AP Syllabus focus:
‘Shifts in sunlight or producer biomass can disrupt ecosystems by changing the number and size of trophic levels.’
Energy entering an ecosystem through producers sets the upper limit on biomass at every higher trophic position. When sunlight or producer biomass shifts, energy supply changes, reorganizing food chains and destabilizing community structure.
Energy supply sets trophic structure
Sunlight is the primary external energy source for most ecosystems because it drives photosynthesis, which builds producer biomass (living mass of autotrophs). That biomass is the energy “budget” that can be converted into new consumer biomass.
Key terms for energy-driven disruption
Trophic level: A feeding position in a food chain or web (e.g., producers, primary consumers, secondary consumers) that reflects where organisms obtain energy.
Because energy is lost as heat and waste at each transfer, higher trophic levels typically have less total biomass and fewer individuals than lower levels.
This figure compares three standard ecological pyramids—numbers, biomass, and energy—across trophic levels. It visually reinforces that available energy declines upward through the food chain, which compresses biomass and abundance at higher trophic positions. Use it to connect “energy loss at each transfer” to why upper trophic levels are typically smaller and more vulnerable to resource shifts. Source
This is why the amount of producer production strongly constrains the number of trophic levels a system can support and the size (biomass and population size) of each level.
Primary productivity links sunlight to biomass
A useful way to connect sunlight to trophic structure is net primary productivity (NPP), which is the rate at which producers store energy as new biomass after meeting their own metabolic needs.
= energy stored as producer biomass available to consumers (e.g., )
= total energy captured by photosynthesis (e.g., )
= energy used by producers for respiration (e.g., )
When sunlight decreases, GPP often drops, lowering NPP and shrinking the energy base that supports the entire food web.
Producer biomass: The total mass of living autotrophs in an ecosystem at a given time, forming the energetic foundation for consumers.
How shifts in sunlight disrupt ecosystems
Decreased sunlight: contraction of food webs
Reduced sunlight can occur through increased canopy shading, turbidity in aquatic systems, seasonal change, or prolonged cloud cover. Common outcomes include:
Lower photosynthetic rate → reduced NPP and reduced producer growth
Smaller producer populations/biomass → less food for herbivores (primary consumers)
Consumer declines that propagate upward because each higher trophic level depends on energy from levels below
Loss of upper trophic levels first, since top predators require large, stable energy inputs to maintain viable populations
Simplification of the food web, often expressed as fewer effective trophic levels (a shorter chain)
As producer input falls, ecosystems may shift toward fewer specialist consumers and more generalists that can persist on limited or variable resources, altering interaction strengths and stability.
Increased sunlight or producer biomass: expansion and instability
Higher sunlight or increased producer biomass can temporarily increase the energy base. Potential outcomes include:
Higher NPP → increased herbivore biomass and potentially more biomass at higher consumer levels
Support for an additional trophic level (or greater persistence of existing high-level consumers) if energy input remains high and continuous
Larger population sizes at multiple levels, because more energy can be converted into growth and reproduction
However, rapid increases in producer biomass can also destabilize trophic structure if the increase is brief or patchy:
Boom-bust dynamics: consumer populations rise with more food, then crash if producer biomass later declines
Mismatched timing: producer peaks may not align with consumer life cycles, reducing energy transfer efficiency
Community reorganization: species better adapted to high-resource conditions may temporarily dominate, changing which pathways energy takes through the web
Changing the number and size of trophic levels
Disruption is often observed as changes in:
This set of pyramids shows how (A) biomass, (B) number of individuals, and (C) energy can be distributed across trophic levels in different ecosystems. The side-by-side examples emphasize that trophic structure depends on producer-based energy supply and turnover, so “size of trophic levels” can shift even when the number of levels remains the same. It’s a concrete visual bridge between biomass pyramids and energy pyramids when discussing ecosystem disruption. Source
Number of trophic levels: energy limitation can remove higher levels (especially large predators), while sustained increases can allow longer chains.
Size of trophic levels (biomass/abundance): reduced producer biomass typically compresses the entire biomass pyramid; increased producer biomass can broaden lower levels and, if sustained, enlarge higher levels too.
Because higher trophic levels are energetically expensive to maintain, they are sensitive indicators of shifts in sunlight and producer biomass. Small reductions at the producer level can translate into large proportional losses near the top, changing predation pressure, survival, and the overall structure of the ecosystem.
FAQ
It depends on generation time and storage. Producers may respond within days to weeks, while long-lived consumers can lag for seasons or years.
Key drivers:
turnover rate of producers
reproductive rate of consumers
persistence of the sunlight change
Top predators sit at the smallest energy pool. They typically need large territories and stable prey supplies, so small declines in lower levels can push them below viable population sizes.
Only if increased sunlight leads to sustained increases in producer biomass and energy transfer supports a stable new consumer level. Short-lived increases usually raise abundance within existing levels rather than adding a persistent level.
Common indicators include declining chlorophyll (aquatic producers), reduced plant cover, falling herbivore densities, shrinking predator ranges, and shorter realised food chains inferred from feeding observations.
Systems with substantial storage (e.g., seeds, woody biomass, fat reserves) can buffer short-term drops in sunlight. Low-storage systems often show faster, larger fluctuations in consumer abundance and trophic structure.
Practice Questions
Explain how a sustained decrease in sunlight could reduce the number of trophic levels in an ecosystem. (2 marks)
Reduced sunlight lowers photosynthesis/primary productivity (1)
Less producer biomass/energy available means higher trophic levels cannot be supported, so top levels are lost and the food chain shortens (1)
An ecosystem experiences a long-term decline in producer biomass. Describe and explain the likely changes in the size (biomass/abundance) of primary, secondary, and tertiary consumer trophic levels. (5 marks)
Primary consumers decline due to reduced food/energy from producers (1)
Secondary consumers decline following reduced primary consumer abundance/biomass (1)
Tertiary consumers decline the most / are most likely to disappear first (1)
Explanation that energy transfer between trophic levels is inefficient, so reductions at the base are magnified at higher levels (1)
Clear linkage to fewer/smaller trophic levels and a simplified food web (1)
