Variation in tiger salamander density within prairie potholes affects aquatic bird foraging behaviour

Through competitive asymmetry, coexisting fish populations can alter aquatic bird distributions and reduce the reproductive success of their offspring. Gray tiger salamanders (Ambystoma mavortium diaboli Dunn, 1940) may function similarly in fishless prairie potholes. To test the hypothesis that tiger salamanders compete with aquatic birds (including ducks, grebes, and American Coot (Fulica americana J.F. Gmelin, 1789)) for prey resources during the breeding season, 16 potholes were divided into halves by an impermeable plastic barrier and tiger salamander densities were increased or decreased on opposite sides of the barrier. From late May until early July, aquatic invertebrates were sampled from both sides of the divider and the foraging activities of birds were observed. Invertebrate samples revealed that densities of most macroinvertebrate taxa were lower in the halves of potholes with relatively higher densities of tiger salamanders. On average, bird occupancy was evenly distributed on either side of the divider. In contrast, birds spent 65% more time foraging where tiger salamander densities were low (71.2% of total time observed) than where they were high (46.2% of total time observed). The birds most responsive to differences in tiger salamander density and food availability were dabbling ducks, followed by American Coot and diving ducks. This ranking corresponds to the degree of dietary overlap documented between tiger salamanders and aquatic bird assemblages in the study area.