Role of Keystone Predators in Trophic Cascade Regulation

Abstract

Keystone predators exert disproportionate regulatory effects on community structure and ecosystem function relative to their biomass, primarily through trophic cascades that propagate top-down control from apex predators through mesopredator and prey communities to primary producers. This study quantified trophic cascade strength, community stability metrics, and ecosystem productivity responses across a predator manipulation gradient in three temperate lacustrine systems (Italy, Germany, Switzerland) encompassing 18 lake basins spanning a natural gradient of piscivore (pike Esox lucius and perch Perca fluviatilis) biomass density (0.8–18.4 kg/ha). Standardised hydroacoustic surveys, gillnet sampling, benthic macroinvertebrate assessments, phytoplankton biomass measurements (chlorophyll-a), and zooplankton size-structure analyses were conducted seasonally over two years (2020–2021). Piscivore biomass density was the strongest predictor of the planktivore-to-total fish biomass ratio (r s = -0.82, p < 0.001), mean zooplankton body size (r s = +0.76, p < 0.001), and chlorophyll-a concentration (r s = -0.74, p < 0.001), confirming a four-level trophic cascade (piscivore fi planktivore fish fi zooplankton fi phytoplankton). Structural equation modelling confirmed direct and indirect cascade pathways with a total cascade amplification factor of 3.8 ± 0.6 (phytoplankton biomass response per unit change in piscivore biomass). Lakes with piscivore biomass ‡ 8 kg/ha showed significantly lower cyanobacterial bloom frequency (mean 1.2 ± 0.8 events/yr vs. 4.6 ± 1.4 in low-piscivore lakes; Mann-Whitney U = 4, p < 0.001), demonstrating a direct ecosystem service benefit of keystone predator maintenance for water quality. These findings provide empirical support for biomanipulation as a cost-effective water quality management tool and quantify keystone predator density thresholds applicable to lake restoration planning under the EU Water Framework Directive.