Date of Award
Master of Science
Tallgrass prairie in North America has been severely degraded over the past century due to anthropogenic changes and is a subject of many restoration projects. Using these restoration projects, it is possible to examine potential drivers that influence community assembly. The environmental heterogeneity hypothesis provides a basis for enhanced diversity as function of resource partitioning and coexistence of potentially competing species. In essence, an area with higher levels of resource heterogeneity would be able to support a higher number of potentially competing species in contrast to an area with lower levels of resource heterogeneity (e.g. agricultural fields). The tallgrass prairie is naturally heterogeneous in abiotic resources such as soil depth and soil nitrogen, native prairie species both drive and exploit this heterogeneity and assemble a highly biodiverse community. Chapter 2 attempted to elucidate the effect of soil resource heterogeneity on plant community assembly, niche availability, and dimensionality. Chapter 3 attempted to examine the indirect influence of soil resources on aboveground Orthoptera herbivores. Both studies were conducted in a 16-year tallgrass prairie restoration experiment over a two-year period. There were no differences in plant community composition on a whole plot level. However, on a subplot level, shallow soil generally resulted in higher species richness and diversity. In contrast to previous studies, I found nitrogen addition increased forb richness and nitrogen reduction reduced forb diversity. As expected, the dominant grass Andropogon gerardii was positively influenced by high nitrogen regardless of soil depth. The multivariate analysis indicated the new species added to the experiment had unique trait spaces. Further analysis indicated niche availability and dimensionality were highest in treatments with nitrogen addition. This study suggests though fine scale spatial heterogeneity influences plant community composition, coarse scale spatial heterogeneity does not. This study also suggests that soil nitrogen may be a poor indicator of plant species diversity in the tallgrass prairie community. Orthoptera richness and biomass were higher in maximum heterogeneity treatments relative to control. The influence of high resource heterogeneity was highest on the richness of mixed-feeder grasshoppers and katydids. This effect, however, was inconsistent between years. Grass-feeder biomass was higher in the maximum heterogeneity treatment than control both years. This was attributed to nitrogen addition resulting in patches of higher quality forage in the maximum heterogeneity plots. Orthopterans are also influenced by the structural complexity of the plant community mediated by varying levels of soil resources. The maximum heterogeneity treatment contained higher variation in the cover and ANPP of a dominant grass, Andropogon gerardii. The positive relationship between plant richness and Orthoptera diversity suggests that maintaining plant richness in restored areas is important for maintaining diversity of higher trophic levels. The negative relationship between light interceptions and Orthoptera abundance suggests the dense vegetation from dominant tallgrass species may impede recruitment of some species. These results suggest suppression of dominant grasses can positively influence the plant community composition and Orthoptera herbivores. Understanding how soil resources influence plant community composition and higher trophic levels can aid our understanding of the community assembly process. Plant species benefited from higher variation in soil resources, particularly soil depth and soil nitrogen, while insect herbivores that depended on these plant species were also indirectly benefited. This study suggests soil heterogeneity is important for the assemblage of species on a multi-trophic level and this knowledge can assist land managers in restoration projects to achieve desired goals.
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