Energy Flow in a Pond Food Web
A pond receives 1,000,000 kcal of sunlight energy per day. Algae capture 2% of that energy. Primary consumers (zooplankton) assimilate 10% of the algae's energy, and secondary consumers (small fish) assimilate 12% of zooplankton energy. A proposal would remove 80% of the small fish to increase zooplankton and algae. A student predicts that removing fish will increase algae because 'fewer fish means more algae eaten by zooplankton, so algae will crash.' Another student predicts algae will increase because 'less grazing pressure.'
1. Calculate the energy available at each trophic level and use it to evaluate both predictions about algae.
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Algae capture 2% of 1,000,000 = 20,000 kcal/day. Zooplankton assimilate 10% = 2,000 kcal/day. Small fish assimilate 12% = 240 kcal/day. Removing 80% of fish reduces fish energy demand, so less zooplankton is consumed by fish, allowing zooplankton biomass to rise. More zooplankton graze more algae, so the first student's direction (algae decline) is plausible under top-down control. The second student's 'less grazing pressure' is wrong: removing fish does not reduce grazing on algae; it can increase it via zooplankton release. The energy numbers show each level is small, so a change at the top can cascade down. The key judgment is distinguishing top-down (predation) from bottom-up (energy) control.
2. If the pond's algae were limited by nutrients rather than by grazing, how would the prediction change?
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If algae are bottom-up limited by nutrients (e.g., phosphorus), then removing fish and increasing zooplankton would not necessarily increase algae; grazing might reduce algae, but the algae cannot grow faster anyway because nutrients cap production. In that case, the cascade is weak or absent. The correct judgment is that trophic cascades depend on which factor limits the basal level. The 2% capture efficiency suggests algae are already far below their light-limited maximum, consistent with nutrient or grazing limitation. So the prediction must specify the limiting factor; without it, neither student's conclusion is safe.
3. Explain why only about 2% of sunlight becomes algal energy and why the 10% and 12% transfer efficiencies are not 'wasteful mistakes' of the ecosystem.
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The 2% reflects limits on photosynthesis: only part of the light spectrum is absorbed, much is reflected or transmitted, and algae cannot convert all captured light to biomass because of nutrient limits and respiration. The 10% and 12% transfer efficiencies reflect the second law of thermodynamics: at each transfer, energy is lost as heat through respiration and as undigested material. This is not a mistake but an inevitable consequence of metabolism. It also explains why food chains are short: there is not enough energy left to support many high-level consumers. The judgment is that low transfer efficiency is a constraint, not a flaw.