Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item How the Availability of Nutrients and Energy Influence the Biodiversity of Cave Ecosystems(2009) Schneider, Katie; Fagan, William F.; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Resource constraints can affect species on multiple levels. In this dissertation, I combine laboratory experiments, an ecosystem-level manipulation experiment and statistical modeling to examine how resources maintain and constrain cave biodiversity and structure cave communities. Chapter I examines how N-limitation may drive morphological adaptations of cave arthropods. By analyzing free amino acid contents, I show that, in comparison to cave-transient millipedes, cave-obligates have decreased concentrations of essential, nonessential and N-rich amino acids, and amino acids associated with pigmentation and cuticular development. Chapter II tests the hypothesis that stoichiometric mismatches impose growth constraints on cave animals. Although results show that cave resource quality is similar to surface leaves, I do show that millipedes experience a strong mismatch to their food. Also, cave-obligate millipedes have lower %P and RNA/DNA (protein synthetic capacity) compared to cave-transient millipedes. Results from these chapters suggest that cave adaptations may reflect stoichiometric challenges of caves. Chapter III describes the manipulation experiment, wherein I removed all organic material from 12 caves, and, while excluding all natural subsidies, I added standardized quantities of leaf packs or rodent carcasses. For 23 months, I monitored the recipient communities to see how subsidies influence species abundance, diversity, and community dynamics. I observed 19,866 arthropods representing 102 morphospecies. Rat treatments supported greater abundances, but the treatments did not differ in richness. Multiple community-level analyses demonstrated that community composition differed drastically depending on treatment. Lastly, the communities changed directionally over time, diverging faster in caves receiving leaves. Chapter IV uses annual bioinventories of 65 caves to investigate occupancy patterns of terrestrial invertebrates. I estimated richness using classical estimators in concert with estimators that incorporate detection. I also used multispecies occupancy models to examine relationships between estimated richness and physical cave characteristics; demonstrating the importance of cave length, entrance geometry (a surrogate for energy input), and connectivity. The results show how inventory data, even if incomplete, can provide valuable information about the distribution of rare species. Resource availability can affect cave ecosystems on multiple levels. Here I illustrate how the biochemical composition, community dynamics, and occupancy patterns of cave species are influenced by resource constraints.Item Photolytic Studies of Aryl and Heteroaryl Nitrenium Ions: Laser Flash Photolysis Studies(2006-05-07) Thomas, Selina Ivan; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of the thesis was to understand the chemical and kinetic behavior of arylnitrenium ions and a heteroaromatic nitrenium ion through photolytic studies. N-(4,4'-dichlorodiphenyl) nitrenium ion and N-(4,4'-dibromodiphenyl) nitrenium ion are the halogenated counterparts of diphenylnitrenium ions and are generated photochemically from their respective N-(4,4'-dihalogenated diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate salts. The halogenated diarylnitrenium ions are ground state singlets that live for more than 1 x 105 ns in acetonitrile. In the absence of nucleophiles, these ions decay to form a dimerized hydrazine. These ions react with nucleophiles such as water and alcohol at a rate constant of 104 - 105M-1s-1 and at diffusion limit with chlorides. With arenes, these ions react via electron transfer mechanism and nucleophilic addition process. The rate constants for the electron transfer mechanisms are between 105 - 109 M-1 s-1 and depend on the Eox of the arenes. Arenes with Eox above 1.78 V showed no reactivity towards the ion. Unlike other diarylnitrenium ions, the halogenated diarylnitrenium ions react with hydrogen atom donors via a hydrogen atom transfer mechanism. The triplet behavior of these ions is attributed to singlet-triplet intersystem crossing facilitated by the lower singlet-triplet energy gap. Therefore, it has been concluded that substituting halogens in diphenylnitrenium ion lowers the singlet-triplet energy gap and increases the lifetime of these ions. (N-Methyl-N-4-biphenylyl) nitrenium ion generated by photolysis was reacted with amino acids and proteins to determine their reactivity with these ions. Eight amino acids were observed to react with the ion at a rate constant of 107 - 109 M-1 s-1. The rate constants depend on the nucleophilicity of the side chains of the amino acids. In addition, this ion also reacts rapidly with proteins with a rate constant of 108 M-1 s-1, comparable to their reactions with ss-DNA. Investigations on generating the quinoline N-oxide nitrenium ions showed that the transient species from the photolysis of 4-azidoquinoline N-oxide (4-AzQO) shows more characteristics of a nitrenium ion. The formation of 4-aminoquinoline N-oxide upon photolysis of 4-AzQO in acetonitrile with 10% sulphuric acid and the electron transfer reactions observed with arenes, indicate that the transient species generated could be a heteroaromatic nitrenium ion. However, more experiments are needed to confirm the assignment.