Direct and Indirect Effects of Fish on Invertebrates and Tiger Salamanders in Prairie Pothole Wetlands

Abstract

Hydrologic changes have facilitated increased fish abundance in wetlands of the North American prairie pothole region (PPR). Declines in PPR wetland condition have been attributed to fish-generated changes in trophic state, and reduced abundance and diversity of plants, invertebrates, and non-fish vertebrates. We sought to gain insight into mechanisms for fish impacts on wetland condition by evaluating their direct and indirect effects on invertebrates and salamanders. We quantified relationships among trophic state variables (chlorophyll a, nitrogen, phosphorus, total suspended solids, turbidity); fish and larval tiger salamander biomass; plant cover and taxon richness; and invertebrate density, biomass, and taxon richness in 34 continuously-flooded wetlands in Iowa. Path analysis results supported hypotheses that fish indirectly caused declines in invertebrate taxon richness by increasing turbidity, which in turn reduced plant cover. Path analysis results also supported the hypothesis that fish directly reduced salamander biomass by predation. Fish biomass increased with increasing water depth. Therefore, deeper wetlands were generally in poorer condition than relatively shallow ecosystems. Our findings support conclusions of previous investigators that regardless of other management strategies, actions limiting fish biomass in PPR wetlands (e.g., eliminating surface-water connections to other aquatic systems) are essential to restore health and functions of these ecosystems.