Biology · Grade 12 · Population Dynamics and Ecology · Term 4

Interspecific Interactions: Competition and Predation

Students examine the ecological consequences of competition and predation, including competitive exclusion, resource partitioning, and predator-prey dynamics.

• Ontario Curriculum Expectations
• HS-LS2-2

About This Topic

Interspecific interactions such as competition and predation drive population dynamics and community structure. Grade 12 students investigate competitive exclusion, where similar species cannot coexist indefinitely on the same limiting resource, and resource partitioning, where species evolve to use resources differently and reduce overlap. They also study predator-prey dynamics, including cyclical fluctuations modeled by Lotka-Volterra equations, co-evolutionary arms races, and the role of keystone species in preventing trophic cascades.

This topic anchors the ecology unit by connecting individual interactions to ecosystem-level effects. Students analyze real data, such as lynx-hare cycles from Hudson Bay Company records, and case studies like sea otter-kelp forest systems. These explorations build skills in quantitative analysis, graphing oscillations, and predicting outcomes from species removal, aligning with HS-LS2-2 standards on interpreting relationships.

Active learning benefits this topic because hands-on simulations and collaborative data modeling reveal complex dynamics that lectures alone cannot convey. When students role-play predator-prey chases or partition resources in group scenarios, they test hypotheses, observe emergent patterns, and link math models to tangible ecological principles.

Key Questions

1. How does the loss of a keystone species trigger a trophic cascade?
2. In what ways does niche partitioning reduce competition between overlapping species?
3. Analyze the co-evolutionary arms race between predators and prey.

Learning Objectives

• Compare the ecological outcomes of competitive exclusion and resource partitioning in simulated ecosystems.
• Explain the cyclical dynamics of predator-prey populations using graphical representations and mathematical models.
• Analyze the ripple effects of removing a keystone species on community structure and trophic levels.
• Evaluate the co-evolutionary adaptations that arise from predator-prey interactions.
• Predict the impact of altered interspecific interactions on ecosystem stability.

Before You Start

Food Webs and Trophic Levels

Why: Students need to understand the flow of energy through ecosystems and the roles of producers, consumers, and decomposers to grasp predation and its effects.

Population Growth Models (Exponential and Logistic)

Why: Understanding how populations grow and are limited provides a foundation for analyzing how interspecific interactions further regulate population sizes.

Key Vocabulary

Competitive Exclusion Principle	States that two species competing for the exact same limited resources cannot coexist indefinitely in the same ecological niche. One species will eventually outcompete the other.
Resource Partitioning	The division of limited resources by species that coexist by using them in different ways, at different times, or in different locations. This reduces direct competition.
Keystone Species	A species that has a disproportionately large effect on its environment relative to its abundance. Its removal can cause significant changes in community structure.
Trophic Cascade	An ecological process that starts at the top of the food chain and tumbles down to lower levels. Often triggered by the removal or addition of a top predator or keystone species.
Co-evolution	The influence of closely associated species on each other in their evolution. This is often seen in predator-prey relationships, leading to an 'arms race' of adaptations.

Watch Out for These Misconceptions

Common Misconception

Competition always results in one species going extinct.

What to Teach Instead

Resource partitioning allows coexistence through trait divergence; card-sorting activities let students manipulate resource use visually, compare scenarios, and see how overlap reduction promotes stability.

Common Misconception

Predator populations directly track prey abundance without lags or cycles.

What to Teach Instead

Dynamics produce oscillations due to time delays in reproduction; graphing simulations in small groups help students plot generations, observe booms and crashes, and refine predictions through iteration.

Common Misconception

Keystone species effects stay within their trophic level.

What to Teach Instead

Impacts cascade through food webs; case study discussions with flow diagrams enable students to trace multi-level changes collaboratively, challenging linear thinking.

Active Learning Ideas

Simulation Game: Predator-Prey Cycles

Scatter paper 'prey' on the floor; 'predator' students collect as many as possible in 1 minute, then switch roles for multiple generations. Vary predator numbers and record data. Graph population trends to identify cycles.

45 min·Small Groups

Case Study Analysis: Yellowstone Wolves

Provide data sets and videos on wolf reintroduction effects on elk, vegetation, and beavers. In groups, map the trophic cascade and predict changes if wolves were removed. Share findings in a class jigsaw.

50 min·Small Groups

Placemat Activity: Resource Partitioning Cards

Distribute cards representing food resources and species traits. Pairs assign traits to niches, first modeling full competition leading to exclusion, then partitioning to show coexistence. Discuss adaptations.

30 min·Pairs

Graphing: Lynx-Hare Data

Provide historical population graphs. Individually plot new data points from tables, then in pairs analyze cycle phases and predict next peaks. Connect to Lotka-Volterra.

35 min·Pairs

Real-World Connections

• Wildlife biologists in Algonquin Provincial Park use population surveys and GPS tracking to study how wolves, as a keystone predator, influence deer and moose populations, which in turn affects vegetation growth.
• Fisheries managers in the Great Lakes analyze predator-prey dynamics, like the relationship between alewife and salmon, to set stocking quotas and fishing limits, aiming to maintain a balanced ecosystem and valuable commercial fishery.
• Conservationists studying the intertidal zones of the Pacific Northwest observe how the sea star Pisaster ochraceus, a keystone species, prevents mussels from dominating, thereby maintaining biodiversity in the rocky shore community.

Assessment Ideas

Quick Check

Provide students with two species profiles that share a similar food source. Ask them to predict whether competitive exclusion or resource partitioning is more likely to occur, and to justify their answer using specific traits of the species and their environment.

Discussion Prompt

Pose the question: 'Imagine a new predator is introduced into an ecosystem with established predator-prey cycles. What are two potential consequences for the existing populations and overall community structure?' Facilitate a class discussion where students share their predictions and reasoning.

Exit Ticket

On an index card, have students draw a simple food web showing a keystone species. Then, ask them to draw an arrow indicating the removal of that species and write one sentence describing a likely trophic cascade that would result.

Frequently Asked Questions

What is competitive exclusion in ecology?

Competitive exclusion occurs when two species compete for the exact same limiting resource, resulting in one species' local extinction. Students learn this principle through Gause's experiments with paramecia. In Ontario Grade 12 Biology, it explains niche differentiation: without partitioning, the species with even a slight advantage dominates. Activities like resource card sorts make this abstract idea concrete by simulating outcomes.

How do trophic cascades work?

Trophic cascades propagate effects across food web levels when a keystone predator is removed or added, altering multiple species abundances. For example, Yellowstone's wolf reintroduction reduced elk numbers, allowing vegetation recovery and benefiting beavers. Students analyze these chains quantitatively, graphing before-and-after data to quantify impacts and predict ecosystem responses.

What is niche partitioning?

Niche partitioning reduces competition by species specializing on different resources, times, or spaces within overlapping habitats. Examples include Darwin's finches with varied beak sizes. In class, students model this with trait assignments, observing how it stabilizes populations compared to full overlap, fostering understanding of coexistence mechanisms.

How can active learning help students grasp interspecific interactions?

Active strategies like predator-prey simulations and trophic cascade case studies engage Grade 12 students kinesthetically and collaboratively. They manipulate variables, collect data, and graph real-time outcomes, making cycles and cascades observable. This builds prediction skills, counters misconceptions through peer debate, and connects math models to ecology, deepening retention over passive methods.