Predator Diversity Effects in an Exotic Freshwater Food Web

Abstract

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.

Predator‐induced morphological defences in two invasive dreissenid mussels: implications for species replacement

“…Kats & Dill, ; Hettyey et al ., ). But in an experiment using both mussel species as herbivores and the same density of predators (three individuals per predator species) in a three‐trophic level experiment, we also found higher non‐consumptive cascading effects of crayfish on phytoplankton than pumpkinseed and round goby (Naddafi & Rudstam, ), supporting our belief that the induced response in dreissenids depends on the identity rather than the density of the predator.…”

“…In this study, we measured the dreissenids' responses to three important North American predators: pumpkinseed sunfish, Lepomis gibbosus , and rusty crayfish, Orconectes rusticus , which are native to North America, and round goby ( Neogobius melanostomus ), which is native in the ancestral range of the mussels. In addition to having different feeding strategies (Naddafi & Rudstam, ), these predators have different evolutionary histories with the prey. We hypothesised that the mussel species with the lowest cost of induced defences would dominate in the new environment where native predators are not well adapted to utilise these two species.…”

A global synthesis of predation on bivalves

“…Through filter feeding, bivalves reduce the abundance of phytoplankton. In a laboratory experiment, Naddafi & Rudstam (2013 a ) showed that the presence of fish and crayfish predators can lead to increased abundance of phytoplankton via a trophic cascade. When these predators consumed non‐native dreissenid mussels, phytoplankton abundance increased, potentially resulting in harmful algal blooms, low water transparency, and reduced oxygen levels that can affect the entire ecosystem (Hallegraeff, 2003; Rensel & Whyte, 2003).…”