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We examined the hypothesis that direct predation is part of the explanation for the displacement of zebra (Dreissena polymorpha) by quagga mussels (Dreissena rostriformis bugensis) in North America. We investigated selectivity and feeding rates by 3 predators (Round Goby, Pumpkinseed Sunfish, rusty crayfish) using 2 or 3 size classes of predators feeding on different size classes of both mussel species. Round Goby, the predator with a long evolutionary history with dreissenids, consumed a larger range of dreissenid sizes than the other 2 predators. Pumpkinseed Sunfish did not show a preference for either mussel species, but both rusty crayfish and gobies preferred quagga mussels over zebra mussels. The size of mussels vulnerable to these predators increased with predator size, but all predators preferred the smallest mussels (4-8 mm). Medium (105-125 mm) and large (135-155 mm) Round Gobies consumed the largest size class (16-20 mm) of quagga mussels, but only large Round Gobies were able to consume 16-20-mm zebra mussels. Crayfish consumed a higher number of quagga than zebra mussels in both the 8-12-mm and 12-16-mm size classes. Large Pumpkinseed Sunfish (170-185 mm) were able to consume the largest size class of quagga but not zebra mussels. Our results do not support direct predation as the cause for quagga mussels replacing zebra mussels. However, predation may affect mussel size structure because small mussels were more vulnerable to predation than larger mussels. [ABSTRACT FROM AUTHOR]

Cascading trophic interactions are often defined as the indirect effects of a predator on primary producers through the effect of the predator on herbivores. These effects can be both direct through removal of herbivores [density-mediated indirect interactions (DMIIs)] or indirect through changes in the behavior of the herbivores [trait-mediated indirect interactions (TMIIs)]. How the relative importance of these two indirect interactions varies with predator diversity remains poorly understood. We tested the effect of predator diversity on both TMIIs and DMIIs on phytoplankton using two competitive invasive dreissenid mussel species (zebra mussel and quagga mussel) as the herbivores and combinations of one, two or all three species of the predators pumpkinseed sunfish, round goby, and rusty crayfish. Predators had either direct access to mussels and induced both TMII and DMII, or no direct access and induced only TMII through the presence of risk cues. In both sets of treatments, the predators induced a trophic cascade which resulted in more phytoplankton remaining with predators present than with only mussels present. The trophic cascade was weaker in three-predator and two-predator treatments than in one-predator treatments when predators had direct access to dreissenids (DMIIs and TMIIs). Crayfish had higher cascading effects on phytoplankton than both pumpkinseed and round goby. Increased predator diversity decreased the strength of DMIIs but had no effect on the strength of TMIIs. The strength of TMIIs was higher with zebra than quagga mussels. Our study suggests that inter-specific interference among predators in multi-species treatments weakens the consumptive cascading effects of predation on lower trophic levels whereas the importance of predator diversity on trait mediated effects depends on predator identity. [ABSTRACT FROM AUTHOR]

Predators may induce changes in prey feeding that indirectly influence both the impact of prey on resource abundances and their interactions with other species in their community. We evaluated whether clearance and excretion (faeces plus pseudofaeces) of phytoplankton by zebra mussels were affected by the presence of predatory cues from roach, Rutilus rutilus, and signal crayfish, Pasifastacus leniusculus. We found that non-lethal effects of predators can alter zebra mussel clearance rate and thus the impact of zebra mussels on phytoplankton. Risk cues released by both predators had similar negative effects on clearance rate of zebra mussels and cascading positive indirect effects on phytoplankton resources. Predation risk had a stronger effect on zebra mussels’ clearance rate of cyanobacteria and diatoms than cryptophytes and chrysophytes. The presence of predators did not significantly affect the rate at which zebra mussels expelled and excreted phytoplankton, although there was a tendency for more chlorophyll to be expelled and excreted in the presence of predators. Our results contribute to the growing evidence that predators indirectly affect resource dynamics and food web structure through their non-lethal effects on consumers. Our results suggest that exotic species such as zebra mussels can show behavioural responses to both native (e.g. roach) and exotic (e.g. crayfish) predators. [ABSTRACT FROM AUTHOR]