Waterfowl are attracted to prairie wetlands or "potholes" to satisfy nutritional demands for reproduction and brood rearing during the breeding season. Although most potholes in the Minnedosa, Manitoba, area are devoid of native fish populations, many are occupied by tiger salamanders (Ambystoma tigrinum). To test the hypotheses that waterfowl influence pothole community structure and indirectly interact with tiger salamanders, I completed a series of observational and manipulative studies.

I determined that tiger salamander occupancy rates of potholes were high (75-80%) but densities ranged widely. Trophic structure varied according to tiger salamander density but greater contrasts in invertebrate abundance and phytoplankton standing crop were obtained between high and low densities rather than simple presence or absence. I interpreted positive relationships between tiger salamander density and phytoplankton standing crop as an indirect effect of nutrient recycling and transfer by tiger salamanders rather than direct consumer-resource interactions between trophic levels. I compared dietary and pothole overlap between ducks (Subfamily Anatinae) and tiger salamanders to estimate the potential for indirect interactions. I found that dabbling ducks had greater overlap in diet with tiger salamanders than diving ducks, but that diving ducks overlapped more among potholes with tiger salamanders than dabbling ducks.

Using exclosures, I determined that invertebrate populations increased and phytoplankton standing crop decreased in the absence of duck, grebe and coot predation. However, the effects of waterfowl exclusion on pothole trophic structure were obscured by the presence of tiger salamanders, especially at high densities. This indirect interaction is an example of compensatory predation and it indicates that either waterfowl or tiger salamanders can control pothole trophic structure. Subsequently, I manipulated tiger salamander densities to test for the effects of food web variation on waterfowl. Within potholes I found that waterfowl preferentially foraged where tiger salamander densities were lower and among potholes I found that American coot (Fulica americana) chick growth rates were decreased in the presence of tiger salamanders. These results indicate that waterfowl and tiger salamanders exert trophic control on prairie potholes by competing for shared prey and that community studies of prairie potholes are incomplete without the inclusion of tiger salamanders.