Predator Diversity Does Not Contribute to Increased Prey Risk: Evidence from a Mesocosm Study

Abstract

Predation plays an important organisational role in structuring aquatic communities. Predator diversity can, however, lead to emergent effects in which the outcomes of predator–prey interactions are modified. The importance of predator diversity in regulating predator–prey interactions was investigated during a 9-day mesocosm study conducted in the middle reach of a temporarily open/closed, temperate, southern African estuary. The zooplankton community, comprising almost exclusively (>95% of total counts) calanoid and cyclopoid copepods of the genera Pseudodiaptomus, Paracartia and Oithona, was subject to three different juvenile fish predator treatments at natural densities: 1. predation by Gilchristella aestuaria, (Gilchrist, 1913; SL 15.3 ± 2.4 mm); 2. predation by Myxus capensis (Valenciennes, 1836; SL 12.8 ± 3.7 mm); and 3. a combination of the two predators. The presence of the predators contributed to a significant decline in the total zooplankton abundances, with a concurrent increase in total chlorophyll-a (Chl-a) concentrations, consistent with the expectations of a trophic cascade (ANCOVA; p < 0.05 in all cases). There were no significant differences in the total Chl-a concentration or total zooplankton abundances between the different predator treatments, suggesting that the increase in predator diversity did not contribute to increased prey risk or to the strength of the trophic cascade.