In a predator manipulation experiment, which outcome would support top-down control of lower trophic levels?

In a predator manipulation experiment, which outcome would support top-down control of lower trophic levels?

• A. Increased phytoplankton biomass following predator removal
• B. Decreased zooplankton due to predator removal
• C. No change in community structure
• D. Increased nutrient concentrations with predator presence

Answer: (A)

Top-down control happens when predators shape the abundance of organisms lower in the food web, producing a cascade that reaches the base producers. In aquatic systems with zooplankton grazing, predators on zooplankton reduce grazing pressure on phytoplankton, allowing phytoplankton to grow. So the clearest sign of strong top-down control is that phytoplankton biomass is higher when predators are present (and lower when predator pressure is released by removing predators). The outcome described as phytoplankton biomass increasing after predator removal would run counter to that pattern: removing predators would typically release herbivores to graze more, which would reduce phytoplankton rather than increase it. In practice, the strongest support for top-down control comes from observing higher phytoplankton when predators are present, not after their removal, because the cascade is driven by predator suppression of the herbivores.

Top-down control happens when predators shape the abundance of organisms lower in the food web, producing a cascade that reaches the base producers. In aquatic systems with zooplankton grazing, predators on zooplankton reduce grazing pressure on phytoplankton, allowing phytoplankton to grow. So the clearest sign of strong top-down control is that phytoplankton biomass is higher when predators are present (and lower when predator pressure is released by removing predators).

The outcome described as phytoplankton biomass increasing after predator removal would run counter to that pattern: removing predators would typically release herbivores to graze more, which would reduce phytoplankton rather than increase it. In practice, the strongest support for top-down control comes from observing higher phytoplankton when predators are present, not after their removal, because the cascade is driven by predator suppression of the herbivores.