Which groups would you monitor to detect a trophic cascade from predator manipulation in freshwater ecosystems?

• A. Plankton, macroinvertebrates, and primary production
• B. Birds and mammals
• C. Bacteria only
• D. Insects only

Answer: (A)

Trophic cascades occur when changes at the top of the food web ripple down to affect producers and other consumers, so you want to watch groups that span key trophic levels in the system. Monitoring plankton, macroinvertebrates, and primary production does exactly that. Plankton covers both phytoplankton, the producers, and zooplankton, their grazers, so predator-driven shifts can show up in both groups. Macroinvertebrates include herbivores and detritivores that feed on algae and organic material, providing a readout of grazing pressure and energy flow in the middle of the web. Primary production measures how much new plant-based biomass is generated, which responds to grazing and nutrient changes caused by predators. Together, these three metrics trace how predator manipulation can cascade from predators down to producers and back through the food web.

Birds and mammals are less direct indicators of aquatic trophic cascades and may reflect broader or delayed ecosystem changes. Bacteria focus on microbial processes and nutrient cycling rather than the broader predator–prey–producer interactions across multiple trophic levels. Insects alone miss other important components, like producers and other vertebrate or invertebrate consumers.