Morphological constraints in the piscivore‐planktivore interaction: Implications for the trophic cascade hypothesis

Results are presented from a 3‐yr, replicated pond study examining how differential planktivore vulnerability can influence planktivore responses to piscivory and ultimately plankton responses via cascading effects. Without piscivores, planktivory from a multispecies planktivore assemblage (fathead minnow Pimephales promelas and pumpkinseed sunfish Lepomis gibbosa ) produced the well‐documented plankton responses. Chaoborus densities, zooplankton biomass, and cladoceran and copepod mean body sizes were reduced, and chlorophyll concentrations and gross primary production were enhanced. With piscivores, levels of planktivory were depressed due to elimination of the highly vulnerable minnow and a shift in the size structure of the relatively less vulnerable sunfish toward larger, benthivorous sizes. Hence, zooplankton biomass and cladoceran and copepod mean body sizes were increased, and chlorophyll concentrations and gross primary production were reduced. However, juvenile sunfish remained abundant in ponds containing piscivores, so Chaoborus densities remained low and small cladocerans ( Bosmina and Ceriodaphnia ) were abundant. Moreover, Daphnia biomass, mean body size, and number of days present during the final year in these ponds were negatively correlated with small sunfish biomass, indicating that piscivory did not simply cancel out the effects of planktivory.