Dispersal adaptations of some acacia species in the Australian arid zone

Journal ArticleMost Australian representatives of the genus Acacia have diaspores with arillate appendages indicative of adaptation for active dispersal by animals. Based on physical and chemical characteristics of these arils and mechanisms of diaspore presentation, a number of arid zone acacias can be distinguished as probable ornithochores, myrmecochores, or species lacking active dispersal by animals. Two factors suggest that dispersal adaptations are evolutionarily labile in the face of changing selection pressures. Both myrmecochores and ornithochores are prevalent in each of the three largest taxonomic sections of the Australian subgenus Phyllodineae. Second, diaspores of at least one species, Acacia ligulata, exhibit geographic variation, resembling those of myrmecochores in one population and ornithochores in a second population. The colorful lipid-rich arils of avian-dispersed species are nutritionally more valuable than the relatively small, white appendages of ant-dispersed species with similar-sized propagules. While ants often collect the diaspores of ornithochorous acacias, we have no evidence that birds use the arils of myrmecochores. Patterns of seedling establishment on ant mounds, and under the canopies of trees where birds perch and defecate, are consistent with these observations. Exploitation of diaspores by birds may help to reduce the destruction of seeds by parasitoids and, in part, compensate for the higher aril expenditures (per milligram of propagule) in avian-dispersed species. Dispersal of seeds by both ants and birds directs seeds to microhabitats where nutrients are concentrated and water resources are more plentiful and/or are used more efficiently. Similarities between mound and subcanopy microhabitats may have facilitated evolutionary transitions between myrmecochory and ornithochory. The nutrient poverty that generally characterizes Australian soils has possibly played a role in selecting for seed dispersal by ants and birds in this genus as well as in many other Australian plants

Comparative structure of harvester ant communities in arid Australia and North America

Journal ArticleIn the Australian arid zone, the species richness of ants is greater and that of mammalian granivores is less than in North American deserts. This study aimed to determine if the structure of harvester ant communities differs from that seen in North American deserts, focussing on differences related to the paucity of rodents. We tested three hypotheses: (1) because there are fewer rodents, Australian harvester ants should be more abundant and diverse in local habitats than in North American deserts of similar productivity; (2) because the absence of rodents would allow ants to use larger seeds that are preferred by rodents in North America, Australian ant communities should include a larger size range and contain larger workers; and (3) that apart from differences resulting from a paucity of rodents, Australian and North American communities would be convergent in characteristics of community structure resulting from competition. We sampled 19 communities across a climatic gradient in the Australian arid zone and compared the results with data previously obtained for 10 North American communities

Tuning gastropod locomotion: Modeling the influence of mucus rheology on the cost of crawling

Common gastropods such as snails crawl on a solid substrate by propagating muscular waves of shear stress on a viscoelastic mucus. Producing the mucus accounts for the largest component in the gastropod's energy budget, more than twenty times the amount of mechanical work used in crawling. Using a simple mechanical model, we show that the shear-thinning properties of the mucus favor a decrease in the amount of mucus necessary for crawling, thereby decreasing the overall energetic cost of locomotion.Comment: Corrected typo

Thin-film flow in helically wound rectangular channels with small torsion

Laminar gravity-driven thin-film flow down a helically-wound channel of rectangular cross-section with small torsion in which the fluid depth is small is considered. Neglecting the entrance and exit regions we obtain the steady-state solution that is independent of position along the axis of the channel, so that the flow, which comprises a primary flow in the direction of the axis of the channel and a secondary flow in the cross-sectional plane, depends only on position in the two-dimensional cross-section of the channel. A thin-film approximation yields explicit expressions for the fluid velocity and pressure in terms of the free-surface shape, the latter satisfying a non-linear ordinary differential equation that has a simple exact solution in the special case of a channel of rectangular cross-section. The predictions of the thin-film model are shown to be in good agreement with much more computationally intensive solutions of the small-helix-torsion Navier–Stokes equations. The present work has particular relevance to spiral particle separators used in the mineral-processing industry. The validity of an assumption commonly used in modelling flow in spiral separators, namely that the flow in the outer region of the separator cross-section is described by a free vortex, is shown to depend on the problem parameters

Coherence of Spin Qubits in Silicon

Given the effectiveness of semiconductor devices for classical computation one is naturally led to consider semiconductor systems for solid state quantum information processing. Semiconductors are particularly suitable where local control of electric fields and charge transport are required. Conventional semiconductor electronics is built upon these capabilities and has demonstrated scaling to large complicated arrays of interconnected devices. However, the requirements for a quantum computer are very different from those for classical computation, and it is not immediately obvious how best to build one in a semiconductor. One possible approach is to use spins as qubits: of nuclei, of electrons, or both in combination. Long qubit coherence times are a prerequisite for quantum computing, and in this paper we will discuss measurements of spin coherence in silicon. The results are encouraging - both electrons bound to donors and the donor nuclei exhibit low decoherence under the right circumstances. Doped silicon thus appears to pass the first test on the road to a quantum computer.Comment: Submitted to J Cond Matter on Nov 15th, 200

Global Optical Control of a Quantum Spin Chain

Quantum processors which combine the long decoherence times of spin qubits together with fast optical manipulation of excitons have recently been the subject of several proposals. I show here that arbitrary single- and entangling two-qubit gates can be performed in a chain of perpetually coupled spin qubits solely by using laser pulses to excite higher lying states. It is also demonstrated that universal quantum computing is possible even if these pulses are applied {\it globally} to a chain; by employing a repeating pattern of four distinct qubit units the need for individual qubit addressing is removed. Some current experimental qubit systems would lend themselves to implementing this idea.Comment: 5 pages, 3 figure

A Study of the Reionization History of Intergalactic Helium with FUSE and VLT

We obtained high-resolution VLT and FUSE spectra of the quasar HE2347-4342 to study the properties of the intergalactic medium between redshifts z=2.0-2.9. The high-quality optical spectrum allows us to identify approximately 850 HeII absorption components with column densities between N~5X10^11 and $ 10^18 cm^-2. The reprocessed FUSE spectrum extends the wavelength coverage of the HeII absorption down to an observed wavelength of 920 A. Approximately 1400 HeII absorption components are identified, including 917 HeII Ly-alpha systems and some of their HeII Ly-beta, Ly-gamma, and Ly-delta counterparts. The ionization structure of HeII is complex, with approximately 90 components that are not detected in the hydrogen spectrum. These components may represent the effect of soft ionizing sources. The ratio Eta=N(HeII)/N(HI) varies approximately from unity to more than a thousand, with a median value of 62 and a distribution consistent with the intrinsic spectral indices of quasars. This suggests that the dominant ionizing field is from the accumulated quasar radiation, with contributions from other soft sources such as star-forming regions and obscured AGN, which do not ionize helium. We find an evolution in Eta toward smaller values at lower redshift, with the gradual disappearance of soft components. At redshifts z>2.7, the large but finite increase in the HeII opacity, Tau=5+/-1, suggests that we are viewing the end stages of a reionization process that began at an earlier epoch. Fits of the absorption profiles of unblended lines indicate comparable velocities between hydrogen and He^+ ions. At hydrogen column densities N<3X10^12 cm^-2 the number of forest lines shows a significant deficit relative to a power law, and becomes negligible below N=10^11 cm^-2.Comment: 40 pages, 10 Postscript figures, uses Aastex.sty The Astrophysical Journal, in pres

Earth resources-regional transfer activity contracts review

A regional transfer activity contracts review held by the Earth Resources Office was summarized. Contracts in the earth resources field primarily directed toward applications of satellite data and technology in solution of state and regional problems were reviewed. A summary of the progress of each contract was given in order to share experiences of researchers across a seven state region. The region included Missouri, Kentucky, Tennessee, Mississippi, Alabama, Georgia, and North Carolina. Research in several earth science disciplines included forestry, limnology, water resources, land use, geology, and mathematical modeling. The use of computers for establishment of information retrieval systems was also emphasized

Should cost effectiveness analyses for NICE always consider future unrelated medical costs?

When developing guidance on the use of new technologies within the NHS, NICE recommends the use of cost effectiveness. Specifically, an intervention is deemed cost effective by NICE if ‘its health benefits are greater than the opportunity costs of programmes displaced to fund the new technology, in the context of a fixed NHS budget. In other words, the general consequences for the wider group of patients in the NHS are considered alongside the effects for those patients who may directly benefit from the technology.’ We argue that the technical guidelines for health technology assessment used by NICE should change given this definition of cost effectiveness. The point at issue is the handling of “unrelated future medical costs”

A Treatise on Diversity in a Dominant Culture University

The authors examine progress in strengthening the Diversity agenda in a school of education within a private Christian university. This agenda is informed by external academic accrediting organizations and principles of social justice congruent with the historical roots of the university. Special emphasis is placed on the unique challenges of confronting how privilege manifests itself in seemingly homogeneous environments. The ultimate goal of the authors is to promote moving beyond cosmetic compliance with accreditation obligations towards a metabolized second order change reflecting internal paradigm shifts in which social justice is a central motivating factor in one’s vocation