Effects of land use and invasive fishes on prairie pothole wetland condition
Michael J. Weber
Prairie pothole wetlands provide many valuable ecosystem services. However, land use changes in the prairie pothole region (PPR) have increased contaminant loadings and facilitated invasions by fishes, leading to declines in biological diversity and wetland condition. Primary objectives of my thesis research were to 1) quantify direct and indirect relationships among nine environmental attributes, including land use, wetland water volume, herbicides, chloride, invasive fishes, turbidity, plant and invertebrate assemblages, and tiger salamander Ambystoma tigrinum abundance, 2) use these relationships to identify a set of wetland condition metrics, and 3) evaluate how wetland ecosystems responded when invasive fishes were eliminated following a drought. In chapter one of my thesis, multiple regression analysis was used to quantify relationships among attribute variables in 45 permanently/semipermanently flooded prairie pothole wetlands. Results from multiple regression indicated that wetland water volume (measured as surface area) and water column chloride concentration increased as crop land and developed land percent cover increased in the watershed. Contrary to predictions, abundance of fishes, primarily fathead minnow Pimephales promelas and black bullhead Ameiurus melas, were unrelated to wetland surface area. However, results suggested that bioturbation or nutrient excretion by fishes caused increases in turbidity, which in turn caused declines in plant cover within wetlands. As predicted, tiger salamander Ambystoma tigrinum abundance and macroinvertebrate taxon richness also declined within wetland basins as fish abundance increased. Additionally, plant cover declined across a gradient of increasing herbicide concentration in the water column. In chapter two of my thesis, a repeated measures ANOVA was used to compare several wetland condition indicator variables (tiger salamander biomass and numerical abundance, turbidity, plant cover and taxon richness) from a subset of 29 prairie pothole wetlands in 2010-2011 (pre-drought period) and 2014-2015 (post-drought period). Pre- and post-drought values were compared among 1) all wetlands where fishes were eliminated by the drought 2) wetlands where large-bodied benthic (black bullhead) fishes were eliminated by the drought, 3) wetlands where small-bodied/open water (native cyprinids, brook stickleback Culaea inconstans) fishes were eliminated by the drought, 4) wetlands where fish abundance was unchanged, and 5) wetlands where fishes were not detected in either the pre- or post-drought study period. Results from pre- and post- drought comparisons suggested water clarity and plant cover increased where large-bodied benthic fish abundance was reduced, likely due to reduced bioturbation. Plant taxon richness and numerical abundance of tiger salamanders increased where total fish abundance was reduced, possibly as a result of reduced predation or competition. Collectively, these findings suggest that while wetlands are negatively affected by fishes through declines in water clarity, plant and salamander abundance, and invertebrate diversity, they have the capacity to quickly recover when fish abundance declines. Results also suggest that chloride concentration is a good indicator of human land use intensity in the watershed, and that elevated herbicide concentrations cause reductions in plant abundance. Efforts to reduce herbicide use and fish abundance would likely improve wetland condition and increase biological diversity.