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Results from our 3 study years indicate that fathead minnows were consistently associated with reduced densities of amphibians, zooplankton, and some nektonic macroinvertebrates in both restored and natural semipermanent wetlands. These effects were dramatic, but variable. Adult tiger salamanders, for example, reflected strong fathead minnow effects in 1997 and 1998, but we observed no similar response in 1996. This disparity likely resulted from sparse catches of adult salamanders in 1996 (and low power of ensuing tests); we captured almost three times as many in 1997. During 1996, our first sampling was considerably later than initial dates in 1997 and 1998. Deutschman and Peterka (1988) reported that adult tiger salamanders began immigration to North Dakota wetlands in early April, reproduced during early May and emigrated by the end of the month; we probably missed these emigrating adults in 1996.

Our finding of decreased amphibian abundance in relation to fathead minnows is especially interesting because it contradicts results of related studies. Wiedenheft (1983) reported larval salamanders in a large wetland with fathead minnows, and Sexton and Phillips (1986) detected only modest effects following minnow introduction to three Missouri ponds.

Decreased amphibian abundance in our sites may result from adult avoidance of wetlands with fatheads, competition with fish for food resources (invertebrates), and fathead minnow predation on amphibian eggs and/or young larvae. Adult avoidance of wetlands with minnows is supported by our collections of adult salamanders during 1997. Unfortunately, adult frogs were captured in low numbers during both years, making it difficult to test related hypotheses.

Deutschman (1984) reported that crustaceans (cladocerans, copepods, and amphipods) comprised 94% of larval tiger salamander diet by weight. Both Held and Peterka (1974) and Deutschman and Peterka (1988) noted the potential for food competition between fatheads and larval salamanders. Information supporting our hypothesis of fathead minnow predation on amphibian eggs and larvae is unavailable and Deutschman and Peterka (1988) suggested that predation on larval amphibians is unlikely due to gape-limitations of fathead minnows. Held and Peterka (1974) found no amphibian larvae or eggs in the stomachs of 541 fathead minnows.

Presence of fathead minnows in our study wetlands was associated with decreased abundance of functionally important groups of invertebrates. Others have demonstrated that fathead minnows consume aquatic invertebrates (Held and Peterka 1974, Price 1991, Duffy 1996); thus the decreased abundance of invertebrates we observed is not surprising and likely due to predation by the fish (as suggested by Hanson and Riggs 1995). Suppression of zooplankton and macroinvertebrates, lower water transparency, and increases in algae and nutrients reflect major functional reorganization of wetland communities, food webs, and basic ecological processes. It is reasonable to expect that these changes may adversely impact numerous other wetland-dependant species (Bouffard and Hanson 1997). For example, these changes have obvious implications for female ducks and young ducklings whose diets consist mostly of aquatic invertebrates (Swanson et al. 1979, Swanson et al. 1985, Krapu and Reinecke 1992, Cox et al. 1997). Tiger salamanders also rely heavily on wetland invertebrates as their major food resource (Deutschman 1984). We expect that salamander larvae are especially vulnerable to lower food availability because they forage almost exclusively on aquatic invertebrates and are restricted to a single wetland until they metamorphose to the adult stage. At the landscape level, competition for food resources may be especially severe in low-rainfall years and in regions where amphibians and waterfowl are forced to use semipermanent wetlands (the type most commonly supporting fathead minnows) as breeding habitat.

We failed to detect effects of fathead minnows on the abundance of submersed macrophytes. This may be due to relatively shallow depths of study wetlands where light available for plant growth is a function of both water clarity and depth. Though turbidity was higher in wetlands with fathead minnows, turbid sites may have been shallow enough to permit sufficient amounts of light to reach plant propagules in sediments. Due to the relation between turbidity and depth, we expect deepest areas of wetlands to be most likely to exhibit decreased plant growth; thus we are currently exploring analyses with plant data collected at only deeper sites.

Finally, fathead minnows influenced most water quality parameters we considered. Higher levels of water-column total phosphorus were evident throughout our study in wetlands with fathead minnows. Fish egestion, excretion, and sediment resuspension may be major sources of phosphorus in lakes (Brabrand et al. 1990, Carpenter et al. 1992). Fish activities modify pathways and cycling rates among nutrient pools and it is plausible that at high fish densities phosphorus accumulates in the water-column pool and is rapidly recycled. Our fish sites also supported higher phytoplankton biomass (chlorophyll a), probably resulting from both decreased herbivory (by large cladocera) and higher nutrient availability associated with fish (Vanni and Layne 1997; Zimmer et al. 1998). Large-bodied cladocerans (like Daphnia) have important functional roles in reducing phytoplankton biomass (Irvine et al. 1989), and were sharply reduced in our sites with fathead minnows. Higher turbidity in wetlands with these fish is likely due to this increased phytoplankton abundance and possibly rates of sediment resuspension by fish. Roles of fathead minnows in nutrient cycling are discussed in a companion summary in this volume (Zimmer et al. 1998).

Effects of Wetland History:
We detected few differences between restored and natural wetlands and this seems to reflect strong functional similarity between restored and natural areas. This ecological similarity between natural and restored wetlands is encouraging and supports the notion that current restoration techniques are successful in reestablishing prairie wetland function, at least in regards to semipermanent wetlands and variables measured in our study. Also, few significant history effects, compared to the large number of fish effects, probably indicates that presence/absence of fathead minnows is a more important determinant of wetland ecosystem structure than whether a wetland is restored or natural. Rather surprising was a general lack of statistical interactions between wetland history and presence of minnows. In other words, fathead minnows have a strong influence on wetland ecosystems, but responses were similar in both restored and natural wetlands.