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Food Chains &
Food Webs

Explore how energy flows through ecosystems, from producers to top predators, and discover how interconnected feeding relationships shape ecological communities.

AP Environmental Science

What Is a Food Chain?

A food chain is a linear sequence that shows the transfer of energy and nutrients from one organism to the next. Each step in the chain is called a trophic level.

Grass Grasshopper Frog Snake Producer Primary Consumer Secondary Consumer Tertiary Consumer

Understanding Trophic Levels

Level 1: Producers
Autotrophs (plants, algae, cyanobacteria) that convert sunlight into chemical energy via photosynthesis.
Level 2: Primary Consumers
Herbivores that feed directly on producers: deer, rabbits, zooplankton.
Level 3: Secondary Consumers
Carnivores or omnivores that eat primary consumers: frogs, small fish, spiders.
Level 4+: Tertiary Consumers
Top predators at the apex of the chain: hawks, sharks, wolves. Also called apex predators.

Why Do Food Chains Matter?

Understanding food chains is essential because they reveal the pathways of energy transfer that sustain all life. Disruption at any trophic level can trigger cascading effects throughout the ecosystem.

1
Energy budgets: Only about \(10\%\) of energy is transferred from one trophic level to the next, limiting the length of food chains.
2
Population regulation: Predator-prey relationships control population sizes and maintain balance.
3
Bioaccumulation: Toxins like DDT and mercury concentrate as they move up the food chain, reaching dangerous levels in top predators.

The 10% Rule of Energy Transfer

At each trophic level, approximately only \(10\%\) of the energy is passed on to the next level. The remaining \(90\%\) is lost as metabolic heat, used in cellular respiration, or lost as waste.

Producers: 10,000 kcal Primary Consumers: 1,000 kcal Secondary Consumers: 100 kcal Tertiary: 10 kcal 10% → 10% → 10% → Ecological Pyramid of Energy

Calculating Energy Transfer

You can calculate the energy available at any trophic level using the formula:

Example: If producers store 20,000 kcal, how much energy is available to a tertiary consumer (\(n = 3\))?

Food Chain vs. Food Web

Feature Food Chain Food Web
Structure Linear, single pathway Interconnected network of chains
Complexity Simple Complex
Realism Simplified model Closer to actual ecosystems
Stability Disruption easily collapses it More resilient due to alternative pathways
Organisms Each has one food source Each may have multiple food sources

A Simple Food Web

Grass Seeds Berries Rabbit Mouse Sparrow Fox Snake Hawk

Locate decomposers in many pathways rather than one final trophic tier.

Use this illustration to explore the concept described in its caption.

A shared grassland cutaway shows grass, a grasshopper, a bird, roots, litter and fungi. Exact arrows route living-grass and herbivore death or waste into litter, then to decomposers; dashed nutrient return is distinct from food-energy transfer.
The detrital web transfers chemical energy from dead organic matter to decomposers and consumers. Nutrients return to biological use, while energy ultimately dissipates as heat.

Keystone Species and Trophic Cascades

A keystone species has a disproportionately large effect on its ecosystem relative to its abundance. Removing a keystone species triggers a trophic cascade: a chain reaction of population changes across multiple trophic levels.

1
Wolves removed from Yellowstone: elk populations exploded.
2
Overgrazing by elk destroyed willow and aspen trees along rivers.
3
Riverbank erosion increased, reducing beaver habitat and altering stream channels.
4
Wolf reintroduction reversed these effects: a top-down trophic cascade.

An otter’s feeding link changes an entire underwater forest

An otter floats above a lush kelp forest beside a conceptual urchin-dominated rocky state with sparse kelp.In this kelp–urchin–otter food web, otter feeding can limit urchin grazing and indirectly favor kelp. Other environmental conditions and interactions also influence kelp-forest outcomes.

Direct feeding can cause indirect effects

Direct feeding can cause indirect effects. In this kelp–urchin–otter food web, otter feeding can limit urchin grazing and indirectly favor kelp. Other environmental conditions and interactions also influence kelp-forest outcomes.In this kelp–urchin–otter food web, otter feeding can limit urchin grazing and indirectly favor kelp. Other environmental conditions and interactions also influence kelp-forest outcomes.

How to Analyze a Food Web

1
Identify all producers: Find the organisms at the base that make their own food (plants, algae).
2
Follow the arrows: Arrows show the direction of energy flow (from prey to predator).
3
Classify trophic levels: Label each organism as producer, primary consumer, secondary consumer, or tertiary consumer.
4
Identify omnivores: Organisms that feed at multiple trophic levels (e.g., a bear eating berries and fish).
5
Predict removal effects: If a species is removed, trace all arrows to predict population increases or decreases.

Example: Energy Calculation

Problem: A grassland ecosystem's producers fix 50,000 kcal of energy per year. How much energy is available to a secondary consumer?

1
Identify trophic level: Secondary consumer is at \(n = 2\) (two levels above producers).
2
Apply the 10% rule: $$E_2 = 50{,}000 \times (0.10)^2$$
3
Calculate: $$E_2 = 50{,}000 \times 0.01 = 500 \text{ kcal}$$

Example: Predicting Removal Effects

Phytoplankton Zooplankton Small Fish Large Fish ✕

Question: If small fish are removed from this marine food web, what happens?

Example: Bioaccumulation of Toxins

Biomagnification occurs when toxins become more concentrated at higher trophic levels. DDT in an aquatic food chain is a classic example:

Water 0.003 ppm Algae 0.04 ppm Small Fish 0.5 ppm Large Fish 2.0 ppm Osprey 25.0 ppm Concentration increases ~10x per level Total magnification from water to osprey: ~8,300x

Bottom-Up vs. Top-Down Control

Bottom-Up Control
Nutrient availability and producer abundance regulate the ecosystem from the base. More sunlight and nutrients = more producers = more consumers at every level.
Top-Down Control
Predators at the top regulate populations below them. Removing a top predator causes prey populations to spike, which can overgraze producers (trophic cascade).

Common Mistakes to Avoid

✕ Arrow Direction
Arrows show energy flow (prey to predator), not "who eats whom." The arrow points toward the consumer, not away from it.
✕ Confusing Levels
An omnivore (e.g., a bear) can occupy multiple trophic levels. Don't assume each organism sits at only one level.
✕ Forgetting Decomposers
Decomposers are part of every food web. They recycle nutrients but are often omitted from diagrams. The AP exam may test this.

Key Concepts Summary

Concept Definition AP Exam Tip
Food Chain Linear energy pathway Simplified model: know limitations
Food Web Interconnected feeding network More realistic: predict cascading effects
10% Rule \(\sim 10\%\) energy transferred per level Use \(E_n = E_0 \times (0.10)^n\)
Trophic Cascade Multi-level population shifts Classic example: wolves in Yellowstone
Biomagnification Toxin concentration increases up chain Top predators most affected
Keystone Species Disproportionate ecological impact Removal triggers trophic cascade

Question 1 of 5

In a food chain, arrows represent the direction of:

Question 2 of 5

If producers in an ecosystem fix 80,000 kcal of energy, approximately how much energy is available to a tertiary consumer?

Question 3 of 5

In a food web, the removal of a keystone predator would most likely result in:

Question 4 of 5

Biomagnification of DDT in an aquatic food chain means that the highest concentrations of DDT would be found in:

Question 5 of 5

Which of the following is the most likely immediate result of removing minnows?