Removal experiments were conducted to determine particular spatial and temporal conditions that can influence competitive interactions in several desert winter annual species. During the 1987-88 season, variation in the magnitude of competition at three habitats along a topographic gradient was demonstrated in two co-occurring species of winter annuals, Plantago patagonica and Pectocarya recurvata. Density effects on the survival and reproductive success of either species were weakest at the slope. However, the habitats where the two species experienced the most intense competition differed. Plantago was most affected by competition at the wash while Pectocarya was most affected at the base of the hill. The most striking pattern observed was that, for both species, the habitat with the highest reproductive success for plants that were not experiencing competition tended to be the worst habitat for plants in competition. A comparison of results from two experiments performed on Plantago patagonica during two growing seasons showed that competition occurred despite large seasonal differences in weather and plant performance. When wet and dry conditions of different year types were simulated by artificial irrigation during a dry season, competition was still detected in both rainfed and irrigated plants regardless of the marked differences in plant size as a result of the irrigation treatment. A neighborhood density roughly equivalent to 8 plants/dm² appeared sufficient to create competitive conditions for Plantago. Effects of competition were consistently manifested in reduced plant growth and fecundity. There was no evidence for density-dependent seedling mortality (self-thinning) even with seedling densities as high as 48 plants/dm². In two pairs of species tested, Plantago patagonica-Schismus barbatus and Plantago patagonica-Pectorcarya recurvata, there was no statistically detectable effect of neighbor species identity on target plants of Plantago and Pectocarya suggesting the possible equivalence of competitive effects in these species of desert winter annuals.

UA Open Access Publishing Fund / We examined a secondary contact zone between two species of desert tortoise, Gopherus agassizii and G. morafkai. The taxa were isolated from a common ancestor during the formation of the Colorado River (4–8 mya) and are a classic example of allopatric speciation. However, an anomalous population of G. agassizii comes into secondary contact with G. morafkai east of the Colorado River in the Black Mountains of Arizona and provides an opportunity to examine reinforcement of species’ boundaries under natural conditions. We sampled 234 tortoises representing G. agassizii in California (n = 103), G. morafkai in Arizona (n = 78), and 53 individuals of undetermined assignment in the contact zone including and surrounding the Black Mountains. We genotyped individuals for 25 STR loci and determined maternal lineage using mtDNA sequence data. We performed multilocus genetic clustering analyses and used multiple statistical methods to detect levels of hybridization. We tested hypotheses about habitat use between G. agassizii and G. morafkai in the region where they co-occur using habitat suitability models. Gopherus agassizii and G. morafkai maintain independent taxonomic identities likely due to ecological niche partitioning, and the maintenance of the hybrid zone is best described by a geographical selection gradient model.

Durch Messungen von spektraler optischer Dicke, Sonnenaureole und Himmelshelligkeit in Kombination mit den Programmen CIRATRA und BILANZ werden die optischen Eigenschaften von Sahara-Wüstenaerosol und dessen klimatische Wirkung unter Berücksichtigung der Nichtsphärizität der Aerosolpartikel untersucht. / By combining measurements of spectral optical thickness, solar aureole and sky brightness with the programs CIRATRA and BILANZ the optical porperties of Saharan desert aerosol and its climatic effects are investigated, including a consideration of the Nonsphericity of the aerosol particles.

An investigation of the life history of the western chuckwalla, Sauromalus obesus, was undertaken in the Colorado Desert of southeastern California during a 7-year period. The study provided for a test of current life history theory and the modeling of population dynamics from environmental parameters. Colorado Desert S. obesus were mainly active February through September, but feeding occurred throughout the year. Coyotes were this species' primary predator. Home range sizes of males and females were correlated with adult female activity and nutrient requirements, respectively. The breeding status of females was determined by the presence of preovulatory follicles, oviductal eggs and copora lutea. The breeding status of males was best determined by the presence of sperm in the vas deferens and by levels of spermatogenic activity. Adult females always oviposited during the first half of July. Size at reproductive maturity for both sexes was about 125 mm snoutvent length. Age at maturity for males and females was 2 and 3 years, respectively. Mean annual reproductive frequency was 51%. Reproduction occurred in 6 of 7 years. Mean clutch size was 6.9 eggs. Clutch masses and egg masses averaged 34.3% and 5.3% of total body mass, respectively. For a given body size, there was no annual variation in clutch size, egg mass, or reproductive effort. The consistency of these traits indicates adaptation to a predictable environment. Relatively high egg masses are an adaptation to counter the harsh environment. Annual recruitment was about 20%, almost exclusively due to reproduction. First year and subsequent annual survivorship rates averaged 40% and 75%, respectively. Most individuals lived no longer than 10 years. Compared with Mojave Desert populations, Colorado Desert S. obesus demonstrated earlier maturity, higher reproductive rates, higher first year survival and lower adult survivorship. High reproductive rates and first year survival were attributable to the predictability of mild winters and summer rainfall. Such conditions promoted lower adult survivorship because of associated costs of reproduction and predation. Attempts were made to predict population age class structure from winter precipitation regression models. Such a procedure appears reliable for predicting age-specific fecundity and therefore is a useful tool for management practices.

Bibliography: leaves 74-80. This study shows techniques for reducing hyperhydricity in the micropropagation of Sturt's desert pea. The effects of support matrix, tube closure and cytokinin regime are examined and the anatomy of hyperhydric shoots is investigated.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 202-224). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 17, 2010). Includes bibliographical references. Also issued in print.

No description available.

One of the important contributions genetic studies have made to conservation is the ability to resolve taxonomy and define relationships among populations. However, this can be complicated when species exhibit hybridization. Hybridization can be an important part of the evolutionary process and a critical component in a species ability to adapt to a changing environment. Most hybrid zones are observed at ecotones between two distinct habitats and this may be important in defining the role of hybrid zones in the evolutionary process. I examined hybridization among the three distinct lineages of desert tortoises in the genus Gopherus. An important aspect of this study system is the presence of areas of overlap between divergent lineages of desert tortoise which allowed me to test hypotheses about which forces influence these taxonomic boundaries. Specifically, I tested hypotheses about the contribution of physical vs. ecological segregation and the relative importance of isolation and gene flow in the formation of these disparate desert tortoise lineages. I used mtDNA sequence data and 25 microsatellite loci to perform Bayesian clustering, clinal analyses and habitat suitability modeling to infer population structure and influence of landscape features at each contact zone. In both instances, I observed ecological niche partitioning and limited hybridization at ecotones. I then used mtDNA and four nDNA loci to perform a multi-locus phylogenetic analysis to estimate the species tree among desert tortoise lineages and tested for ancestral admixture with RNA-seq data using demographic inference employed in the software package ∂a∂i. My results validate taxonomic distinction among all three lineages without evidence of ancestral introgression. These data suggest that despite the presence of contemporary hybridization and incomplete reproductive isolation, divergence among these lineages is consistent with species-level differentiation. By clarifying the evolutionary processes that influence the distribution of desert tortoise lineages, this study will directly inform efforts to preserve the evolutionary potential of these threatened species. Ultimately, understanding the evolutionary history of desert tortoises not only clarifies the forces that have driven speciation in this group, but it also contributes to our knowledge of the biogeographic history of the southwestern deserts and how diversity is maintained within them.

No description available.