The Influence of Multiple Introduced Predators on a Littoral Pond Community

Abstract

In a replicated field experiment we studied the effects of natural densities of two exotic consumers, the predatory and herbivorous signal crayfish (Pacifastacus leniusculus) and the predatory rainbow trout (Oncorhynchus mykiss), on multiple trophic levels of a pond community. The goals were to: (1) determine the individual and combined effects of predators on macroinvertebrates, macrophytes, and periphytic algae; (2) evaluate the strength of direct and indirect interactions in a food web influenced by omnivores; and (3) evaluate the relative importance of direct and indirect predator effects on mortality and growth of a native frog species, Rana temporaria. The experiment showed that both signal crayfish and rainbow trout had strong effects on multitrophic levels of a littoral pond community, through direct consumption and indirect effects on lower trophic levels. Crayfish had weak but significant negative effects on the biomass of predatory invertebrates and greatly reduced the biomass of snails, the most abundant invertebrate grazers. Although the number of active herbivorous tadpoles tended to be higher in crayfish cages relative to control cages, the proportion of surviving froglets was lower in crayfish cages than in control cages, possibly due to crayfish predation on injured tadpoles. The size of surviving froglets did not differ from controls, but tadpoles in crayfish cages often suffered tail injuries. Macrophyte coverage decreased as a result of crayfish consumption and nonconsumptive fragmentation. However, the biomass of periphyton increased in crayfish cages relative to controls, probably due to reduced grazing from snails. In contrast, trout had strong negative effects on the biomass of both predatory invertebrates and insect grazers, whereas trout had less effect on snail biomass than did crayfish. Also, in contrast to crayfish cages, the number of active tadpoles in trout cages was lower than in controls, probably due to a combination of trout predation and trout‐induced reduced tadpole activity. Trout had a strong negative impact on froglet survival, and froglets in trout cages metamorphosed at a smaller size and had reduced growth rates compared to froglets in crayfish and control cages. As with crayfish, the biomass of periphyton increased in trout cages relative to controls, which may be due to a combination of both density and trait‐mediated trout effects on tadpole grazing. In treatments with multiple predators the effects of crayfish and trout on caged communities were independent, and there were few interactions. Mostly effects of combined predators reflected those in single predator cages. Our results demonstrate that noninteracting, introduced multiple predators can have strong direct and indirect effects on multiple trophic levels in pond communities. Trophic cascades may develop in aquatic food webs even with omnivores such as crayfish, and in complex habitats with trout. These strong indirect effects are mediated through both predation on important grazers (i.e., the crayfish–snail–pe...

Interactions among invaders: community and ecosystem effects of multiple invasive species in an experimental aquatic system

“…Orconectes substantially increased the amount of periphyton, most likely as an indirect result of a trophic cascade: crayfish predation reduced the abundance of grazing native snails, thereby increasing the amount of periphyton in the system. Similar results from crayfish predation have been reported by Nyström et al (1999Nyström et al ( , 2001. Grazing by Bellamya, in contrast, negatively affected periphyton levels on the mesocosm walls and especially the sediment, where Bellamya were most commonly observed during the experiment.…”

“…As found in several previous studies, predation by invasive crayfish sharply reduced native snail biomass (Lodge et al 1994;Nyström et al 1999Nyström et al , 2001; by virtue of its large size and thick shell, however, Bellamya was relatively unaffected by Orconectes, but did have significant competitive consequences for two of the three native snail groups. How results from these experimental systems scale up to entire lake ecosystems is an important but as of yet unanswered question.…”

Effects of stream predator richness on the prey community and ecosystem attributes

“…To quantify predator eVects on invertebrate activity we used active-fauna traps that are modiWed plastic beakers with a mesh (1.0 mm) funnel entrance and a removable mesh lid (1.0 mm) at the other end (see Nyström et al 2001 for further details). Three traps were placed in each stream channel for both day-(1000-1700 hours) and nighttime activity (2200-0700 hours) once for each trial period before ending the experiments.…”

“…First, predatory species that have diVerent foraging strategies may aVect diVerent parts of the food web (Chalcraft and Resetarits 2003). Hence, predators with diVerent foraging strategies can, in combination, have emergent eVects on lower trophic levels and reduce or enhance prey predation risk (Sih et al 1998;Eklöv and VanKooten 2001;Nyström et al 2001). Indeed, increasing predator diversity has been shown to alter trophic relationships and induce trophic cascades both in terrestrial and marine systems (Finke and Denno 2004;Bruno and O'Connor 2005;Byrnes et al 2006).…”

Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis

“…ecosystem engineering) (Lodge and Lorman 1987;Nyström and Strand 1996;Nyström et al 2001;Gherardi and Acquistapace 2007). Crayfish destroy much more plant tissue than they eat, using chelipads in the established stage of macrophytes, while direct consumption is the main mechanism for macrophyte reduction in the seedling stage (Nyström and Strand 1996).…”