Science · 6th Grade · Energy Flow in Ecosystems · Weeks 19-27

Competition and Predation

Students investigate how organisms interact through competition and predation for resources.

• Common Core State Standards
• MS-LS2-2

About This Topic

Competition and predation are two of the most powerful forces shaping population sizes and community structure in ecosystems. Students investigate both intraspecific competition (within a species) and interspecific competition (between species) for limited resources including food, water, shelter, and mates. They also examine predator-prey dynamics, which produce the characteristic oscillating population cycles documented in classic cases like the snowshoe hare and lynx in North America. This content aligns with MS-LS2-2, which requires students to construct explanations for how interactions determine population sizes.

The counterintuitive insight here is that predators can be beneficial for ecosystem health. By controlling herbivore populations, predators prevent overgrazing and maintain habitat structure, which supports biodiversity. This ecological role of predation challenges the common perception that predators are destructive.

Simulation and modeling activities are ideal for this topic because they let students manipulate variables and observe population dynamics in real time, which is far more effective than reading about oscillating curves.

Key Questions

1. Explain in what ways different species compete for or share limited resources.
2. Analyze the role of predation in maintaining population balance within an ecosystem.
3. Predict how an increase in a predator population might affect its prey.

Learning Objectives

• Compare and contrast intraspecific and interspecific competition for resources like food, water, and shelter.
• Analyze the impact of predator-prey relationships on population dynamics, using examples like the snowshoe hare and lynx.
• Predict the cascading effects on an ecosystem when a predator or prey population significantly increases or decreases.
• Explain how predation can maintain biodiversity by preventing any single herbivore species from dominating an area.

Before You Start

Food Chains and Food Webs

Why: Students need to understand the flow of energy through ecosystems and identify producers, consumers, and decomposers before analyzing predator-prey interactions.

Basic Needs of Organisms

Why: Understanding that organisms require resources like food and shelter is fundamental to grasping the concept of competition for those resources.

Key Vocabulary

competition	The struggle between organisms for limited resources such as food, water, shelter, or mates. This can occur within the same species (intraspecific) or between different species (interspecific).
predation	An interaction where one organism, the predator, hunts and kills another organism, the prey, for food. This is a key factor in regulating population sizes.
niche	The specific role an organism plays in its ecosystem, including its habitat, food sources, and interactions with other species. Competition often arises when species share overlapping niches.
population dynamics	The study of how and why populations of organisms change in size and composition over time, influenced by factors like birth rates, death rates, immigration, and emigration.
carrying capacity	The maximum population size of a biological species that can be sustained by that specific environment, considering available resources and environmental conditions.

Watch Out for These Misconceptions

Common Misconception

Students often believe competition always results in one species driving the other to extinction.

What to Teach Instead

Introduce the concept of niche partitioning: species often evolve or adjust behaviors to reduce direct competition rather than fight until one disappears. Darwin's finches and warbler warblers in the same forest feeding at different heights are classic examples. Collaborative analysis of real population data helps students see coexistence as a common outcome.

Common Misconception

Many students see predators as harmful to ecosystems and assume ecosystems would be better off without them.

What to Teach Instead

Use the Yellowstone wolf case or similar documented trophic cascade to show that removing top predators destabilizes ecosystems by allowing prey populations to explode and overgraze. The simulation activity gives students direct experience with how predator removal changes population dynamics.

Common Misconception

Students sometimes think predator and prey populations are independent, with predators simply 'choosing' how many prey to eat.

What to Teach Instead

Predator population size is directly dependent on prey availability. When prey populations crash, predator populations follow. When prey rebound, predators rebound. The lynx-hare data activity makes this interdependence clear through real historical patterns.

Active Learning Ideas

Simulation Game: Predator-Prey Tag

In an open space, designate most students as rabbits and a few as foxes. Rabbits must collect resource tokens (paper clips on the ground) to survive each round. Foxes tag rabbits to 'eat' them. After each round, adjust populations based on the rules (starved rabbits removed, well-fed rabbits reproduce, foxes that ate enough reproduce). Track population sizes per round and graph the oscillation.

40 min·Whole Class

Collaborative Analysis: Lynx and Snowshoe Hare

Provide groups with the historical Hudson's Bay Company population data for lynx and snowshoe hare spanning 90 years. Groups graph the data, identify the oscillating cycles, and develop an explanation for the pattern using cause-and-effect reasoning. Groups then predict what would happen if lynx were removed from the ecosystem.

35 min·Small Groups

Think-Pair-Share: What Is Competition Really About?

Give students two scenarios: two species of sparrow competing for nest sites in the same territory, and two wolves within a pack competing for access to food. Pairs identify the type of competition in each case and discuss what resource is actually limited. Class discussion surfaces the distinction between intraspecific and interspecific competition and explores when competition leads to niche partitioning rather than exclusion.

20 min·Pairs

Real-World Connections

• Wildlife biologists in Yellowstone National Park study the reintroduction of wolves (predators) and their impact on elk populations (prey), observing how this affects vegetation regrowth and overall ecosystem health.
• Farmers and ranchers often manage predator populations, like coyotes or foxes, to protect livestock, balancing the need for protection with the ecological role these predators play in controlling rodent populations.
• Conservationists working to protect endangered species, such as the California condor, must consider the entire food web, including the availability of prey and the impact of competing scavengers.

Assessment Ideas

Quick Check

Provide students with a scenario describing a simple ecosystem (e.g., a forest with deer, wolves, and oak trees). Ask them to identify one example of competition and one example of predation, and explain how each interaction might affect population sizes.

Discussion Prompt

Pose the question: 'Can predators be beneficial to an ecosystem?' Facilitate a class discussion where students use evidence from their learning about population balance and biodiversity to support their arguments, referencing specific predator-prey examples.

Exit Ticket

Ask students to draw a simple food web with at least three organisms. Then, have them write two sentences explaining how competition between two of the organisms might occur and one sentence describing the predator-prey relationship between two others.

Frequently Asked Questions

What is the difference between intraspecific and interspecific competition?

Intraspecific competition occurs between members of the same species competing for the same limited resources, like two robins fighting for the same territory. Interspecific competition occurs between different species using the same resource, like coyotes and red foxes both hunting rabbits. Intraspecific competition is typically more intense because individuals have identical resource needs.

How does predation help maintain balance in an ecosystem?

Predators prevent any one prey species from consuming all available resources by keeping population sizes in check. Without predators, herbivore populations can explode, overgraze vegetation, and eliminate habitat that many other species depend on. This is why top predators are called keystone species: removing them destabilizes the entire community structure below them.

How can active learning help 6th graders understand predator-prey dynamics?

Population dynamics involve time-delayed cause-and-effect relationships that are difficult to understand from a static graph. A simulation where students experience predator and prey population changes in real time, then graph their own data, gives them an embodied understanding of the cycle. Analyzing real data like the lynx-hare record then connects that experience to documented ecological patterns.

Why do predator and prey populations oscillate rather than staying constant?

When prey are abundant, predators thrive and their population grows. More predators means more prey are eaten, so prey populations decline. Fewer prey means predators begin to starve and their population falls. With fewer predators, prey populations recover, and the cycle begins again. This time-lagged feedback loop produces the characteristic oscillating curves seen in real population data.