IB DP Environmental Systems & Societies

Energy & Biomass
in Ecosystems

Subtopic 2.2 — New Syllabus (First Assessment 2026)
Standard Level + Higher Level

⚡
21 SL Syllabus Points
2.2.1 – 2.2.21
🔬
7 HL Extensions
2.2.23 – 2.2.29
Overview

What You Need to Know

This subtopic examines how energy flows and matter cycles through ecosystems, why energy transfer is inefficient, and how human activities disrupt these natural processes.

Guiding Questions

Content Map

A. Energy Fundamentals
Laws of thermodynamics, photosynthesis, respiration
B. Trophic Transfer
Food chains, webs, pyramids, productivity
C. Human Impacts
Bioaccumulation, pollution, human activities
Time allocation: minimum 6 hours for this subtopic. The exam often tests energy flow calculations and bioaccumulation chains.
2.2.1

Ecosystems Are Sustained by Energy & Matter

Key UnderstandingEcosystems are open systems — they continuously exchange both energy and matter with their surroundings. Energy flows through ecosystems (one direction), while matter is recycled within them.
☀️ Solar
Energy
→
🌿 Producers
(Autotrophs)
→
🐾 Consumers
(Herbivores → Carnivores)
→
🍄 Decomposers
& Detritivores

Energy Flow

Enters as sunlight → converted to chemical energy by producers → passed through consumers → exits as heat. Energy is not recycled — a constant supply from the sun is required.

Matter Cycling

Elements (C, N, P, etc.) cycle through biotic and abiotic components via biogeochemical cycles. Decomposers break down dead organic matter, returning nutrients to the soil/atmosphere for reuse.

If asked why ecosystems need a constant energy input but not a constant matter input — energy is lost as heat at each transfer, but matter is recycled.
2.2.2

The First Law of Thermodynamics

Key UnderstandingEnergy can be transformed from one form to another but cannot be created or destroyed. The total energy in a closed system remains constant.
In ecosystems Solar Energy → Chemical Energy → Kinetic Energy + Heat

Application to Ecosystems

The first law is why you can never have an ecosystem that recycles energy. You always need the sun. This contrasts with matter, which cycles via biogeochemical cycles.
2.2.3

Photosynthesis & Cellular Respiration

Key UnderstandingThese two complementary processes transform energy and matter in ecosystems, forming the foundation of all energy flow.

🌿 Photosynthesis

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

Location: Chloroplasts (in chlorophyll)
Input: CO₂, water, sunlight
Output: Glucose + Oxygen
Transformation: Light energy → chemical energy

🔬 Cellular Respiration

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

Location: Mitochondria
Input: Glucose + Oxygen
Output: CO₂ + Water + ATP
Transformation: Chemical energy → usable energy + heat

The CyclePhotosynthesis produces the glucose and O₂ that respiration needs. Respiration produces the CO₂ and H₂O that photosynthesis needs. Together they close the matter cycle while driving energy flow.
2.2.4

Photosynthesis: Light Energy → Biomass

Key UnderstandingPhotosynthesis converts light energy to chemical energy (glucose). Some glucose is stored as biomass by autotrophs — this is the energy available to the rest of the food chain.

What Happens to the Glucose?

  • Immediate use: Glucose is broken down via respiration for the plant's own metabolic needs
  • Storage: Converted to starch for later energy use
  • Biomass formation: Converted to cellulose, proteins, lipids → structural components of the plant
Biomass = the stored energy and organic matter in living (or recently living) organisms. It is what gets passed along to the next trophic level when an organism is eaten.
Biomass is not the same as "weight." It specifically refers to the organic content (excluding water) — this is why we dry samples before measuring it.
2.2.5

Producers: The First Trophic Level

Key UnderstandingProducers (autotrophs) form the first trophic level in every food chain. They are the only organisms that introduce new energy into the ecosystem.

Types of Producers

TypeHabitatExamples
Terrestrial plantsLand ecosystemsGrasses, trees, shrubs, mosses
AlgaeFreshwater & marineDiatoms, kelp, seaweed
Photosynthetic bacteriaVarious (incl. extreme environments)Cyanobacteria, purple sulfur bacteria

Why Producers Are Essential

2.2.6

Cellular Respiration: Releasing Energy

Key UnderstandingCellular respiration releases energy from glucose by converting it into ATP (adenosine triphosphate) — a chemical form that can easily be used in carrying out active processes within living cells.

What is ATP?

ATP is the "energy currency" of cells. It powers:

• Muscle contraction — movement
• Cell division — growth & repair
• Active transport — moving molecules across membranes
• Biosynthesis — building proteins, DNA
• Nerve impulses — signal transmission
• Reproduction — gamete production
Simplified equation Glucose + Oxygen → CO₂ + Water + ATP (+ heat)
Every organism that performs cellular respiration — plants included — loses some energy as heat. This is why plants also contribute (slightly) to heat loss in ecosystems.
2.2.7

Energy Transformed to Heat

Key UnderstandingEnergy transformations are inefficient. In living cells, cellular respiration transforms chemical energy into work, but a large portion is lost as heat energy to the surroundings.

Heat released during respiration cannot be converted back into chemical energy by living things — it is lost from the biological system forever.

2.2.8

The Second Law of Thermodynamics

Key UnderstandingWhen energy is transformed, the amount of usable energy decreases as entropy (disorder) increases. No energy transformation is 100% efficient.
The second law explains why food chains rarely have more than 4 or 5 trophic levels — so much energy is lost as heat at each step that there isn't enough left to support a viable population of higher-level predators.
2.2.9

Consumers: Diverse Energy Strategies

Key UnderstandingConsumers gain chemical energy from carbon (organic) compounds obtained from other organisms. They have diverse strategies for obtaining these energy-containing compounds.
StrategyDescriptionExamples
HerbivoryFeed on living plants/algaeDeer, rabbits, caterpillars, zooplankton
PredationHunt and kill other animalsLions, eagles, spiders, sharks
ParasitismLive on/in host, extract nutrientsTapeworms, fleas, mistletoe
DetritivoryFeed on dead organic matter (detritus)Earthworms, millipedes, woodlice
SaprotrophySecrete enzymes onto dead matter, absorb nutrientsFungi, decomposer bacteria
ScavengingFeed on already-dead animalsVultures, hyenas, crabs
In exam questions, being able to classify an organism's feeding strategy is often required. "Detritivore" and "saprotroph" are commonly confused — detritivores ingest dead matter; saprotrophs absorb externally digested nutrients.
2.2.10

Food Chains: Producers to Consumers

Key UnderstandingProducers make their own carbon compounds by photosynthesis → they start food chains. Consumers obtain carbon compounds from producers or other consumers → they form subsequent trophic levels.
Forest Food Web Energy Flow
Example food chain Grass → Grasshopper → Frog → Snake → Hawk

Reading Food Chains

  • Arrows show the direction of energy flow (not "who eats whom" — though that's implied)
  • The arrow points from the food → to the eater
  • Each organism feeds at a specific trophic level

Trophic Levels

  • Level 1: Producers (grass)
  • Level 2: Primary consumers (herbivore)
  • Level 3: Secondary consumer (carnivore)
  • Level 4: Tertiary consumer (top carnivore)
Exam skill: Given data, construct a food chain and label all trophic levels. Arrows always point in the direction of energy transfer.
2.2.11

Carbon Compounds & Trophic Levels

Key UnderstandingCarbon compounds and the energy they contain are passed from one organism to the next in a food chain. The stages in a food chain are called trophic levels.

What Moves Between Trophic Levels?

It's not just "energy" in the abstract — it is carbon compounds (organic molecules like glucose, proteins, fats) that carry both matter AND energy. When a rabbit eats grass, it gains carbon-based molecules that the grass built from CO₂.

Role of Decomposers

2.2.12

Energy Losses Along Food Chains

Key UnderstandingThere are losses of energy and organic matter as food is transferred along a food chain. Not all energy available at one level reaches the next.
Trophic Energy Flow & 10% Rule Pyramid

Where Does Energy Go?

Stage of LossMechanism
1. Not eatenSome organisms/parts are never consumed (thorns, bones, roots)
2. Not digestedIndigestible material passes through as faeces (cellulose, shells)
3. Not assimilatedSome absorbed energy used immediately for metabolism, not stored as biomass
4. Respiration heat lossCellular respiration converts energy to ATP + heat; heat is lost permanently
Net result: Only about 10% of the energy stored as biomass at one trophic level is converted to biomass at the next level. The rest is lost through the mechanisms above.
2.2.13

Gross Productivity & Net Productivity

Key UnderstandingGross productivity (GP) is the total gain in biomass. Net productivity (NP) is what remains after losses due to cellular respiration.
The Formula NP = GP − R
Net Productivity = Gross Productivity minus Respiration losses

Gross Productivity (GP)

  • Total biomass produced by an organism/trophic level over a given time
  • For producers: total photosynthesis (all glucose made)
  • For consumers: total energy from all food eaten

Net Productivity (NP)

  • Biomass remaining after the organism uses some for respiration
  • Available for growth, reproduction, and consumption by the next level
  • This is what's available to herbivores/carnivores

Worked Example

A plant captures 100 units of energy via photosynthesis (GP = 100). It uses 40 units for respiration (R = 40).
NP = 100 − 40 = 60 units → available for growth and for herbivores to consume.

2.2.14

Limited Trophic Levels

Key UnderstandingThe number of trophic levels in ecosystems is limited due to cumulative energy losses at each level.

Why Food Chains Are Short

With ~10% transfer efficiency, energy diminishes rapidly:

Level 1 (Producers): 100,000 kJ
Level 2: 10,000 kJ
Level 3: 1,000 kJ
Level 4: 100 kJ
Level 5: 10 kJ ← barely enough
Level 6: 1 kJ ← cannot sustain a population
Maximum: 4–5 trophic levels
🔺
Fewer organisms at each level
Implication for top predators: Apex predators (eagles, sharks, lions) are always rare — there simply isn't enough energy at the top of the pyramid to support large populations. This also makes them vulnerable to ecosystem disturbance.
1,350 kg of corn fed directly to humans supports ~22 people. Fed to cattle first (as beef), the same amount supports only ~1 person. This illustrates the energetic cost of eating at higher trophic levels.
2.2.15

Food Webs: Real-World Complexity

Key UnderstandingFood webs show the complexity of trophic relationships in communities. Organisms rarely feed on just one species — they participate in multiple food chains simultaneously.

Food Chains vs Food Webs

  • Chain: Linear, simple, one path (grass → rabbit → fox)
  • Web: Interconnected, realistic, many overlapping paths
  • Web shows that most organisms feed at multiple trophic levels (omnivores)

Why Food Webs Matter

  • Stability: More connections = more alternative pathways = more resilience
  • Impact prediction: Removal of one species has cascading effects through the web
  • Biodiversity indicator: Complex webs = healthy, diverse ecosystems
Key concept: If a species in a food web is removed, energy can flow through alternative pathways — this is why biodiversity confers ecosystem stability. Loss of a keystone species, however, can collapse entire sections of the web.
2.2.16

Measuring Biomass

Key UnderstandingBiomass of a trophic level can be measured by collecting and drying samples. The dry mass approximates the stored energy available at that trophic level.

Method: Collection & Drying

  1. Collect representative samples of organisms from the ecosystem
  2. Dry samples in an oven at constant temperature until stable weight is reached
  3. Weigh the dry mass (removes all water content)
  4. Extrapolate from sample to estimate total biomass of the trophic level

Measuring Energy Content: Calorimetry

  1. Burn dried sample in a calorimeter
  2. Heat released warms a known mass of water
  3. Measure temperature change → calculate energy content
  4. Extrapolate to estimate total energy in the trophic level's biomass
For exams: know that we use dry mass (not wet mass) because water content varies enormously between organisms and doesn't represent stored energy.
2.2.17

Ecological Pyramids

Key UnderstandingEcological pyramids are graphical representations of relative numbers, biomass, or energy at each trophic level in an ecosystem.

Pyramid of Numbers

Individual organisms per level
Top predator
Secondary C.
Primary C.
Producers

Can be inverted (e.g., one tree → many insects)

Pyramid of Biomass

Total dry mass per level
Tertiary
Secondary
Primary
Producers

Usually upright; can be temporarily inverted in aquatic systems

Pyramid of Energy

Energy flow per unit time
Tertiary
Secondary
Primary
Producers

Always upright — the most reliable pyramid type

The energy pyramid is ALWAYS upright because of the second law of thermodynamics. The numbers and biomass pyramids can occasionally be inverted (e.g., one large tree supporting many insects).
2.2.18

Bioaccumulation & Biomagnification

Key UnderstandingNon-biodegradable pollutants (PCB, DDT, mercury) cause changes to ecosystems through bioaccumulation (buildup in individuals) and biomagnification (increasing concentration up the food chain).

Bioaccumulation

Pollutant concentration increases within an individual organism over time because the organism absorbs it faster than it can excrete or metabolize it.

Example: A fish swimming in water with low DDT concentrations absorbs DDT throughout its life → tissue concentration builds up.

Biomagnification

Pollutant concentration increases at each trophic level because predators consume many contaminated prey, concentrating the pollutant.

Example: A bird eating 100 contaminated fish accumulates the DDT from all 100 fish.

Classic Example: DDT

Water (0.000003 ppm) → Plankton (0.04 ppm) → Small fish (0.5 ppm) → Large fish (2 ppm) → Eagle (25 ppm)

DDT caused eggshell thinning in raptors (eagles, pelicans), leading to reproductive failure. This was famously documented by Rachel Carson in Silent Spring (1962).

2.2.19

Microplastics & Pollutant Transmission

Key UnderstandingNon-biodegradable pollutants are absorbed within microplastics, which increases their transmission in the food chain.

How Microplastics Amplify Pollutant Transfer

  • Microplastics (plastic particles <5mm) absorb and concentrate hydrophobic pollutants like PCBs, DDT, and heavy metals from surrounding water
  • Organisms mistake microplastics for food (zooplankton, filter feeders, fish)
  • Pollutants desorb from the plastic inside the organism's gut → absorbed into tissues
  • Microplastics act as "Trojan horses" — carrying concentrated pollutants into the food chain
  • Because microplastics are virtually indestructible, they persist and transmit pollutants through multiple trophic levels
Key consequence: Even areas with low ambient pollution can have organisms contaminated by pollutants delivered via microplastics. This is a global, not local, problem — microplastics have been found in the deepest ocean trenches and in Arctic ice.
2.2.20

Human Activities & Energy/Matter Flows

Key UnderstandingHuman activities — such as burning fossil fuels, deforestation, urbanization, and agriculture — have impacts on flows of energy and transfers of matter in ecosystems.
ActivityImpact on Energy FlowImpact on Matter Transfer
Burning fossil fuelsReleases stored solar energy (from ancient producers) as heatReleases ancient carbon → increased atmospheric CO₂
DeforestationReduces photosynthetic capacity → less energy capturedReduces carbon fixation; releases stored carbon when burned/decays
UrbanizationCreates impervious surfaces → alters local energy budgetsSeals soil → disrupts nutrient cycling and decomposition
AgricultureSimplifies food chains → reduces energy flow pathwaysFertilizer inputs disrupt N and P cycles; monocultures reduce matter cycling
When answering exam questions on human impacts, always specify which flow (energy or matter) is affected and how the natural cycle is disrupted.
2.2.21 KEY TERM

Autotrophs vs Heterotrophs

Key UnderstandingAutotrophs synthesize carbon compounds from inorganic sources (CO₂). Heterotrophs obtain carbon compounds from other organisms.

Autotrophs ("self-feeders")

  • Produce own organic compounds from CO₂ + H₂O
  • Use light energy (photoautotrophs) or chemical energy (chemoautotrophs)
  • Examples: Plants, algae, cyanobacteria
  • Form the base of all food chains

Heterotrophs ("other-feeders")

  • Cannot fix carbon from CO₂ — must consume other organisms
  • Include all consumers and decomposers
  • Examples: Animals, fungi, most bacteria
  • Obtain carbon compounds by eating, absorbing, or parasitizing
2.2.22 KEY TERM

Photoautotrophs vs Chemoautotrophs

Key UnderstandingPhotoautotrophs use light as an external energy source. Chemoautotrophs use exothermic chemical reactions as an energy source in ecosystems with little or no light.

Photoautotrophs

  • Drive carbon fixation using solar energy (photosynthesis)
  • Contain pigments like chlorophyll to capture light
  • Examples: Terrestrial plants, green algae, kelp
  • Dominant producers in sunlit environments

Chemoautotrophs

  • Fix carbon using energy from oxidation of inorganic molecules
  • Synthesize organic matter without any solar radiation
  • Examples: Deep-sea vent bacteria, nitrifying bacteria
  • Foundation of deep ocean floor and cave ecosystems
HL — 2.2.23

Primary ProductivityHL

Key UnderstandingPrimary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements.

Gross Primary Productivity (GPP)

The total rate of biomass production by producers through photosynthesis. This includes all glucose made — before any is used for respiration.

GPP = Total photosynthesis per unit time

Net Primary Productivity (NPP)

The rate of biomass production remaining after producers' own respiration. This is what's available to consumers.

NPP = GPP − Rautotroph
NPP is the key metric — it represents the energy available to all heterotrophs in the ecosystem. Higher NPP = more food available = more consumers supported.
HL — 2.2.24

Secondary ProductivityHL

Key UnderstandingSecondary productivity is the gain in biomass by consumers using carbon compounds absorbed and assimilated from ingested food.

Assimilation Efficiency

Not all food eaten is assimilated. Secondary productivity depends on:

  • Ingestion: Total food consumed
  • Assimilation: What is absorbed (minus faeces)
  • Respiration: Energy used for metabolism (minus heat loss)
  • NP (consumer) = Assimilated food − Respiration

Example

A bird eats 100g of seeds. It assimilates 80g (20g lost as faeces). It uses 60g for respiration. Secondary NP = 80 − 60 = 20g of new biomass (available for growth, reproduction, and predators).

HL — 2.2.25

NPP: The Basis for Food ChainsHL

Key UnderstandingNet primary productivity is the basis for food chains because it is the quantity of carbon compounds sustainably available to primary consumers.

Why NPP and Not GPP?

  • GPP includes glucose that the plant uses for its own respiration — that energy is not available to herbivores
  • NPP = what's left over after the plant's metabolic needs → this is the biomass available for consumption
  • If herbivores consumed more than NPP, they would be depleting the producer's biomass, reducing future productivity
Sustainable consumption means that consumers can only harvest biomass up to the NPP level without degrading the producer population. Exceeding NPP leads to ecosystem degradation.
This connects to the concept of Maximum Sustainable Yield (next slide) — it's the same principle applied to fisheries and forestry management.
HL — 2.2.26 & 2.2.27

Maximum Sustainable YieldsHL

Key Understanding (2.2.26)Maximum sustainable yields (MSYs) are the net primary or net secondary productivity of a system — the maximum that can be harvested without depleting the resource.
Key Understanding (2.2.27)Sustainable yields are higher for lower trophic levels. It is more energy-efficient to harvest producers directly than to raise consumers.

MSY Applied to Fisheries

MSY = the highest catch that can be taken indefinitely without causing population decline. Exceeding MSY leads to fishery collapse.

🐟🐟🐟🐟🐟

Eating fish (Level 3)

Low NPP available
Small sustainable yield

🌾🌾🌾🌾🌾🌾🌾🌾🌾🌾

Eating plants (Level 1)

High NPP available
Large sustainable yield

This is why plant-based diets are more energy-efficient — less energy is lost between trophic levels, so more people can be fed from the same land area.
HL — 2.2.28

Ecological EfficiencyHL

Key UnderstandingEcological efficiency is the percentage of energy received by one trophic level that is passed on to the next level.
Formula Ecological Efficiency = (Energy at Level n+1 ÷ Energy at Level n) × 100%

Worked Example

Producers receive 20,000 kJ of solar energy and convert it to 2,500 kJ of biomass.
Primary consumers receive 2,500 kJ of this as food.

Ecological Efficiency = (2,500 ÷ 20,000) × 100% = 12.5%

Typical range: 1–20% depending on ecosystem and species. The "10% rule" is an average approximation.

Exam questions may ask you to calculate ecological efficiency from given data. Always check which units are being compared (kJ of sunlight vs kJ of biomass).
HL — 2.2.29

Entropy & Biomass FlowHL

Key UnderstandingThe second law of thermodynamics shows how the entropy (disorder) of a system increases as biomass passes through ecosystems.

Entropy in Ecosystems

  • Entropy is a measure of disorder or randomness in a system
  • When sunlight (ordered electromagnetic radiation) is converted to chemical energy (glucose) and then to heat (disordered molecular motion), entropy increases
  • Each trophic transfer further increases entropy — energy becomes less concentrated, less organized, less useful
  • The total entropy of the universe always increases — this is the essence of the second law
Connection to ecosystem structure: This is why ecosystems have a pyramid shape — at each level, energy becomes more dispersed (higher entropy), so less energy is available to support biomass at the next level.

The Big Picture

Ecosystems are dissipative structures — they maintain internal order (low entropy) by continuously importing low-entropy energy (sunlight) and exporting high-entropy energy (heat). When the energy input stops, the system collapses.

Key Terms Glossary

Essential Vocabulary

TermDefinition
AutotrophOrganism that synthesizes its own carbon compounds from inorganic sources (CO₂)
HeterotrophOrganism that obtains carbon compounds from other organisms
BiomassStored organic matter (dry mass) in living or recently living organisms
Trophic levelA feeding position/stage in a food chain
Food chainLinear sequence of organisms through which energy and matter transfer
Food webNetwork of interconnected food chains in a community
Gross productivity (GP)Total biomass produced per unit time (before respiration losses)
Net productivity (NP)Biomass remaining after respiration: NP = GP − R
Ecological efficiency% of energy at one trophic level passed to the next
BioaccumulationBuildup of pollutant in an individual organism over time
BiomagnificationIncreasing pollutant concentration at successive trophic levels
MicroplasticsPlastic particles <5mm that absorb and transport pollutants
Maximum sustainable yieldMaximum harvestable biomass without depleting the resource
EntropyMeasure of disorder; increases with each energy transfer
Exam Preparation

Key Takeaways & Exam Tips

Must-Know Calculations

NP = GP − R

Calculate net productivity from gross and respiration values

Efficiency = (next level ÷ current level) × 100%

Calculate ecological efficiency between trophic levels

Common Exam Patterns

  • Construct food chains/webs from data → label trophic levels, identify producers/consumers
  • Calculate energy transfer between levels → apply the ~10% rule with real data
  • Bioaccumulation chains → show increasing concentration at each level
  • Explain energy losses → use second law of thermodynamics, heat loss, respiration
  • Compare GP vs NP → know the formula and what each represents
  • Human impact analysis → identify which flows (energy/matter) are disrupted and how
For 9-mark structured essays: Always use specific examples, link to the laws of thermodynamics, and consider both the energy and matter aspects of any ecosystem process. Show understanding, not just knowledge.

You've covered all 29 syllabus points ✅

2.2.1 – 2.2.21 (SL) + 2.2.23 – 2.2.29 (HL)