Biology · 11th Grade · Ecology and Environmental Dynamics · Weeks 19-27

Community Interactions: Competition and Predation

Explores interspecific and intraspecific competition, predator-prey relationships, and their ecological consequences.

• Common Core State Standards
• HS-LS2-6

About This Topic

Competition and predation are the two most studied interspecific interactions in ecology, and both drive population dynamics and community structure in ways students can trace through real data. Interspecific competition occurs when different species compete for the same limited resource, while intraspecific competition occurs between individuals of the same species and is often more intense because niche overlap is total. The competitive exclusion principle, formalized by Gause using Paramecium cultures, states that two species with identical niches cannot stably coexist, and that ecological differences between species allow diverse communities to persist.

Predator-prey dynamics introduce students to coupled population oscillations. The Lotka-Volterra model and the famous lynx-snowshoe hare data from Canadian trappers show how predator and prey populations track each other with a time lag. This coevolutionary arms race is central to HS-LS2-6 and supports explanations of how population-level interactions ripple through entire communities. Keystone predators can maintain community diversity by preventing any single prey species from competitively excluding others.

Active learning is essential here because the mathematical and conceptual models only become intuitive when students can manipulate them. Running population simulations, analyzing real time-series data, and tracing trophic cascades step by step helps students build a working model of community dynamics.

Key Questions

1. Differentiate between interspecific and intraspecific competition and their effects on populations.
2. Analyze the coevolutionary arms race between predators and prey.
3. Explain how the competitive exclusion principle influences species distribution.

Learning Objectives

• Compare and contrast the mechanisms and impacts of interspecific and intraspecific competition on population size and distribution.
• Analyze the cyclical dynamics of predator and prey populations, explaining the role of coevolutionary adaptations.
• Explain the competitive exclusion principle using Gause's Paramecium experiments and predict its effect on species coexistence.
• Evaluate the role of keystone predators in maintaining community biodiversity by preventing competitive exclusion.
• Synthesize data from population simulation models to illustrate the ripple effects of community interactions on trophic levels.

Before You Start

Population Growth and Limiting Factors

Why: Students need to understand basic population dynamics and the concept of carrying capacity to analyze how competition and predation affect population sizes.

Food Webs and Trophic Levels

Why: Understanding how energy flows through ecosystems is fundamental to grasping predator-prey relationships and their impact on community structure.

Key Vocabulary

Interspecific Competition	Competition for limited resources that occurs between individuals of different species, potentially leading to reduced population growth or exclusion.
Intraspecific Competition	Competition for limited resources that occurs between individuals of the same species, often more intense due to identical resource needs.
Competitive Exclusion Principle	The principle stating that two species competing for the exact same limited resources cannot stably coexist; one species will eventually outcompete and eliminate the other.
Predator-Prey Dynamics	The interaction where one organism (predator) hunts and kills another organism (prey) for food, influencing the population sizes of both.
Coevolution	The process where two or more species reciprocally influence each other's evolution, often seen in predator-prey relationships as adaptations develop in response to each other.
Keystone Species	A species that has a disproportionately large effect on its environment relative to its abundance, often by controlling populations of other species and maintaining community structure.

Watch Out for These Misconceptions

Common Misconception

Predators always reduce prey populations over time until the prey goes extinct.

What to Teach Instead

Predator-prey relationships typically produce oscillating cycles rather than extinction. Predators become less efficient as prey populations shrink, which allows prey to recover and predators to follow. The lynx-hare time series data is the most direct evidence for this dynamic and is far more convincing than a verbal explanation alone.

Common Misconception

Competition between species always results in one species being eliminated.

What to Teach Instead

Competitive exclusion requires nearly identical niches. Species with even modest niche differences can coexist through resource partitioning. The classic example of warblers partitioning the vertical feeding zones of a spruce tree shows students that coexistence is the common outcome when species have sufficiently differentiated niches.

Common Misconception

Intraspecific competition is less important than interspecific competition.

What to Teach Instead

Intraspecific competition is often more intense because individuals of the same species have exactly the same resource requirements with no niche differentiation possible. It is a major driver of density-dependent population limitation. Comparing per-capita growth rate at low versus high population density makes this visible quantitatively.

Active Learning Ideas

Simulation Game: Predator-Prey Population Dynamics

Pairs of students simulate lynx-hare cycles using index cards over 12 rounds, recording population sizes after each round and plotting them on shared graph paper. The class compiles data to produce the characteristic oscillating curves, then discusses what drives the time lag between predator and prey peaks.

45 min·Pairs

Inquiry Circle: Competitive Exclusion vs. Coexistence

Groups analyze a published dataset of barnacle settlement patterns or Paramecium growth curves to determine whether competitive exclusion or niche-based coexistence occurred. They must identify which resource was limiting and whether niche differentiation was sufficient to allow coexistence.

40 min·Small Groups

Think-Pair-Share: Sea Otter Trophic Cascade

Students read a one-page summary of the sea otter-urchin-kelp system in the Pacific Northwest. Pairs must explain the chain of effects that follows sea otter removal and articulate why this qualifies as a trophic cascade rather than a simple two-species interaction.

20 min·Pairs

Gallery Walk: Arms Race Adaptations

Stations display paired photographs of predator-prey arms races: the bombardier beetle and its predators, crypsis in walkingsticks, the death's-head hawkmoth infiltrating bee colonies. Students explain the reciprocal adaptations at each station and predict the likely next evolutionary step in the arms race.

35 min·Small Groups

Real-World Connections

• Wildlife biologists studying wolf and elk populations in Yellowstone National Park observe how predation by wolves influences elk grazing patterns, which in turn affects plant regeneration and overall ecosystem health.
• Marine ecologists investigating tide pools analyze how different species of barnacles and mussels compete for space on rocks, and how the presence of predatory sea stars can prevent competitive exclusion and maintain species diversity.
• Agricultural scientists research pest management strategies, considering how introducing natural predators or competitors can control insect populations and reduce reliance on chemical pesticides.

Assessment Ideas

Quick Check

Present students with two scenarios: one describing competition between two plant species for sunlight, and another describing a fox hunting a rabbit. Ask students to identify the type of interaction (interspecific competition, intraspecific competition, predation) and explain their reasoning in one sentence for each.

Discussion Prompt

Pose the question: 'How might the introduction of an invasive predator species disrupt the established community interactions in a local ecosystem?' Facilitate a discussion where students apply concepts of predator-prey dynamics, competitive exclusion, and coevolution to predict potential outcomes.

Exit Ticket

Ask students to draw a simple graph showing the oscillating population sizes of a hypothetical predator and prey over time. Below the graph, they should write two sentences explaining the relationship between the two population curves and one example of a coevolutionary adaptation one of the species might possess.

Frequently Asked Questions

What is the competitive exclusion principle?

The competitive exclusion principle states that two species with identical ecological niches cannot stably coexist because one will always be a slightly more efficient competitor for the limiting resource. In practice, most coexisting species have enough niche differences to allow resource partitioning, which is why high biodiversity is the norm in real communities rather than the exception.

How do predator and prey populations cycle?

As prey become abundant, predators have more food and reproduce more. The growing predator population then reduces prey numbers. With less prey available, predators decline due to food shortage. With fewer predators, the prey population recovers, and the cycle repeats. The Canadian lynx and snowshoe hare trapping data is the most-cited real-world example of this oscillation.

What is a coevolutionary arms race between predator and prey?

A coevolutionary arms race occurs when an adaptation in one species creates selection pressure on another, which evolves a counter-adaptation that in turn creates new pressure on the first. Milkweed producing toxins and monarchs evolving toxin resistance is a well-studied example. Predator-prey systems commonly follow this pattern, producing escalating adaptations in both offense and defense over evolutionary time.

How can active learning help students understand competition and predation?

Running a predator-prey simulation and graphing the resulting population cycles gives students an empirically grounded picture that diagrams alone cannot match. When students have personally watched a prey population crash and then seen the predator population follow, the Lotka-Volterra model becomes an explanation of something they experienced rather than a formula to memorize for an exam.