Deserts comprise the largest terrestrial biome, making up approximately one third of the Earth’s land mass. They are defined in terms of moisture deficit using the Aridity Index with values <1. A further delineation based on mean annual temperatures into hot (>18°C), cold (<18°C) and polar (<0°C) deserts is employed. In the absence of significant macrobiota, microorganisms are key to desert ecosystems. They are located in near-surface soils, and include a widespread hypolithic mode of colonization, where microbial biomass develops on the ventral surfaces of quartz and other translucent stones. A literature review was conducted to appreciate the status of existing knowledge on these systems. Amongst unresolved questions that arose were the following, which form the basis of this inquiry: What are the taxonomic and functional differences between hypolithic and near-soil communities? Do hypolithic communities assemble differently in deserts of different xeric and thermal stresses? Can the keystone cyanobacterial taxa be cultivated under laboratory conditions to allow manipulative studies? The Mojave Desert in the USA was used as a model to test the extent to which hypolithic and near-surface soil communities vary in both taxonomic and putative functional composition. A common phylogenetic marker (16S rRNA gene ITS region) was used to conclude that soil and hypolithic communities are significantly different, although both were dominated by cyanobacteria. The ubiquitous hypolithic cyanobacterial taxon Chroococcidiopsis was encountered, although communities appeared to be dominated functionally by the diazotrophic genus Nostoc. The data strongly suggest that carbon and nitrogen fixation pathways in desert soils are mediated by the same taxa, although heterotrophic pathways may differ and support distinct assemblages of heterotrophic bacteria. An opportunistic sampling of three sites along a latitudinal gradient in China allowed some inference about adaptations in hypoliths. Communities recovered from the cold Tibetan Desert, Taklamakan Basin Desert, and exposed hillsides in tropical Hong Kong, did not display significant differences at the level of community assembly. This suggests that hypolithic taxa undergo strong selection for xeric and extreme thermal stresses. A cultivation strategy for the keystone taxon Chroococcidiopsis has been lacking and is an obvious impediment to manipulative physiological studies. Here various methods for laboratory cultivation were attempted. This bacterium proved extremely fastidious and displayed slow growth rates. After extensive trials a novel cultivation method was developed. This involved using plastic petri dishes containing liquid growth medium, into which glass coverslips were introduced along with cell suspensions. The surface energy of glass served as a nucleation site for Chroococcidiopsis biofilms (which do not develop on plastic surfaces) and this method was evaluated in growth studies as a means of quantifying growth. This research includes key advances to demonstrate that hypoliths and soil, whilst supporting different communities, likely perform similar functional roles in the desert soil. Selection due to the severe environmental stresses results in similar communities across large latitudinal and environmental gradients. The development of a cultivation strategy paves the way for manipulative physiological studies on these important organisms. / published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy

"June 2002" / Bibliography: leaves 172-198. / viii, 198 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Proposes that the overall outcome of plant interactions along a temporal gradient of resource availability changes from positive during interpulses to negative during pulses. Examines negative interactions between 4 co-dominant chenopod scrubs in arid Acacia papyrocarpa woodlands. Negative interactions were more intense when conditions were least productive. Positive interactions between seedlings also changed over time, depending on the facilitation mechanism. Plant interactions seem to be most intense at the beginning of interpulses when plants are competing for diminishing water, or survivorship is enhanced in the favorable microsites provided by other plants. Later in the interpulse, interactions become less intense as conditions become more stressful and therefore survivorship and growth are affected more by abiotic conditions than plant interactions. / Thesis (Ph.D.)--University of Adelaide, Dept. of Environmental Biology, 2002

"June 2002" / Bibliography: leaves 172-198. / viii, 198 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Proposes that the overall outcome of plant interactions along a temporal gradient of resource availability changes from positive during interpulses to negative during pulses. Examines negative interactions between 4 co-dominant chenopod scrubs in arid Acacia papyrocarpa woodlands. Negative interactions were more intense when conditions were least productive. Positive interactions between seedlings also changed over time, depending on the facilitation mechanism. Plant interactions seem to be most intense at the beginning of interpulses when plants are competing for diminishing water, or survivorship is enhanced in the favorable microsites provided by other plants. Later in the interpulse, interactions become less intense as conditions become more stressful and therefore survivorship and growth are affected more by abiotic conditions than plant interactions. / Thesis (Ph.D.)--University of Adelaide, Dept. of Environmental Biology, 2002

Thesis (M.S. - Renewable Natural Resources)--University of Arizona, 1985. / Includes bibliographical references (leaves 64-69).

Thesis (Ph. D. - Renewable Natural Resources)--University of Arizona, 1986. / Includes bibliographical references (leaves 165-174).

Thesis (Ph. D.)--University of Adelaide, 1987. / Includes bibliographical references (leaves 146-152).

Thesis (Ph. D. - Anthropology)--University of Arizona. / Includes bibliographical references (leaves 317-339).

Thesis (Ph.D.)--University of Adelaide, Dept. of Environmental Biology, 2002. / Accompanying CD-ROM inside back cover, includes Appendices. Bibliography: leaves 242-266.

Thesis (M. Sc. Agric(Pasture science))--University of Pretoria, 2005. / Includes bibliographical references. Available on the Internet via the World Wide Web.

The proposition that environment is of equal importance to time and space in the study of culture suggests that a three-dimensional approach may be a useful method for studying the process of cultural evolution. This possibility is tested by detailed investigations into the history and natural habitat of the prehistoric Hohokam Culture. This culture developed an extensive system of irrigation agriculture in the Gila River valley of southern Arizona about A. D. 1-1400. The study is carried out on the processual as well as analytical level of enquiry whereby primary attention was given to the relationship between environment and culture. This emphasis is particularly relevant in the case of the arid conditions of the region in which the Hohokani Culture developed. The interaction of the primary environmental agents, water and soil, and the cultural agent, prehistoric man, is delineated in terms of the natural processes of soil development and movement of water in the region as well as the cultural process of irrigation agriculture. During all phases of the River liohokarn Culture the relationship between soil, water, and man was interdependent and their functional interaction was exposed to the forces of the active environment, which may have become critical. An attempt is made to arrive at an interpretation along these lines for the terminal period of the Hohokam Culture at Snaketown, a major prehistoric site near Chandler, Arizona. While no definite conclusions are derived, the evidence, presented by means of inferential analysis, points strongly towards environmental causation. The impact of the prevailing climatic conditions at the end of the 13th century may have upset the precarious balance between water and soil and thus the livelihood of the desert dwellers. These events led inevitably to the termination of the Hohokam Culture.