Biology · 12th Grade · Ecological Interactions · Weeks 28-36

Interspecific Interactions: Competition and Predation

Study the dynamics of competition and predation between different species in a community.

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

About This Topic

When species in a community share resources or interact through consumption, they exert selective pressure on each other that shapes community structure. Interspecific competition, where different species require overlapping resources, is a primary driver of niche partitioning: over time, competing species evolve to use slightly different portions of a resource gradient, reducing direct competition and allowing coexistence. Predation, where one organism consumes another, drives evolutionary arms races between predator and prey lineages over generations. HS-LS2-2 and HS-LS2-6 require students to evaluate the impact of these interactions on ecosystem structure and function.

Real US examples are abundant: character displacement in Appalachian salamander communities, the trophic cascade triggered by gray wolf reintroduction in Yellowstone, and competitive exclusion in Great Lakes fish communities all provide data-rich cases for investigation. These examples help students connect abstract ecological principles to conservation decisions and ecosystem management debates that appear regularly in current news and policy discussions.

Active learning is especially valuable for competition and predation because these concepts are inherently dynamic and interactive. Students who simulate predator-prey cycles or physically model competitive exclusion can reason about the timing, magnitude, and reversibility of ecological interactions in ways that lecture and reading alone do not support.

Key Questions

1. Explain how niche partitioning and competition shape community structure.
2. Analyze the evolutionary adaptations that arise from predator-prey relationships.
3. Predict the outcome of introducing a new predator or competitor into an ecosystem.

Learning Objectives

• Analyze the impact of niche partitioning on the coexistence of competing species within a defined community.
• Evaluate the evolutionary adaptations that have arisen in predator and prey populations due to reciprocal selective pressures.
• Predict the potential consequences for community structure and stability following the introduction of an invasive competitor or predator.
• Compare and contrast the outcomes of competitive exclusion and resource partitioning using case studies.
• Synthesize information from ecological data to explain how interspecific interactions shape biodiversity.

Before You Start

Species Interactions: Symbiosis and Food Webs

Why: Students need a foundational understanding of how species interact and the flow of energy through ecosystems before analyzing more complex dynamics like competition and predation.

Population Dynamics

Why: Understanding population growth, carrying capacity, and limiting factors is essential for grasping how competition and predation affect population sizes and community structure.

Key Vocabulary

Interspecific Competition	A type of interaction where individuals of different species compete for limited resources, such as food, water, or territory.
Niche Partitioning	The process by which competing species evolve to use different resources or use shared resources in different ways, reducing direct competition and allowing them to coexist.
Competitive Exclusion Principle	The concept that two species competing for the exact same limited resources cannot coexist indefinitely; one species will eventually outcompete and eliminate the other.
Predation	An interaction where one organism, the predator, hunts and kills another organism, the prey, for food.
Trophic Cascade	An ecological process that starts at the top of the food chain and tumbles down to lower levels, often initiated by the addition or removal of a top predator.

Watch Out for These Misconceptions

Common Misconception

The strongest competitor always wins and drives all others extinct.

What to Teach Instead

Competitive exclusion occurs only when two species occupy an identical ecological niche with no differentiation. In practice, niche partitioning often allows apparent competitors to coexist by specializing on different parts of a resource gradient. Most real communities show coexistence maintained by niche differences, not competitive elimination of all but one species per resource.

Common Misconception

Predators harm their ecosystems by reducing prey populations.

What to Teach Instead

Predators play a regulatory role that often increases ecosystem health and biodiversity by preventing prey from overconsuming vegetation, allowing multiple prey species to coexist, and facilitating nutrient cycling. The Yellowstone wolf reintroduction is a well-documented example where predator presence increased rather than decreased overall biodiversity and ecosystem function.

Common Misconception

Prey species are passive victims in predator-prey interactions.

What to Teach Instead

Prey species evolve active defenses including cryptic coloration, chemical deterrents, behavioral adaptations, alarm signals, and group defense strategies. The evolutionary arms race between predators and prey means both lineages are under constant reciprocal selection. Students underestimate prey agency until they examine the full diversity of anti-predator adaptations directly.

Active Learning Ideas

Simulation Game: Predator-Prey Population Cycles

Student groups simulate predator and prey populations using cards or beans representing individual organisms. Each round, students calculate survival and reproduction, then adjust population counts. Groups graph population sizes over 10 rounds and identify the phase relationship between predator and prey peaks, comparing their results to Lotka-Volterra model predictions.

45 min·Small Groups

Case Study Analysis: Yellowstone Trophic Cascade

Pairs read a data summary of wolf reintroduction effects in Yellowstone and construct a cause-and-effect diagram tracing how wolf predation on elk affected riparian vegetation, beaver populations, and stream hydrology. Pairs present their diagrams and compare which indirect effects each pair identified, noting any that were overlooked.

35 min·Pairs

Think-Pair-Share: Competitive Exclusion vs. Coexistence

Present data from two species of barnacles occupying different zones on a tidal rock face. Pairs explain which competitive mechanisms are operating, predict what would happen if one species were removed, and design a simple field experiment to test competitive exclusion in this system.

25 min·Pairs

Gallery Walk: Niche Partitioning Examples

Post six stations showing data on niche partitioning in different communities, including MacArthur's warblers, lizard communities, and African savanna grazers. Student groups rotate, identify the resource axis along which each community is partitioned, and classify each as character displacement or ecological niche differentiation.

40 min·Small Groups

Real-World Connections

• Wildlife biologists in national parks, such as Yellowstone, study predator-prey dynamics, like the reintroduction of wolves, to understand how these interactions maintain ecosystem health and biodiversity.
• Conservationists working to protect endangered species often analyze competitive interactions to determine if introduced species are outcompeting native ones for vital resources, impacting recovery efforts.
• Fisheries managers monitor fish populations in the Great Lakes to understand how invasive species, like zebra mussels, compete with native species for food, affecting the entire aquatic food web and local economies.

Assessment Ideas

Discussion Prompt

Present students with a scenario: 'A new, highly efficient herbivore is introduced into a grassland ecosystem with several native herbivore species.' Ask: 'What are two ways this introduction could impact the existing herbivore community? How might niche partitioning or competitive exclusion play a role?'

Quick Check

Provide students with a diagram showing two predator species and their shared prey. Ask them to label the types of interactions occurring (e.g., interspecific competition for prey, predation). Then, ask them to write one sentence explaining a potential evolutionary adaptation one of the predators might develop to reduce competition.

Exit Ticket

On an index card, have students define 'niche partitioning' in their own words and provide one specific example of how it helps species coexist. They should also list one real-world profession that uses this ecological concept.

Frequently Asked Questions

What is competitive exclusion and when does it actually occur?

Competitive exclusion occurs when two species with identical resource requirements occupy the same ecological niche and one consistently outcompetes the other, driving it locally extinct or forcing a niche shift. It is most likely in simple environments with limited niche diversity. In complex natural communities, niche differentiation usually prevents complete exclusion, and coexistence is the more common outcome.

What is a trophic cascade?

A trophic cascade occurs when a change at one level of a food web produces indirect effects that travel through multiple levels. Removing top predators often causes prey populations to explode, overgraze vegetation, and reduce habitat complexity, affecting many other species indirectly. The Yellowstone wolf reintroduction is a well-documented example of a top-down trophic cascade with effects reaching stream hydrology.

How do prey populations avoid being driven to extinction by predators?

Several mechanisms prevent prey depletion. Predator populations decline when prey is scarce, reducing predation pressure and allowing prey recovery. Spatial refuges in dense vegetation, deep water, or underground provide escapes. Behavioral flexibility allows prey to avoid high-predator areas. These mechanisms create the oscillating dynamics described by Lotka-Volterra models rather than steady predator-driven declines.

How does active learning help students understand competition and predation?

Simulating predator-prey cycles and building cause-and-effect diagrams for trophic cascades requires students to trace indirect ecological effects and reason about timing, skills difficult to develop from static diagrams. When students predict a population change, run a simulation, and compare outcomes with their prediction, they practice the evidence-based reasoning required by HS-LS2-2 and HS-LS2-6.