Doctor of Philosophy / Department of Biology / Keith B. Gido / Global climate change could lead to less frequent but more severe precipitation events in the Great Plains, altering the hydrologic regimes of streams. It is important to quantify species roles in these dynamic systems, because changes in stream communities are likely to accompany predicted changes in hydrology. The effects of species on ecosystem processes also are limited by the frequency of disturbance, because prairie streams are harsh, nonequilibrium systems characterized by a wide range of disturbances. In particular, frequent floods that reset the ecosystem to an early successional state can override the influence of consumer populations because the availability of resources is too unpredictable to maintain stable populations of those species or because species are absent following the flood. As flood frequency decreases, potential consumer effects may intensify. Using a combination of field and experimental stream mesocosm experiments, I (1) characterized the ecosystem effects of southern redbelly dace (Phoxinus erythrogaster), a grazing minnow, (2) tested the interactive effects of flood frequency and the presence of water column (red shiner; Cyprinella lutrensis) or grazing minnows (Phoxinus) on ecosystem processes, and (3) tested the effects of species loss from the grazer functional feeding group on stream ecosystem structure and function. I found that dace affected some aspects of ecosystem structure but not function, which suggested that grazer effects in prairie streams may not be consistent across taxa. In the context of flood frequency, both the water column omnivore and dace affected recovery of prairie stream primary producers following flooding disturbance by stimulating production, presumably through nutrient remineralization. However, some of these effects were transient or dependent on flood frequency, and my results indicate that consumer effects depend not only on environmental venue but also on the balance between consumptive losses and nutrient stimulation. In a comparison of the effects of removing different taxa from a grazer assemblage, the loss of crayfish, snails, or dace from a grazer assemblage did not differentially affect ecosystem processes, suggesting overlap in the ecosystem roles of these species in the context of this experiment.

North American prairie headwater streams are highly threatened and relatively poorly studied. Most studies on prairie streams have occurred at the Konza Prairie Biological Station, a tallgrass prairie Long Term Ecological Research site in the Flint Hills ecoregion near Manhattan, KS. According to the Stream Biome Gradient Concept, several ecosystem factors vary along a gradient from more allochthonous forested streams to more autochthonous desert streams, with grassland streams often intermediate in several key ecological factors including litter inputs, primary production, and invertebrate abundance and biomass. However, few studies have examined the degree of variation that exists within prairie headwater streams, and whether this variation occurs along a longitudinal gradient as well, with more mesic tallgrass prairie streams differing from more xeric shortgrass prairie streams, and mixed-grass sites intermediate between the two. I examined thirteen prairie headwater stream sites in the central United States from 2014 to 2017. My objective was to determine whether basal resource composition – including standing stocks of coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and very fine particulate organic matter (VFPOM), sestonic and benthic chlorophyll-a levels, and sources of CPOM – differed significantly among streams in tallgrass, mixed-grass, and shortgrass prairie regions. In addition, I examined whether invertebrate communities differed among tallgrass, mixed-grass, and shortgrass prairie regions, and whether this was reflected in the functional feeding group composition, habit composition, voltinism, and dispersal ability of invertebrate communities. There were no significant differences in total CPOM, FPOM, and VFPOM standing stocks among regions. However, CPOM composition did differ with region, with tallgrass sites having higher standing stocks of leaf litter, but lower standing stocks of grass litter and macrophyte litter than the other regions. Benthic chlorophyll-a did not differ significantly among regions, but there were lower sestonic chlorophyll-a levels in tallgrass sites. Given higher light availability and nutrient levels in shortgrass and mixed-grass streams, lack of stable substrata may be limiting benthic algae in these regions. Invertebrate abundance and biomass were highest in mixed-grass sites and lowest in tallgrass sites, with shortgrass sites intermediate. Mixed-grass sites also had significantly higher Shannon diversity and taxa richness than tallgrass sites. A NMDS revealed that sites differed in overall community structure. Functional feeding group composition did not differ significantly across regions, with collector-gatherers, followed by predators, dominating in all sites. High Predator-Prey Index (predator biomass: other invertebrate biomass) values in each region indicates strong top-down pressure and high turnover rates of prey taxa. While there was a weak correlation between leaf litter AFDM and invertebrate community structure, the correlation between latitude and longitude and invertebrate community structure was much higher, indicating that basal resources may not be the main drivers in these systems. Invertebrate habit composition did not differ with region; all regions were dominated by taxa preferring fine substrata (burrowers and sprawlers). All regions showed selection against semivoltine invertebrates and were dominated by high dispersing insect taxa, though the proportion of the insect community that consisted of high dispersers did not differ significantly with region. My results suggest that generalizing about prairie streams based on studies from one or a handful of sites may not be prudent, at least for some aspects. The higher proportion of autochthonous inputs in shortgrass and mixed-grass regions may drive increased invertebrate abundance, biomass, richness, and diversity, but the relative hydrologic stability of the mixed-grass sites might also explain these results or have an interactive or additive relationship with primary production. Overall, the link between basal resources and communities across prairie types was somewhat weak, and all regions were dominated by collector-gatherers with rapid life-cycles and high dispersal abilities, indicating that disturbance may be a more important community filter than basal resource composition. Streams in all three regions have highly variable hydrology, and this may be an overriding factor that results in similarity in communities.