Ecosystem processes in grasslands are largely determined by dominant species’ response to ecological drivers. In the Great Plains, large grazers selectively feed on dominant grasses to affect grassland structure and functioning. However, the degree to which grazing acts as a selective force on genetic variation in grasses, the role of grazers in evolutionary change in plant species, and consequences for ecosystem processes is unknown. Thus, our overall goal is to test whether grazing alters the genetic structure of foundation grass species and results in genetically-based variation in functional traits that cascades to affect ecosystem processes. We investigated whether grazing alters the genetic structure, genetic diversity, and functional traits of a foundation grass species in tallgrass prairie using 30 year old grazing exclosures. We hypothesize that grazing, known to alter local abiotic and biotic environmental conditions, is a selection pressure that changes genotypic frequencies and functional traits with consequences for ecosystem processes. This knowledge will deepen our conceptual understanding of linkages between evolutionary and ecosystem processes, and how evolutionary trajectories may influence ecosystem responses to environmental change.
We collected leaf samples from 43 Andropogon gerardii plants on 4 transects from each of side-by-side grazed (n=43; grazed for 30+ y) and ungrazed watersheds (n=43; ungrazed for 30 y) at the Konza Prairie LTER site, where the only variable differing is grazing history. We used Illumina Hi-Seq and identified ~12,000 single nucleotide polymorphisms (SNPs) mutations in the DNA. We found that genetic variation differed between grazed and ungrazed grassland. STRUCTURE analyses identified 3 genetic clusters within grazed and ungrazed watersheds. However, the relative proportion of each genetic cluster differed, with grazed areas containing a balanced mix of genetic clusters, and ungrazed areas had primarily only one. Genetic outlier analysis identified ~15 outlier SNPs showing a signature of divergent selection. Furthermore, out of 12,000 SNPs identified, ~500 private alleles were found in the grazed grassland with approximately 20 found at frequency of about 25%, in comparison to their virtual absence in the ungrazed grassland. These data suggest that evolutionary change and genetic differentiation is occurring in response to a key ecological driver. These studies will reveal evolutionary responses to an ecological driver and if there is selection for genotypes and phenotypes on contemporary timescales.