Analyze ecosystem energy flow and symbiosis.

SECTION 1

Historical Context & Motivation

For most of human history, people understood that animals eat plants and other animals, but they lacked a framework to explain why energy decreases as it passes from one organism to the next. Early naturalists observed that predators were far less common than the prey they fed upon, yet no one could explain this pattern with a unifying scientific idea. The development of ecology as a formal science changed that, giving us tools to map exactly how energy enters, flows through, and eventually leaves an ecosystem.

1789

Lavoisier's Conservation of Mass

Antoine Lavoisier established that matter is neither created nor destroyed in chemical reactions, laying the groundwork for understanding energy transformations in living systems.

1877

De Bary Defines Symbiosis

German biologist Anton de Bary coined the term 'symbiosis' to describe different species living together in close, long-term relationships—whether helpful, harmful, or neutral.

1927

Elton's Food Chains

Charles Elton published 'Animal Ecology,' introducing the concepts of food chains and the pyramid of numbers to explain why top predators are always rare.

1942

Lindeman's 10% Rule

Raymond Lindeman studied energy transfer in a lake ecosystem and proposed that roughly 10% of energy passes from one trophic level to the next, with the rest lost as heat.

1969

Odum's Ecosystem Energetics

Eugene Odum published detailed energy flow diagrams, establishing the modern approach to measuring how energy moves through producers, consumers, and decomposers.

These discoveries converge on a central question that the GED Science exam frequently explores: How does energy flow through an ecosystem, and how do organism relationships influence that flow? Understanding this helps you interpret food webs, energy pyramids, and symbiotic relationships—all common topics on the test.

SECTION 2

Core Principles & Definitions

Every ecosystem runs on energy, and understanding how that energy behaves requires a few foundational ideas. These core principles appear repeatedly on the GED Science exam, especially in questions that ask you to analyze a diagram or explain why an ecosystem changes when one species is removed.

Trophic Levels

Every organism occupies a feeding level. Producers (plants, algae) form the base by converting sunlight into chemical energy through photosynthesis. Primary consumers (herbivores) eat producers. Secondary consumers eat herbivores, and tertiary consumers sit at the top.

The 10% Rule

When one organism eats another, only about 10% of the energy stored in the food is passed to the consumer. The remaining 90% is used for the organism's own life processes (movement, body heat, digestion) or lost as heat to the environment.

Food Webs vs. Food Chains

A food chain is a single linear pathway of energy. A food web is a network of interconnected food chains in an ecosystem. Real ecosystems are always food webs because most organisms eat more than one type of food.

Decomposers

Decomposers (bacteria, fungi) break down dead organisms and waste at every trophic level. They recycle nutrients back into the soil and atmosphere, making those nutrients available to producers again. Without decomposers, ecosystems would run out of essential elements.

Symbiosis

Symbiosis describes close, long-term interactions between two species. The three main types are mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed). These relationships influence how energy and nutrients move through communities.

✦ KEY TAKEAWAY

Think of energy in an ecosystem like money passing through a chain of businesses. A farmer grows $1,000 worth of crops (producer). A grocery store buys the crops but only gets to keep about $100 after paying its own costs (primary consumer). A restaurant buys from the grocery store and keeps only about $10 (secondary consumer). By the time you reach the top, very little of the original 'money' remains. That is exactly how the 10% rule works: each level keeps only a fraction of the energy it receives.

SECTION 3

Energy Pyramid — Visual Explanation

The energy pyramid is one of the most important diagrams you will encounter on the GED Science exam. It shows how energy decreases at each trophic level. The wide base represents producers, which capture the most energy from the sun. Each level above is narrower because roughly 90% of the energy is lost as heat before it reaches the next consumer.

This energy pyramid shows how 10,000 kcal of energy captured by producers is reduced at each trophic level. Only about 10% passes upward—the rest is lost as heat through cellular respiration and life processes.

Notice how the pyramid narrows dramatically. If producers capture 10,000 kcal of energy from sunlight, only about 1,000 kcal reaches herbivores, 100 kcal reaches secondary consumers, and just 10 kcal reaches top predators. This is why ecosystems can support far fewer predators than prey. On the GED, you might be asked to calculate the energy available at a specific level or to explain why removing a trophic level affects the entire ecosystem.

SECTION 4

How Energy Flows Through Ecosystems

Energy enters nearly every ecosystem through photosynthesis, where producers convert light energy from the sun into chemical energy stored in glucose. This process can be summarized with a simple equation.

PHOTOSYNTHESIS

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Carbon dioxide and water are converted into glucose (chemical energy) and oxygen using sunlight. This is the foundation of nearly all food webs on Earth.

Once producers store energy in glucose, that energy moves through the ecosystem when consumers eat producers or other consumers. Every organism uses some of that energy through cellular respiration, which releases the chemical energy in glucose to power life processes. This process also releases heat, which is the primary reason energy is "lost" at each trophic level.

CELLULAR RESPIRATION

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (usable energy) + heat

Glucose is broken down to release ATP (the energy currency of cells). Much of the energy is released as heat, which cannot be reused by other organisms.

ENERGY TRANSFER BETWEEN TROPHIC LEVELS

Energy available at next level = Energy at current level × 0.10

The 10% rule: if producers hold 10,000 kcal, then primary consumers receive approximately 10,000 × 0.10 = 1,000 kcal. This is an approximation—actual transfer efficiency ranges from 5% to 20% depending on the ecosystem.

💡 GED TIP

The GED frequently presents a food web and asks what happens when one species is removed. Remember: energy flows in one direction (from producers upward), but removing any organism affects organisms both above and below it in the web. Also, notice that energy does not cycle—it flows through and leaves the ecosystem as heat. Nutrients (like carbon and nitrogen) do cycle, but energy does not.

SECTION 5

Types of Symbiosis

Energy flow is only part of the picture. Organisms in an ecosystem also form close, lasting relationships with each other. Symbiosis (from the Greek for "living together") refers to a close biological interaction between two different species. These relationships can powerfully influence how energy and nutrients move through an ecosystem. The GED tests three main categories of symbiosis.

The three types of symbiosis differ in how each species is affected. In mutualism (+/+), both benefit. In commensalism (+/0), one benefits while the other is unaffected. In parasitism (+/−), one benefits at the expense of the other.

The diagram above uses the shorthand +/+, +/0, and +/− to represent how each species is affected. On the GED, you may be given a scenario describing two species and asked to identify the type of symbiosis. The key is to determine whether each organism is helped, harmed, or unaffected. Remember that symbiosis is not the same as predation—in predation, the prey is killed and consumed; in parasitism, the host is harmed but typically kept alive because the parasite depends on it.

⚠ WATCH OUT

A common GED mistake is confusing parasitism with predation. A lion killing a zebra is predation, not parasitism. A tapeworm living inside a zebra's gut is parasitism. The difference is that parasites live on or inside their host for an extended time without immediately killing it.

SECTION 6

Worked Example — Energy Pyramid Calculation

Let's walk through a typical GED-style problem step by step. This kind of question gives you information about one trophic level and asks you to calculate the energy available at another level.

📋 SCENARIO

In a grassland ecosystem, producers store 20,000 kcal/m²/year of energy. Using the 10% rule, how much energy is available to secondary consumers (the third trophic level)?

Calculating Energy at the Third Trophic Level

Step 1 — Identify the trophic levelsThe first trophic level is producers (20,000 kcal). The second trophic level is primary consumers (herbivores). The third trophic level is secondary consumers—this is the level we need to find.

Step 2 — Calculate energy at the second levelApply the 10% rule to the producers' energy: 20,000 kcal × 0.10 = 2,000 kcal available to primary consumers.

Primary consumers: 2,000 kcal

Step 3 — Calculate energy at the third levelApply the 10% rule again: 2,000 kcal × 0.10 = 200 kcal available to secondary consumers.

Secondary consumers: 200 kcal

Step 4 — Verify the patternNotice the pattern: 20,000 → 2,000 → 200. Each level has one-tenth of the energy of the level below. If the question asked about tertiary consumers, you would continue: 200 × 0.10 = 20 kcal.

Answer: 200 kcal/m²/year

⚡ SHORTCUT

To jump multiple trophic levels at once, multiply by 0.10 for each step. Going from producers to secondary consumers (2 steps up) means multiplying by 0.10 × 0.10 = 0.01. So 20,000 × 0.01 = 200 kcal. This shortcut saves time on the GED.

SECTION 7

Comparing Energy Flow & Symbiosis Concepts

The GED often tests whether you can distinguish between similar-sounding concepts. The following table highlights common comparisons and distinctions that appear on the exam.

Common GED comparison questions about ecosystem concepts

Concept A	Concept B	Key Difference
Food chain	Food web	A chain is a single linear path; a web is a network of interconnected chains showing all feeding relationships.
Energy flow	Nutrient cycling	Energy flows in one direction (sun → producers → consumers → heat) and is NOT recycled. Nutrients (carbon, nitrogen) cycle repeatedly through ecosystems.
Parasitism	Predation	A parasite lives on/in its host over time without immediately killing it. A predator kills and consumes its prey.
Mutualism	Commensalism	In mutualism, both species benefit. In commensalism, only one benefits while the other is neither helped nor harmed.
Producer	Decomposer	Producers make energy from sunlight. Decomposers break down dead matter and recycle nutrients back into the soil.

✦ KEY TAKEAWAY

The single most important distinction for the GED is that energy flows one way and does not recycle, while nutrients cycle. Think of energy like a waterfall—it only goes downhill and cannot flow back up. Nutrients are more like a conveyor belt that loops around, delivering materials back to the starting point.

SECTION 8

Connections to Broader Ecology

The ideas in this lesson connect to several larger ecological topics that appear on the GED, including biodiversity, population dynamics, and human impacts on ecosystems. Understanding energy flow and symbiosis gives you the foundation to analyze these more complex scenarios.

How energy flow and symbiosis connect to broader GED topics

This Lesson's Concept	Broader Application
The 10% rule limits energy at top levels	Explains why ecosystems support fewer predators and why large predator populations are more vulnerable to extinction
Food webs show interconnected feeding	Removing one species (through pollution, hunting, or habitat loss) creates cascading effects throughout the entire web
Mutualism between pollinators and plants	Declining bee populations threaten crop production, illustrating how symbiosis has direct economic consequences for humans
Decomposers recycle nutrients	Connects to biogeochemical cycles (carbon, nitrogen, water) — a related GED topic about how matter cycles through Earth systems

On the GED, you may encounter short-answer questions that ask you to explain how a change—like the introduction of an invasive species or the removal of a predator—affects an ecosystem. Use the energy flow and symbiosis concepts from this lesson to build your response. Strong answers reference specific trophic levels, name the type of relationship involved, and explain the chain of effects step by step.

📝 SHORT-ANSWER STRATEGY

When writing a short-answer response about ecosystems, follow this structure: (1) State which organisms are directly affected. (2) Identify the trophic level or symbiotic relationship involved. (3) Explain what happens to organisms above and below in the food web. (4) Support your reasoning with evidence from the passage or data provided.

SECTION 9

Practice Problems

A researcher studies a coral reef and observes clownfish living among the stinging tentacles of sea anemones. The clownfish are protected from predators by the anemone's stings, and the anemone benefits from nutrients in the clownfish's waste. Which type of symbiotic relationship does this describe?

In a pond ecosystem, algae (producers) store 50,000 kcal/m²/year. Small fish (primary consumers) eat the algae, and larger fish (secondary consumers) eat the small fish. Using the 10% rule, approximately how much energy is available to the larger fish?

A scientist studying a forest food web discovers that a disease has wiped out most of the rabbit population. Rabbits are primary consumers that eat grasses and are prey for foxes (secondary consumers). Based on your understanding of energy flow, which of the following is the most likely short-term effect on the ecosystem?

PROBLEM 4 — APPLIED

Read the following scenario and write a response of 3–7 sentences. A marine biologist observes that when sea otters were removed from a kelp forest ecosystem by hunting, the populations of sea urchins (which otters eat) exploded. The urchins then consumed most of the kelp, collapsing the underwater forest. Many fish species that depended on the kelp for shelter disappeared. Using your knowledge of food webs and energy flow, explain why the removal of a single predator caused such widespread changes in this ecosystem. Include at least one reference to trophic levels in your answer.

PROBLEM 5 — CRITICAL THINKING

Study the data table below and write a response of 3–7 sentences. A research team measured the energy stored at each trophic level in two different ecosystems. | Trophic Level | Forest Ecosystem (kcal/m²/year) | Aquatic Ecosystem (kcal/m²/year) | |---|---|---| | Producers | 40,000 | 8,000 | | Primary Consumers | 4,000 | 1,200 | | Secondary Consumers | 500 | 180 | | Tertiary Consumers | 50 | 18 | Compare the energy transfer efficiency between trophic levels in the two ecosystems. Do both ecosystems follow the 10% rule exactly? Cite specific data from the table to support your analysis and propose one possible reason for any differences you find.

SUMMARY

Lesson Summary

Energy enters ecosystems through photosynthesis in producers and flows upward through trophic levels — from primary consumers to secondary consumers to tertiary consumers. The 10% rule tells us that roughly 90% of energy is lost as heat at each level, which is why energy pyramids are wide at the base and narrow at the top. Decomposers recycle nutrients (but not energy) back into the ecosystem. Remember: energy flows in one direction and does not cycle.

Symbiosis describes close, long-term relationships between two species: mutualism (+/+) where both benefit, commensalism (+/0) where one benefits and the other is unaffected, and parasitism (+/−) where one benefits and the other is harmed. On the GED, identify symbiosis by asking: Is each organism helped, harmed, or unaffected? These energy flow and symbiosis concepts appear in multiple-choice questions, data interpretation tasks, and short-answer responses across the Life Science portion of the exam.