Photo: Jacob W. Frank, National Park Service, 2017, public domain, via Wikimedia Commons.
In 1963, biologists counted just 417 nesting pairs of bald eagles left in the entire lower 48 United States, down from an estimated population in the hundreds of thousands before European settlement.
The pesticide blamed for the collapse, DDT, was never sprayed anywhere near an eagle's nest. So how did it reach the birds at all?
It concentrated as it moved up the food chain. DDT sprayed on farmland washed into rivers and lakes, where it was absorbed by algae in tiny concentrations. Small fish ate large numbers of algae, and larger fish ate large numbers of small fish, each step re-concentrating the DDT residue already stored in the organisms below it. Bald eagles, fish-eating apex predators, ended up with the highest concentration of all, stored in their fat tissue as a DDT breakdown product called DDE.
DDE disrupted the calcium metabolism the eagles needed to build strong eggshells. Shells became so thin they cracked under the parent's own weight during incubation. This entire mechanism, a substance becoming more concentrated at each successive trophic level, is called biomagnification, and it depends directly on how energy and matter move (and fail to move efficiently) through food chains, exactly what this subtopic covers.
- DDT came into widespread use after the mid-1940s; by 1963, the lower-48 bald eagle population had fallen to just 417 known nesting pairs.
- DDE, a persistent DDT breakdown product, accumulated in the eagles' fatty tissue and disrupted eggshell formation, causing shells to thin and break during incubation.
- The United States banned DDT in 1972; the Endangered Species Act, passed in 1973, gave the recovery effort legal force.
- The bald eagle was removed from the US endangered and threatened species list on 28 June 2007. By a 2024 federal report, the lower-48 population had recovered to more than 71,400 nesting pairs and an estimated 316,700 individual birds.
Sources: U.S. Fish and Wildlife Service, Bald Eagle species profile and 2024 nesting population report; American Bird Conservancy, "The Bald Eagle: The Ultimate Endangered Species Act Success Story."
Photo: USDA Forest Service, Pacific Northwest Region, 1955, public domain, via Wikimedia Commons.
Ecosystems run on energy and matter
Ecosystems are sustained by continuous supplies of energy and matter. As established in 1.2, ecosystems are open systems: energy and matter are constantly exchanged with the surrounding environment, most obviously as sunlight entering and heat leaving.
The first law of thermodynamics
The first law of thermodynamics states that energy cannot be created or destroyed, only transformed from one form into another, as it flows through an ecosystem. Light energy becomes chemical energy through photosynthesis; chemical energy becomes heat through respiration. The total amount of energy is always conserved, even as its useful form changes.
This is the same first law introduced in 1.2 as a general property of energy flows through any system. Here it is applied specifically to ecosystems: light to chemical, chemical to heat, over and over, at every trophic level.
Photosynthesis and respiration transform energy and matter
Two processes account for almost every transformation of energy and matter in an ecosystem: photosynthesis and cellular respiration.
Photosynthesis
Photosynthesis is the conversion of light energy into chemical energy, stored in the form of glucose. Autotrophs (organisms that make their own food) can convert glucose into other carbon compounds, some of which are stored as biomass.
Producers
Producers, typically plants, algae and photosynthetic bacteria, form the first trophic level in a food chain by making their own food through photosynthesis.
Cellular respiration and heat loss
Cellular respiration releases the chemical energy stored in glucose, converting it into a form living cells can actually use to carry out active processes. This conversion is never fully efficient: some of the chemical energy released is always transformed into heat. That heat cannot be converted back into usable chemical energy, and is eventually lost from the organism's body entirely.
The second law of thermodynamics
The second law of thermodynamics states that energy transformations are inherently inefficient: whenever energy changes form, some of it is degraded into a less useful form, almost always heat. In ecosystems, the largest single loss happens during cellular respiration. This law is the reason no energy transfer between trophic levels is ever 100% efficient, a fact that will matter throughout the rest of this page.
5.2 makes essentially the same point from the agriculture side: raising livestock for meat wastes far more of the sun's original energy than eating plants directly, precisely because each additional trophic level loses more of it to respiration.
Consumers and their feeding strategies
Consumers gain chemical energy from carbon compounds obtained by feeding on other organisms, and use a range of different strategies to do it: herbivores (feed on producers), predators (kill and eat other animals), parasites (feed on a living host without immediately killing it), scavengers (feed on animals they did not kill), saprotrophs (feed by absorbing nutrients directly from dead organic matter) and detritivores (feed by ingesting fragments of dead organic matter). Saprotrophs and detritivores are together often called decomposers.
Trophic levels and food chains
Because producers make their own carbon compounds, they sit at the start of every food chain. Consumers obtain carbon compounds from producers, or from other consumers, forming each subsequent stage. These stages are called trophic levels, and the compounds and the energy they contain pass from one trophic level to the next along a food chain. Decomposers are traditionally left off food chain diagrams, since they draw from many sources at once rather than one specific trophic level, though their role in returning energy and matter to the system still matters.
Sort each feeding description into the correct consumer strategy.
A caterpillar that eats only the leaves of a single species of tree.
A lion that hunts, kills and eats a wildebeest.
A tapeworm living in an animal's intestine, absorbing nutrients from its host over months or years.
A vulture feeding on a zebra carcass it found already dead.
A fungus growing through a fallen log, absorbing nutrients directly from the decaying wood.
A dung beetle collecting and eating fragments of animal droppings.
Losses at every transfer
There are losses of energy and organic matter every time food is transferred along a food chain. Not all of what is available at one trophic level is actually harvested by the next; of what is harvested, not all is consumed; of what is consumed, not all is absorbed; and of what is absorbed, not all is stored, since some is lost as heat through cellular respiration. There is never 100% transfer of organic matter from one trophic level to the next.
Gross and net productivity
Gross productivity (GP) is the total gain in biomass by an organism. Net productivity (NP) is what remains after losses to cellular respiration are subtracted: NP = GP − respiratory loss. Respiratory losses are typically greater in consumers than in producers, since consumers spend more energy on active movement and behavior. The NP of any trophic level is also the maximum sustainable yield that can be harvested from it without diminishing what is available in the future.
Why food chains are short
The number of trophic levels in an ecosystem is limited because of these repeated energy losses. Energy lost as heat during respiration is permanently unavailable to organisms further up the chain, so typically 10% or less of the energy flowing into a trophic level is available to the next one, limiting how many links a food chain can support.
Organisms at higher trophic levels do not need to eat more food than organisms lower down in order to get enough energy. What limits the number of trophic levels is the shrinking total supply of energy available at each successive level, not an increased appetite among predators. A tertiary consumer is not eating unusually large amounts; there is simply far less total energy left for it to eat from.
Food webs
Real communities are rarely a single, simple chain. Food webs show the full complexity of trophic relationships in a community: arrows indicate the direction energy and biomass flow, and a single species can feed at more than one trophic level depending on what it eats at a given time.
Measuring biomass
Biomass at a given trophic level can be measured directly by collecting and drying samples: since water makes up most of an organism's mass, dry mass is a close approximation of the actual organic matter (biomass) present. The energy stored in that biomass can then be measured by burning (combusting) samples and extrapolating from the heat released.
Ecological pyramids
Ecological pyramids represent the relative numbers, biomass or energy of each trophic level in an ecosystem, stacked from producers at the base to the highest consumers at the tip. Pyramids of numbers and pyramids of biomass show the standing crop present at one point in time; pyramids of energy show the rate of energy flow through each level per unit area, per unit time.
Pyramids of numbers and biomass can sometimes be inverted, a single large tree can support many thousands of insects, or a small mass of fast-reproducing phytoplankton can sustain a larger standing mass of zooplankton. A pyramid of energy can never be inverted, because the second law of thermodynamics guarantees an energy loss at every single transfer, no exceptions.
Diagram: Thompsma, 2011, redrawn from Fundamentals of Ecology (Brooks Cole, 2005), CC BY-SA 3.0, via Wikimedia Commons.
Bioaccumulation and biomagnification
Non-biodegradable pollutants, such as PCBs, DDT and mercury, cause harm through two related processes. Bioaccumulation is the increasing concentration of a pollutant within a single organism over time, as it keeps absorbing more than it can excrete. Biomagnification is the increasing concentration of that same pollutant along a food chain, since biodegradable biomass is lost at every trophic transfer (to respiration, for example) while the non-biodegradable pollutant stored within it is not.
Diagram: via Wikimedia Commons, CC BY-SA 3.0.
Applying it to the hook. DDT bioaccumulated within each individual fish over its lifetime, then biomagnified again as it passed from algae to small fish to large fish to eagles. Both processes operating together are exactly why an apex predator, feeding at the top of the chain and never sprayed directly, ended up with the highest concentration of all.
Microplastics
Non-biodegradable pollutants can also be absorbed within microplastics, tiny plastic fragments under 5 mm across, which increases their transmission through the food chain. Filter feeders such as mussels and small fish ingest microplastics directly from the water column, and any pollutants absorbed onto those particles are then transferred to whatever eats them.
Human impacts on energy and matter flows
Human activities disrupt the flow of energy and the transfer of matter through ecosystems in several ways. Burning fossil fuels can raise atmospheric carbon dioxide available for photosynthesis, but the accompanying pollution and warming reduce primary productivity overall. Deforestation, urbanization and agriculture all reduce standing ecosystem biomass, disrupt existing food webs, and shrink the total capacity for photosynthesis across a landscape.
Autotrophs and heterotrophs
Every organism can be classified by how it obtains carbon compounds. Autotrophs synthesize their own carbon compounds from inorganic sources of carbon and other elements. Heterotrophs obtain carbon compounds by consuming other organisms.
A maize plant converting carbon dioxide and water into glucose using sunlight.
A fungus decomposing a fallen branch and absorbing the carbon compounds it contains.
A pond ecosystem
A small farm pond contains phytoplankton, water fleas (zooplankton) that eat the phytoplankton, minnows that eat the water fleas, and herons that eat the minnows. Farm runoff introduces a persistent, non-biodegradable pesticide into the pond.
Think it through, then check your reasoning against the model answer below. (1) Construct the food chain, from producer to top consumer. (2) Predict which organism in the chain will carry the highest concentration of the pesticide, and explain why, using the terms bioaccumulation and biomagnification correctly.
- Food chain: phytoplankton (producer) → water fleas (primary consumer) → minnows (secondary consumer) → herons (tertiary consumer).
- The herons will carry the highest pesticide concentration, since they occupy the highest trophic level in this chain.
- Bioaccumulation: each individual organism absorbs more of the pesticide over its lifetime than it excretes, so the concentration within any one organism rises over time.
- Biomagnification: at each transfer, biodegradable biomass is lost (to respiration and incomplete digestion) while the non-biodegradable pesticide is retained, so the concentration re-forms at a higher level in a smaller mass of tissue at each successive trophic level.
- A strong answer explicitly connects this back to the herons' position as the top, fish-eating predator, exactly the same mechanism that concentrated DDT in bald eagles.
- Bald eagle population figures, DDT/DDE mechanism, and recovery timeline: U.S. Fish and Wildlife Service, Bald Eagle species profile and 2024 nesting population report; American Bird Conservancy, "The Bald Eagle: The Ultimate Endangered Species Act Success Story."
- Photos and diagrams: via Wikimedia Commons, individually credited beneath each image.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.2, statements 2.2.1-2.2.21.
Build a food chain from data
Application of skills, statement 2.2.10: create a food chain from given data. Click each organism below, in feeding order, starting with the producer. One organism in the list does not belong in this particular grazing food chain: think about why before you include it.
Calculate productivity and ecological efficiency
Application of skills, statements 2.2.13 and 2.2.14: work out gross productivity (GP), net productivity (NP), and the efficiency of energy transfer between trophic levels, from given data. Work each answer out yourself before checking it.
Try it yourself · question 1
A population of producers has a gross productivity (GP) of 1,000 kJ m-2 yr-1. Energy lost to respiration (R) is 600 kJ m-2 yr-1. Calculate the net productivity (NP).
Try it yourself · question 2, multi-step
A grassland ecosystem has the following measured energy values:
- Producers, gross productivity (GPP): 12,000 kJ m-2 yr-1
- Producers, energy lost to respiration: 7,500 kJ m-2 yr-1
- Primary consumers receive the producers' entire net productivity as their own gross productivity, and lose 3,150 kJ m-2 yr-1 to respiration
- Secondary consumers receive 90 kJ m-2 yr-1 from the primary consumers
"Ecological efficiency" specifically means energy passed to the next trophic level, divided by the energy received by the current trophic level. It is not automatically 10%: this figure is commonly cited but highly variable between ecosystems, trophic levels and species, never state it as a fixed law.
Match the pyramid shape
Application of skills, statement 2.2.17: consider pyramid diagrams and the reasons for variation in their shape. For each scenario, decide whether the pyramid described would be upright (wide base, narrowing upward) or inverted.
A pyramid of numbers for a single large oak tree that supports thousands of individual insects.
A pyramid of biomass for the open ocean, where a small standing mass of fast-turnover phytoplankton supports a larger standing mass of zooplankton.
A pyramid of energy for that same open-ocean ecosystem.
- Illustrative productivity and efficiency figures are for teaching purposes; the underlying formulas (NP = GP − R; ecological efficiency = energy to next level ÷ GP) follow the IB ESS Guide directly.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.2, statements 2.2.10, 2.2.13, 2.2.14, 2.2.17, 2.2.28.
Glossary
- First law of thermodynamics
- Energy cannot be created or destroyed, only transformed from one form into another.
- Second law of thermodynamics
- Energy transformations are inherently inefficient; some energy is always degraded into a less useful form, usually heat.
- Photosynthesis
- The conversion of light energy into chemical energy, stored as glucose.
- Cellular respiration
- The release of chemical energy from glucose into a form usable by living cells; always partly lost as heat.
- Producer
- An organism, typically a plant, alga or photosynthetic bacterium, that forms the first trophic level by making its own food.
- Consumer
- An organism that gains chemical energy from carbon compounds obtained by feeding on other organisms.
- Trophic level
- A stage in a food chain, defined by how many feeding steps separate an organism from the producers.
- Food chain
- A single linear sequence showing the transfer of carbon compounds and energy from one trophic level to the next.
- Food web
- A diagram showing the full complexity of trophic relationships within a community.
- Gross productivity (GP)
- The total gain in biomass by an organism or trophic level.
- Net productivity (NP)
- Gross productivity minus the losses due to cellular respiration.
- Ecological pyramid
- A diagram representing the relative numbers, biomass or energy of each trophic level in an ecosystem.
- Bioaccumulation
- The increasing concentration of a non-biodegradable pollutant within a single organism over time.
- Biomagnification
- The increasing concentration of a non-biodegradable pollutant along a food chain, from one trophic level to the next.
- Autotroph
- An organism that synthesizes its own carbon compounds from inorganic sources.
- Heterotroph
- An organism that obtains carbon compounds by consuming other organisms.
- Primary productivity (HL)
- The rate of production of biomass by autotrophs, using an external energy source and inorganic carbon.
- Secondary productivity (HL)
- The gain in biomass by consumers, from carbon compounds absorbed and assimilated from ingested food.
- Net primary productivity (HL)
- The quantity of carbon compounds sustainably available to primary consumers; the basis for all food chains.
- Maximum sustainable yield, MSY (HL)
- The net primary or net secondary productivity of a system; the largest harvest a system can sustain without diminishing its future output.
- Ecological efficiency (HL)
- The percentage of energy received by one trophic level that is passed on to the next.
- Photoautotroph (HL)
- An autotroph that uses light as its external energy source, via photosynthesis.
- Chemoautotroph (HL)
- An autotroph that uses exothermic inorganic chemical reactions as its external energy source, via chemosynthesis.
- Entropy (HL)
- A measure of disorder within a system; increases overall as biomass and energy pass through an ecosystem, chiefly through cellular respiration.
The questions and markschemes below attempt to mimic IB wording, phrasing and expectations, but are not IB-written questions.
Quick quiz
Score: 0 / 5
1. Which law explains why no energy transfer between trophic levels is ever 100% efficient?
2. Net productivity (NP) is calculated as:
3. Which type of ecological pyramid can never be inverted?
4. What process caused DDE to reach its highest concentration in bald eagles, at the top of the food chain?
5. An organism that absorbs nutrients directly from dead organic matter, rather than ingesting solid particles of it, is best described as a:
Written practice questions
Total score: 0 / 21
Calculate how many times greater the 2024 nesting-pair population of bald eagles was than the 1963 population, given 417 known nesting pairs in 1963 and more than 71,400 nesting pairs by 2024.
Award the mark for the correct working and answer. Do not accept only "increased a lot."
Your score: 0 / 1
Discuss why the recovery of bald eagles after the DDT ban may be considered evidence of successful environmental management.
Check what your answer covers, up to 3 points per side:
Arguments that it was successful
Arguments/limitations
Award up to 3 marks for arguments and up to 3 for limitations, capped at 4 marks total to match this question. A full-mark response also reserves 1 mark for a supported judgement weighing both sides: do not credit a conclusion if only one side has been considered. This checklist cannot detect that judgement, so weigh it yourself.
Your score: 0 / 4
Distinguish between the first and second laws of thermodynamics in the context of ecosystems.
Do not accept that energy "disappears" or is "destroyed."
Your score: 0 / 2
Explain why energy transfer between trophic levels is never 100% efficient.
Do not accept only "energy is lost" without identifying how it is lost.
Your score: 0 / 3
Calculate the net productivity of a producer trophic level with a gross productivity of 18,000 kJ m-2 yr-1 and a respiratory loss of 11,500 kJ m-2 yr-1.
Award the mark for correct substitution and final answer.
Your score: 0 / 1
A student draws the food chain algae → water flea → small fish → heron for a lake, leaving decomposers out of the diagram. Suggest why decomposers are often left out of simple food chain diagrams.
Award 1 mark for a valid reason and 1 for development; a single developed suggestion may earn both marks. Do not accept "decomposers are not important."
Your score: 0 / 2
Farm runoff introduces a persistent, non-biodegradable pesticide into a pond food chain: phytoplankton → zooplankton → minnows → herons. Explain why persistent pesticides biomagnify along a food chain.
Do not accept only "the pesticide moves up the food chain" without explaining persistence and concentration increase.
Your score: 0 / 3
Outline two ways human activities can reduce standing ecosystem biomass.
Do not accept only "humans damage ecosystems" without linking to biomass.
Your score: 0 / 2
Explain how deforestation can reduce energy flow through an ecosystem.
Do not accept only "deforestation destroys habitats" unless linked to energy flow/photosynthesis/biomass.
Your score: 0 / 3
To what extent do energy losses between trophic levels explain the structure and vulnerability of ecosystems?
Essays like this are marked holistically against markbands, not ticked off point by point. Read the sections below, then rate your own answer against the bands.
Answers may demonstrate
- understanding of concepts and terminology such as trophic level, food chain, food web, producer, consumer, decomposer, gross productivity, net productivity, respiration, biomass, energy transfer efficiency, ecological pyramid, pyramid of energy, pyramid of biomass, second law of thermodynamics, bioaccumulation, biomagnification and persistent pollutants;
- breadth in addressing and linking a range of ecosystem structures and processes, including short food chains, smaller higher-trophic-level populations, pyramids of energy, limits to apex predators, productivity, sustainable yield, food webs, decomposers and pollutant movement;
- examples of energy losses, such as respiration losses as heat, uneaten biomass, undigested material, waste/egestion/excretion, death before consumption and movement/active behavior in consumers;
- examples of ecosystem structure shaped by energy loss, such as producers forming the base of food chains, fewer trophic levels, lower biomass/energy at higher trophic levels, or pyramids of energy that cannot be inverted;
- examples of vulnerability linked to energy transfer, such as apex predators being vulnerable to low energy availability, habitat loss reducing producer biomass, or persistent pollutants such as DDT/DDT breakdown products biomagnifying in eagles;
- balanced analysis of why energy losses are important, because repeated inefficiency limits food-chain length and reduces the energy available to higher trophic levels;
- balanced analysis of limitations, such as ecosystem vulnerability also being influenced by habitat loss, climate change, invasive species, disease, hunting, pollution type, life-history strategy and food-web complexity;
- evaluation of the idea that energy losses explain some major patterns in ecosystems but cannot explain all ecosystem vulnerability alone;
- a conclusion that is consistent with, and supported by, the analysis and examples given, for example: energy losses between trophic levels are a major reason food chains are short, pyramids of energy narrow and apex predators are energetically limited, but ecosystem vulnerability also depends on other ecological and human factors, so energy transfer is an important but incomplete explanation.
Indicative content
- Energy is transformed from light to chemical energy by photosynthesis.
- Respiration releases chemical energy but loses some energy as heat.
- The second law of thermodynamics means every energy transformation is inefficient.
- Not all biomass is consumed, digested or assimilated.
- Only a small proportion of energy, often 10% or less, is passed to the next trophic level.
- Food chains are usually short because little energy remains for higher trophic levels.
- Pyramids of energy cannot be inverted.
- Pyramids of biomass or numbers may be inverted because they show standing crop at one point in time.
- Net productivity is the energy/biomass remaining after respiration and represents the maximum sustainable yield.
- Higher trophic levels often contain fewer organisms/less biomass because less energy is available.
- Persistent pollutants may biomagnify because biomass is lost between trophic levels but the pollutant is retained.
- DDT/DDE in bald eagles is an example of apex predators being vulnerable to biomagnification.
- Food webs may complicate predictions because organisms feed through multiple pathways.
- Human activities such as deforestation, urbanization, agriculture, pollution and warming can reduce producer biomass and disrupt energy flow.
Markbands
The response shows limited understanding of energy transfer or trophic structure. The answer may state that "energy is lost" without explaining how or why. Examples may be absent, vague or inaccurate. The response may be one-sided, with little or no consideration of other causes of ecosystem vulnerability. There is little or no supported judgement on "to what extent."
The response shows sound understanding of energy loss between trophic levels. There is some explanation of how energy loss affects food-chain length, trophic levels or ecological pyramids. At least one relevant example is used. There is some consideration of ecosystem vulnerability, such as biomagnification or reduced biomass. There is some recognition that factors other than energy loss also affect ecosystems. The response includes some judgement, although it may be uneven or partly supported.
The response gives a balanced and well-developed evaluation of the role of energy losses in explaining ecosystem structure and vulnerability. It clearly explains mechanisms of energy loss, including respiration, heat loss, uneaten biomass, undigested material or waste. It links energy loss to food-chain length, ecological pyramids, productivity and availability of energy to higher trophic levels. It uses relevant examples effectively, such as DDT/bald eagles, pyramids of energy, food webs or producer biomass loss. It evaluates limitations and recognizes that ecosystem vulnerability is also affected by habitat loss, climate change, pollution, disease, invasive species and human management. There is a clear and supported conclusion that directly answers "to what extent."
Self-assessed band: not yet rated
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.2, statements 2.2.1-2.2.29.
Photoautotrophs and chemoautotrophs
Not every autotroph relies on light. Photoautotrophs use light as their external energy source in photosynthesis, the pathway covered so far in this subtopic. Chemoautotrophs instead use exothermic inorganic chemical reactions as their external energy source, a process called chemosynthesis.
Photoautotrophs
- External energy source: light
- Process: photosynthesis
- Dominant in ecosystems with access to sunlight
Chemoautotrophs
- External energy source: inorganic chemical reactions
- Process: chemosynthesis
- Principal energy source in ecosystems with little or no light
Diagram: Cburnett, English Wikipedia, CC BY-SA 3.0 / GFDL, via Wikimedia Commons.
- In February 1977, scientists aboard the research vessel Knorr discovered dense communities of life around hydrothermal vents on the Galapagos Rift, on the seafloor far below where any sunlight reaches.
- Vent water is rich in hydrogen sulfide (H2S). Chemoautotrophic bacteria oxidize this hydrogen sulfide to obtain energy, in place of the light energy a plant would use.
- Giant tube worms (genus Riftia) host these bacteria symbiotically: the bacteria receive a stable, chemical-rich habitat, and the worm receives all of its nutrition in return, growing as much as 85 cm per year.
- These chemoautotrophic bacteria are the base of the entire vent food web, standing in for producers in an ecosystem that has no light at all.
Sources: Woods Hole Oceanographic Institution, "The Discovery of Hydrothermal Vents"; MarineBio Conservation Society.
Photo: NOAA Okeanos Explorer Program, Galapagos Rift Expedition, 23 July 2011, public domain, via Wikimedia Commons.
Sort each organism by its external energy source.
Bacteria living inside a giant tube worm oxidize hydrogen sulfide from vent water to obtain energy, in total darkness.
A cyanobacterium in a sunlit pond uses light energy, captured by chlorophyll, to fix carbon dioxide into glucose.
Iron-oxidizing bacteria in a deep, permanently dark cave system obtain energy by oxidizing dissolved iron compounds in the groundwater.
A rainforest canopy tree captures light energy through chlorophyll to build new biomass.
Primary productivity
Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements. It is usually expressed in kg carbon m-2 yr-1. Estimates can be made in a laboratory, using photosynthesizing samples, or in the field, by measuring the change in biomass of samples such as grassland over time.
Secondary productivity
Secondary productivity is the gain in biomass by consumers, from carbon compounds absorbed and assimilated from ingested food. It is calculated as ingested food minus fecal waste: the fraction that passed through undigested and unabsorbed is not included, since it never actually became part of the consumer's own biomass.
Net primary productivity
Net primary productivity (NPP) is the basis for every food chain, because it represents the quantity of carbon compounds sustainably available to primary consumers. NPP can be thought of as the plant growth that primary consumers, in a natural ecosystem, or farmers and foresters, in an agricultural or silvicultural system, can sustainably harvest.
Use laboratory and field techniques for measuring primary and secondary productivity, and work out GP and NP from data. This is one of the most commonly tested skills at HL: know the actual methods, not just the definitions.
Measuring primary productivity in the field is usually done by the harvest method: mark out a known area (a quadrat), cut and collect all the plant material inside it, then oven-dry the sample to a constant mass to remove water, since dry mass closely approximates true biomass. Repeating this at a second time point, in a second, adjacent, uncut quadrat, gives the change in biomass over that interval, an estimate of NPP for the area, once expressed per unit area per unit time. Comparing paired cut and uncut plots, or bagging some samples to exclude grazers, also lets losses to herbivory be estimated alongside growth.
Measuring primary productivity in the laboratory uses photosynthesizing samples under controlled conditions, most often with an oxygen probe or an infrared gas analyzer (IRGA). A sample is enclosed in a sealed, transparent chamber and its rate of O2 release or CO2 uptake in the light gives an estimate of gross productivity; the same sample in the dark, where only respiration occurs, gives the respiration rate, so GP and NP can be separated from a single pair of readings.
Measuring secondary productivity most often uses a controlled feeding trial: a known mass of food is offered to a consumer, and the mass of food ingested, faecal waste egested, and any further weight change of the consumer itself are all recorded over a fixed period. Secondary productivity is then the biomass gained by the consumer, calculated from what was ingested minus what was egested as faecal waste and lost to respiration.
2.2.16 already introduced the drying and combustion methods used to convert a harvested sample into a biomass and energy value: oven-drying to constant mass gives biomass, and burning a sample in a calorimeter and measuring the heat released gives its energy content. Productivity measurements build directly on these two techniques, applied at two or more time points instead of just one.
Outline a method a student could use to estimate the net primary productivity of a grassland ecosystem, using the harvest method.
Check each point your answer covers:
Your score: 0 / 4
Sort each scenario into primary or secondary productivity.
Chemoautotrophic bacteria at a hydrothermal vent fix carbon from CO2, using energy released by oxidizing hydrogen sulfide.
A population of zooplankton gains biomass by absorbing and assimilating carbon compounds from the phytoplankton it has eaten, after subtracting undigested waste.
A field of wheat converts inorganic carbon dioxide into 1,200 kg of new plant biomass per hectare, per year.
A herd of wildebeest gains body mass by digesting and assimilating the grass they graze.
Maximum sustainable yield
Maximum sustainable yield (MSY) is the net primary or net secondary productivity of a system: the largest harvest that can be taken from a natural or agricultural system without reducing what is available to harvest in the future.
1.3 covers exactly this idea in practice, without naming MSY directly: the Newfoundland northern cod fishery collapsed after decades of harvesting well beyond what the stock's net productivity could sustain, and its 2024 reopening, against the explicit advice of government scientists, is a live example of a harvest decision made in tension with the science-based MSY figure. 7.1 makes the same underlying point about renewable resources generally: "renewable does not mean cannot run out," a resource stays renewable only while its withdrawal rate does not exceed its regeneration rate, which is just another way of describing MSY.
Quick check. Newfoundland cod were harvested for decades at rates that exceeded the stock's net secondary productivity. In MSY terms, what does this represent?
Harvesting above the maximum sustainable yield: taking more than the stock could replace, which is exactly what triggered the 1992 population collapse.
Sustainable yields are higher for lower trophic levels
Because energy is lost at every trophic transfer, sustainable yields are consistently higher for lower trophic levels. Food production is easier to sustain the closer to producers it stays, which is why plant-based food production can sustainably support far more people, from the same land area, than food production based on animals higher up the food chain.
5.2 covers this directly from the food-security side: diets built around lower trophic levels require less land and energy per calorie than diets built around meat from higher trophic levels, the same efficiency logic driving the numbers here.
Try it yourself
One hectare of farmland produces 5,000 kg of harvestable wheat biomass (its NPP) per year. Instead of eating the wheat directly, a farmer feeds all of it to cattle. The ecological efficiency of the wheat-to-cattle energy transfer is 10%. Calculate how much cattle biomass would be available to harvest from that same hectare.
Ecological efficiency, revisited
Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next. This percentage varies substantially between ecosystems, trophic levels and species. The commonly cited value of 10% is neither a fixed rule nor a true average: treat it as a rough illustration, not a constant to quote as fact.
Try it yourself
A trophic level receives 850 kJ m-2 yr-1 as gross productivity (GP). Energy lost to respiration (R) is 610 kJ m-2 yr-1. Of the energy remaining, only 18 kJ m-2 yr-1 is passed on to the next trophic level. Calculate the net productivity (NP) and the ecological efficiency of this transfer.
Entropy and the second law, revisited
The second law of thermodynamics also explains how entropy, the amount of disorder within a system, increases as biomass passes through an ecosystem. Living organisms maintain a high degree of internal organization, low entropy, but only by continuously exporting disorder elsewhere: the net effect of cellular respiration across an ecosystem is a steady increase in the entropy of the wider system, even while individual organisms stay highly organized.
Quick check. As biomass and energy pass through a food chain, what happens to the entropy of the wider system?
It increases steadily. Organisms keep their own internal order low-entropy, but only by continuously exporting disorder to the wider system through respiration, so the system as a whole becomes more disordered over time.
- Hydrothermal vent chemosynthesis: Woods Hole Oceanographic Institution, "The Discovery of Hydrothermal Vents"; MarineBio Conservation Society, "Hydrothermal Vents & Chemosynthetic Ecosystems."
- Newfoundland cod cross-link: covered in full on 1.3 Sustainability.
- Photos and diagrams: via Wikimedia Commons, individually credited beneath each image.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 2, Subtopic 2.2, statements 2.2.22-2.2.29.