Trace energy pathways and test a food-web explanation

Read arrows as transfers, not rank

A food chain traces one path from a resource to its consumer. A food web joins several such paths. Producers form organic matter from inorganic sources; primary consumers feed on producers; secondary and tertiary consumers feed farther along particular paths. Omnivores can occupy more than one trophic position depending on what they eat. An arrow points from the consumed resource toward the consumer receiving matter and energy.

Detritivores ingest dead material; decomposers break down organic material and release nutrients into forms that can be reused. Both are essential to cycling matter. They do not recycle usable energy back into the producer level. Energy ultimately dissipates as heat, so energy accounting and nutrient cycling require different claims.

An explicit two-route energy model

Assign producer NPP of 10,000 kJ/m²/yr. The original synthetic web sends 60% into a grazed route and 40% directly into a detrital route. Those inputs are 6,000 and 4,000. With a stipulated production-transfer efficiency of 10%, herbivore production is 600 and first detrital-consumer production is 400. Predator production via herbivores is 60 after another 10% transfer.

The “10% rule” is a useful approximation, not a universal fixed efficiency. Our control lets you compare 5%, 10% and 20% efficiencies. At 20%, herbivore production is 1,200 and predator production is 240. Doubling both consumer-step efficiencies quadruples the two-step output because two factors of two multiply.

The first grazing-step residual is 6,000−600=5,400. Do not label all of it immediate heat: it includes material not converted into consumer production, such as waste entering detrital pathways, as well as respiration. This limited graph does not follow all later detrital transfers. It displays the boundary honestly rather than inventing a closed whole-web budget.

A static transfer graph does not predict a cascade

Removing a predator can release herbivores from predation and thereby increase grazing pressure, but this is a hypothesis about population and interaction dynamics. The arithmetic ledger has no births, deaths, adaptive feeding or alternative prey. It cannot prove that every predator removal causes the same vegetation response.

Competition, disease, immigration and resource limitation offer alternative explanations for changed counts. For a virtual study design, compare replicated treatment and reference plots before and after a predator-access manipulation, with ethically appropriate methods designed by qualified investigators. A sham treatment can test whether the enclosure itself affects light, moisture or movement. The learner is not asked to trap, remove or harm organisms.

Transfer with two competing explanations

Suppose herbivore abundance rises after a predator decline, but producer production also rises following unusual rainfall. Predator release and increased food supply may both contribute. Describe observations or a factorial design that could distinguish their effects. State the response variable, replication, treatment timing and one remaining limitation.

For NPP 20,000 and 5% efficiency, calculate 12,000 entering grazing, 600 herbivore production and 30 predator production. Compare that with the original 10,000/10% case: the higher producer production does not guarantee greater top-consumer production when transfer efficiencies differ. Justify the comparison with both the energy path and its assumptions, not a memorized pyramid alone.

Conservation and feedback do not mean energy recycling

A positive indirect effect is not itself a feedback loop: a loop must return to its starting variable. In a hypothetical lake, more submerged vegetation traps sediment, reducing turbidity; reduced turbidity increases light, which supports more submerged vegetation. Vegetation→turbidity is negative, turbidity→light negative, and light→vegetation positive: the closed product is positive, a reinforcing loop under these assumptions. A loss of vegetation can therefore reinforce further loss as well. Predict the effect of changing the final link so greater light no longer increases vegetation because nutrients are limiting: the reinforcing response weakens or disappears. This is a mechanism hypothesis, not proof of inevitable runaway growth or collapse.

The first law requires energy accounting: transferred chemical energy is transformed, not destroyed. The second law explains why transformations spread energy into less available forms such as heat; organisms require continuing usable energy input. In most surface ecosystems sunlight supplies that input. Nutrient atoms can return through decomposition while the previously dissipated energy does not return as new usable sunlight.

An original signed-effect chain gives predators a negative effect on herbivores and herbivores a negative effect on plant biomass. Predict the indirect predator effect on plants: multiplying two negative links gives a positive effect under those assumptions. A rise in plants that supports more herbivores creates a negative plant–herbivore feedback loop because more herbivores subsequently reduce plants. Change the packet so herbivores mainly eat a competing plant rather than the focal plant: the focal-plant response can reverse. Explain which link changed rather than assuming every food web has the same cascade. These qualitative signs are hypotheses requiring evidence, not population forecasts from the static energy ledger.

Sources and investigation boundary

Original explanations and synthetic calculations aligned with AP Environmental Science topics 1.9–1.11. Framework checked 30 October 2026: College Board course at a glance and Fall 2026 clarifications. These are public teaching materials, not official exam questions. Computational captures do not establish field measurements, independent mastery or supervised hands-on laboratory time.

Clause reconciliation source checked 31 October 2026: official course framework, interpreted with the linked Fall 2026 corrections. The added comparisons are original supplied-data teaching exercises.