The influence of multiple introduced predators on a littoral pond community

In a replicated field experiment we studied the effects of natural densitie s of two exotic consumers, the predatory and herbivorous signal crayfish (P acifastacus leniusculus) and the predatory rainbow trout (Oncorhynchus myki ss), on multiple trophic levels of a pond community. The goals were to: (1) determine the individual and combined effects of predators on macroinverte brates, macrophytes, and periphytic algae; (2) evaluate the strength of dir ect 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 stron g effects on multitrophic levels of a littoral pond community, through dire ct consumption and indirect effects on lower trophic levels. Crayfish had w eak but significant negative effects on the biomass of predatory invertebra tes and greatly reduced the biomass of snails, the most abundant invertebra te grazers. Although the number of active herbivorous tadpoles tended to be higher in crayfish cages relative to control cages, the proportion of surv iving froglets was lower in crayfish cages than in control cages, possibly due to crayfish predation on injured tadpoles. The size of surviving frogle ts did not differ from controls, but tadpoles in crayfish cages often suffe red tail injuries. Macrophyte coverage decreased as a result of crayfish co nsumption and nonconsumptive fragmentation. However, the biomass of periphy ton increased in crayfish cages relative to controls, probably due to reduc ed grazing from snails. In contrast, trout had strong negative effects on t he biomass of both predatory invertebrates and insect grazers, whereas trou t 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-in duced reduced tadpole activity. Trout had a strong negative impact on frogl et survival, and froglets in trout cages metamorphosed at a smaller size an d had reduced growth rates compared to froglets in crayfish and control cag es. As with crayfish, the biomass of periphyton increased in trout cages re lative to controls, which may be due to a combination of both density and t rait-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. Ou r results demonstrate that noninteracting, introduced multiple predators ca n have strong direct and indirect effects on multiple trophic levels in pon d communities. Trophic cascades may develop in aquatic food webs even with omnivores such as crayfish, and in complex habitats with trout. These stron g indirect effects are mediated through both predation on important grazers (i.e., the crayfish-snail-periphyton link) and a combination of density an d behavioral responses of grazers to predators (i.e., the trout-tadpole-per iphyton link). When two noninteracting predators have strong but different effects on prey survival or activity, their combined effects on intermediat e trophic levels reflect responses to the more dangerous predator.