coexist. This can involve using different resources, habitats, or times of activity.
Predation	An interaction where one organism, the predator, hunts and kills another organism, the prey, for food.
Coevolution	The process where two or more species reciprocally affect each other's evolution. This is often seen in predator-prey relationships, where prey develop defenses and predators develop ways to overcome them.
Interspecific Competition	Competition for resources that occurs between individuals of different species.
Intraspecific Competition	Competition for resources that occurs between individuals of the same species.

Watch Out for These Misconceptions

Common Misconception

Predators always reduce prey populations toward extinction.

What to Teach Instead

Predators and prey typically stabilize in dynamic population cycles. When prey is scarce, predator numbers decline, allowing prey to recover, which then supports predator recovery. Analyzing real predator-prey oscillation data helps students see that natural predation regulates rather than eliminates prey populations, and that ecosystem stability actually depends on this interaction.

Common Misconception

Competition always results in one species winning and the other going extinct.

What to Teach Instead

Competitive exclusion only applies when two species occupy exactly the same niche in the same location with no alternative resources. In practice, coexisting species almost always partition resources through behavioral differences, microhabitat preferences, or activity timing. Running a competitive exclusion simulation and then modifying it with resource partitioning lets students observe both outcomes directly.

Common Misconception

Defensive adaptations are always physical structures like shells or spines.

What to Teach Instead

Many prey species rely on behavioral defenses such as alarm calls, group mobbing of predators, and feigning death, rather than physical structures. Sorting a set of adaptation examples into physical, chemical, and behavioral categories during a gallery walk broadens students' view of the full range of strategies that natural selection produces.

Active Learning Ideas

Graphing Lab: Predator-Prey Population Cycles

Using Hudson Bay Company records of lynx and snowshoe hare pelts (a classic proxy for population size), students graph both populations on the same axes and analyze the lagged oscillations. They identify which population leads the cycle, explain the cause-and-effect relationship that sustains the cycle, and predict what happens if a secondary prey source for lynx is added.

45 min·Pairs

Simulation Game: Competitive Exclusion Experiment

Based on G.F. Gause's paramecium experiments, two 'species' of students compete for the same resource chips with slightly different success rates. After 10 rounds, students graph population sizes, observe competitive exclusion, and then modify the experiment by adding a second resource type to simulate resource partitioning and test whether coexistence becomes possible.

40 min·Small Groups

Gallery Walk: Coevolution in Action

Create stations featuring paired coevolved predator-prey relationships (cheetah and Thomson's gazelle speed, monarch butterfly and milkweed cardenolide toxins, mimic octopus and predators). At each station, students identify the evolutionary pressure, list one defensive and one offensive adaptation the relationship has produced, and note how removing one species would affect the other's evolution.

35 min·Small Groups

Think-Pair-Share: Classifying Competition

Students receive six competition scenarios and classify each as interspecific or intraspecific. They explain to a partner why intraspecific competition is generally more intense (identical resource requirements) and discuss one example from a US ecosystem where intraspecific competition has shaped observable behavior or morphology.

20 min·Pairs

Real-World Connections

• Wildlife biologists in Yellowstone National Park study wolf-elk dynamics to understand how predator reintroduction impacts ecosystem balance and vegetation recovery.
• Conservationists managing invasive species like the zebra mussel in the Great Lakes use knowledge of competition to predict how native mussels are affected and to develop control strategies.
• Agricultural scientists research pest management by studying the competitive interactions between crop plants and weeds, as well as the predator-prey relationships between pests and beneficial insects.

Assessment Ideas

Quick Check

Provide students with two hypothetical species descriptions, including their food sources and habitat preferences. Ask them to write a short paragraph predicting whether these species would compete, and if so, whether they might coexist based on niche partitioning.

Discussion Prompt

Pose the question: 'Imagine a new predator is introduced into a forest ecosystem with established prey populations. What are two specific adaptations the prey might evolve over time, and how would these adaptations then influence the predator's own evolution?' Facilitate a class discussion where students share their ideas.

Exit Ticket

On an index card, have students define 'interspecific competition' and 'intraspecific competition' in their own words. Then, ask them to provide one real-world example for each type of competition they might observe in a local park or backyard.

Frequently Asked Questions

What is the competitive exclusion principle?

The competitive exclusion principle states that two species occupying the same ecological niche in the same location cannot coexist indefinitely. The species even slightly more efficient at using the shared resource will eventually outcompete and eliminate the other. In practice, coexisting species almost always show some degree of resource partitioning that reduces direct competition enough for both to persist.

How do defensive adaptations in prey influence predator evolution?

Prey and predator evolution are locked in a coevolutionary arms race. When prey evolve a defense (such as a toxin), predators that can tolerate or circumvent it are more successful and pass those traits to offspring. The improved predator then selects for prey with stronger defenses, continuing the cycle. Over time, this produces increasingly specialized adaptations on both sides of the relationship.

What is the difference between interspecific and intraspecific competition?

Interspecific competition occurs between different species competing for overlapping resources. Intraspecific competition occurs between members of the same species competing for the exact same resources. Intraspecific competition is typically more intense because members of the same species have identical resource needs, leaving no possibility for the niche partitioning that softens interspecific rivalry.

How does active learning help students understand competition and predation?

Physical simulations are especially effective for community interaction topics. When students compete for resource chips with slightly different success rates, they experience competitive exclusion as an emergent outcome of the rules rather than a stated principle to memorize. Adding resource partitioning to the same simulation and watching coexistence emerge gives students a direct, manipulable model they can apply to new ecological scenarios.