AP Syllabus focus:
‘Energy flows and matter cycles through trophic levels, linking producers, consumers, decomposers, and biogeochemical cycles.’
Ecosystems can be understood by tracking two connected processes: how energy moves one-way through feeding relationships and how matter (atoms and nutrients) is repeatedly recycled among organisms and the environment.
Trophic levels and feeding relationships
Trophic levels organise energy flow
Trophic level: A feeding position in an ecosystem (for example, producers, primary consumers, secondary consumers) that indicates how an organism obtains energy.
Producers (autotrophs) capture external energy (usually sunlight) and convert it to chemical energy stored in organic molecules. Consumers (heterotrophs) obtain that chemical energy by eating other organisms. Decomposers and detritivores obtain energy from dead organic matter and wastes, while also releasing inorganic nutrients back to the environment.
This food web diagram emphasizes that energy moves from organisms that are eaten to organisms that eat them, linking multiple trophic pathways at once. It also highlights the decomposer/detritus route: all producers and consumers ultimately become inputs for decomposers, which return nutrients to the ecosystem. Source
Food chains and food webs
A food chain shows one pathway of energy transfer (producer → consumer levels). A food web shows many interconnected pathways, better reflecting real ecosystems where most organisms have multiple food sources and predators. Trophic levels are still useful even in a food web because they help predict broad patterns of energy availability and nutrient movement.
Energy transfer through trophic levels
Energy flows, but is not recycled
Energy enters ecosystems primarily through producers and then moves to consumers and decomposers. At each transfer, a large fraction of energy is lost from the perspective of the next trophic level because organisms use energy for:
Cellular respiration (releasing heat)
Movement and maintenance (homeostasis)
Incomplete consumption (not all biomass is eaten)
Incomplete digestion (some ingested material is egested as waste)
Because energy is continually lost as heat, ecosystems require a постоян input of energy (typically sunlight). This one-way flow helps explain why higher trophic levels have less available energy and typically support fewer individuals and less total biomass.
Primary productivity links producers to the rest of the web
Net primary productivity (NPP): The energy captured by producers and stored as biomass after subtracting the energy producers use for respiration; it represents energy available to consumers.
NPP is a key driver of how much consumer biomass an ecosystem can support because it is the main “budget” of new biomass entering the food web.
= net primary productivity (energy per area per time)
= gross primary productivity, total energy captured by producers (energy per area per time)
= energy used in producer respiration (energy per area per time)
Higher NPP generally allows longer food chains and/or greater biomass at higher trophic levels, but transfer losses still constrain how much energy reaches top predators.
Ecological efficiency and energy pyramids
Ecological efficiency (trophic transfer efficiency) describes the fraction of energy (or biomass energy) transferred from one trophic level to the next. It varies by ecosystem and organism type, but it is always far below 100% due to the losses above. As a result, energy pyramids are always upright: each higher trophic level contains less energy than the level below it.
Ecological pyramids compare how number of organisms, biomass, and energy change across trophic levels. The energy pyramid is always upright because only a fraction of energy is transferred upward at each step, limiting biomass and population sizes at higher trophic levels. Source
Matter cycling through trophic levels
Matter is recycled, not lost
Biogeochemical cycle: The movement of elements and compounds between living organisms and the nonliving environment through biological, geological, and chemical processes.
Unlike energy, matter cycles because atoms are conserved.
The carbon cycle diagram shows carbon moving between atmospheric , living biomass (via photosynthesis and food webs), and returns through respiration and decomposition. It also includes long-term reservoirs (sediments/fossil carbon) and processes that return carbon to the atmosphere, connecting biological and geological cycling. Source
Carbon, nitrogen, phosphorus, and water move into producers from the environment, pass through consumers via feeding, and return to the environment through:
Excretion (nitrogenous wastes, dissolved ions)
Egestion (undigested material)
Death and decomposition
Producers, consumers, and decomposers connect cycles
Producers incorporate inorganic molecules (for example, , water, mineral nutrients) into organic biomass.
Consumers rearrange that matter into their own biomass and release some back to the environment as wastes.
Decomposers (especially fungi and bacteria) break down complex organic matter and mineralise nutrients—converting them into inorganic forms producers can reuse.
Decomposition is therefore a crucial link between trophic levels and biogeochemical cycles, preventing essential nutrients from remaining locked in dead biomass and allowing continued primary production.
Detritus pathways are central to cycling
Not all primary production is consumed by herbivores. A substantial fraction becomes detritus (dead organic material) that enters decomposer-based pathways. These pathways strongly influence how quickly nutrients return to producers, shaping ecosystem productivity and the structure of trophic levels.
FAQ
Phytoplankton can have very low standing biomass yet extremely high turnover.
Energy pyramids remain upright because transfer efficiency is low, even if producer biomass at any moment is small.
Key factors include food quality (C:N:P ratios), digestibility (cellulose, lignin), consumer metabolism, and temperature.
Short-lived, easily digested producers often support higher efficiencies than woody plants.
$^{15}N$ tends to become enriched at higher trophic positions due to fractionation during excretion.
Measuring $\delta^{15}N$ across organisms can estimate relative trophic level within a food web.
Dissolved organic matter can be taken up by bacteria, which are then eaten by protists and small consumers.
This pathway retains carbon and nutrients in the web rather than losing them directly to the environment.
Faster decomposition increases nutrient availability to producers, raising potential growth.
Slow decomposition can trap nutrients in detritus, lowering producer access to limiting elements despite adequate light.
Practice Questions
Explain why energy transfer between trophic levels is inefficient, giving two biological reasons. (3 marks)
States that energy is lost as heat via respiration (1)
Gives one additional valid reason (e.g. not all biomass is eaten; some is indigestible and egested; energy used for movement/maintenance) (1)
Links inefficiency to less energy available at higher trophic levels / upright energy pyramid (1)
Describe how matter cycles through trophic levels and explain the role of decomposers in linking trophic interactions to biogeochemical cycles. (6 marks)
Producers take up inorganic substances and convert them into organic biomass (1)
Consumers obtain matter by feeding and incorporate some into their biomass (1)
Matter is returned via excretion/egestion and death (1)
Decomposers break down dead organic matter and wastes (1)
Decomposers release inorganic nutrients back to the environment (mineralisation) (1)
These nutrients are then available again to producers, completing the cycle (1)
