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Search Results: 1 - 10 of 261759 matches for " David L Smith "
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A test of the herbivore optimization hypothesis using muskoxen and a graminoid meadow plant community
David L. Smith
Rangifer , 1996,
Abstract: A prediction from the herbivore optimization hypothesis is that grazing by herbivores at moderate intensities will increase net above-ground primary productivity more than at lower or higher intensities. I tested this hypothesis in an area of high muskox {Ovibos moschatus) density on north-central Banks Island, Northwest Territories, Canada (73°50'N, 119°53'W). Plots (1 m2) in graminoid meadows dominated by cottongrass (Eriophorum triste) were either clipped, exposed to muskoxen, protected for part of one growing season, or permanently protected. This resulted in the removal of 22-44%, 10-39%, 0-39% or 0%, respectively, of shoot tissue during each growing season. Contrary to the predictions of the herbivore optimization hypothesis, productivity did not increase across this range of tissue removal. Productivity of plants clipped at 1.5 cm above ground once or twice per growing season, declined by 60+/-5% in 64% of the tests. The productivity of plants grazed by muskoxen declined by 56+/-7% in 25% of the tests. No significant change in productivity was observed in 36% and 75% of the tests in clipped and grazed treatments, respecrively. Clipping and grazing reduced below-ground standing crop except where removals were small. Grazing and clipping did not stimulate productivity of north-central Banks Island graminoid meadows.
Effect of Land Use Change on Carbon Content and CO2 Flux of Cloud Forest Soils, Santa Elena, Costa Rica  [PDF]
Lawrence H. Tanner, David L. Smith, Jessica Curry, Justin Twist
Open Journal of Soil Science (OJSS) , 2014, DOI: 10.4236/ojss.2014.42009
Abstract:

We investigated the effects of land-use changes on soil carbon storage and soil CO2 flux by comparing soils from mature cloud forest and 31-year-old secondary forest, both in the Santa Elena Forest Reserve, a municipallyowned reserve at an elevation of 1600 to 1700 m near the town of Monteverde, and a clear-cut pasture near the reserve. Soils in the mature forest exhibit only weak horizonation but typically thick A horizons; they also consistently yield the highest carbon contents in the upper 30 cm. Soil CO2 flux was the highest in these soils, but also displayed the highest spatial variability. Secondary forest soils contain substantially less soil carbon than mature forest soils, but more than pasture soils. CO2 flux in the secondary forest soils was more similar to that of the mature forest, but displayed lower spatial variability. The pasture soils contain less soil carbon and produced lower CO2 flux levels than either of the forest soils. The pasture soils typically contain a well-defined coarse sandy layer 10 to 20 cm below the surface that we interpret as a sediment layer deposited across much of the landscape following a widespread erosion event, likely a consequence of the clear-cutting. Soil nitrogen concentrations are more than an order of magnitude lower than soil carbon concentrations, and display no trends between the different landscapes examined. Our preliminary results suggest that reforestation does restore soil carbon to clear-cut landscapes, but returning soil carbon levels to pre-land use levels occurs at a time scale of centuries, rather than decades.

Regularity of volume-minimizing flows on 3-manifolds
David L. Johnson,Penelope Smith
Mathematics , 2005,
Abstract: In this article, we show that, for any compact 3-manifold, there is a $C^{1}$ volume-minimizing one-dimensional foliation. More generally, we show the existence of mass-minimizing rectifiable sections of sphere bundles without isolated "pole points" in the base manifold. This same analysis is used to show that the examples, due to Sharon Pedersen, of potentially volume-minimizing rectifiable sections (rectifiable foliations) of the unit tangent bundle to $S^{2n+1}$ are not, in fact, volume minimizing.
Partial regularity of mass-minimizing Cartesian currents
David L. Johnson,Penelope Smith
Mathematics , 2004,
Abstract: Let B be a fiber bundle with compact fiber F over a compact Riemannian n-manifold M. There is a natural Riemannian metric on the total space B consistent with the metric on M. With respect to that metric, the volume of a rectifiable section s:M--> B is the mass of the image s(M) as a rectifiable n-current in B. Theorem: For any homology class of sections of B, there is a mass-minimizing Cartesian current T representing that homology class which is the graph of a C^1 section on an open dense subset of M.
Immunoresponse to Allogeneic Synovial or Xenogenic Mesenchymal Stromal Cells in a Co-Culture Model  [PDF]
Seth S. Jump, David S. Smith, David C. Flanigan, Alicia L. Bertone
Open Journal of Cell Biology (OJCB) , 2012, DOI: 10.4236/ojcb.2012.21001
Abstract: The purpose of our investigations was to measure, in a co-culture condition, the immunoresponse to allogeneic or xeno-genic cells, selected as potential sources for cell therapy of arthritis. We challenged human spleen-derived cells (hSpl) by three different mechanisms: 1) exposure to donor allogeneic or xenogeneic cellular antigens; 2) exposure to donor cells transduced with adenoviral antigens (Ad) and 3) lipopolysaccharide (LPS), a known inflammatory immunostimulant. The immunoresponse to allogeneic human synovial-derived mesenchymal stromal cells alone or transduced with adenoviral green fluorescent protein (hSD-MSC or hSD-MSC/GFP) or the immunoresponse to xenogeneic equine mesenchymal stromal cells (eqMSC) or equine dermal fibroblasts (eqDFb), characterized by the proportion of CD3+, CD4+, and CD8+ human splenocytes (hSpl), was measured on Day 0 and Day 6 of co-culture by flow cytometry. In culture with hSD-MSC, hSD-MSC/GFP, eqDFb, or eqMSC, the proportion of CD3+ and CD8+ hSpl increased with time in culture but not with exposure to cell allo- or xeno-antigens. Both hSD-MSC and hSD-MSC/GFP increased in number during culture and were not affected in viability or proliferation by co-culture with allogeneic hSpl. In this in vitro, primary exposure study, hSpl demonstrated a natural selection and adaptation to a short-term cell culture environment, and that neither allogeneic nor xenogeneic cell antigens incited a greater cellular immunoactivation than co-cultured hSpl alone.
Immunoresponse to Allogeneic Synovial or Xenogenic Mesenchymal Stromal Cells in a Co-Culture Model  [PDF]
Seth S. Jump, David S. Smith, David C. Flanigan, Alicia L. Bertone
CellBio (CellBio) , 2012, DOI: 10.4236/ojcb.2012.21001
Abstract:

The purpose of our investigations was to measure, in a co-culture condition, the immunoresponse to allogeneic or xenogenic cells, selected as potential sources for cell therapy of arthritis. We challenged human spleen-derived cells (hSpl) by three different mechanisms: 1) exposure to donor allogeneic or xenogeneic cellular antigens; 2) exposure to donor cells transduced with adenoviral antigens (Ad) and 3) lipopolysaccharide (LPS), a known inflammatory immunostimulant. The immunoresponse to allogeneic human synovial-derived mesenchymal stromal cells alone or transduced with adenoviral green fluorescent protein (hSD-MSC or hSD-MSC/GFP) or the immunoresponse to xenogeneic equine mesenchymal stromal cells (eqMSC) or equine dermal fibroblasts (eqDFb), characterized by the proportion of CD3+, CD4+, and CD8+ human splenocytes (hSpl), was measured on Day 0 and Day 6 of co-culture by flow cytometry. In culture with hSD-MSC, hSD-MSC/GFP, eqDFb, or eqMSC, the proportion of CD3+ and CD8+ hSpl increased with time in culture but not with exposure to cell alloor xeno-antigens. Both hSD-MSC and hSD-MSC/GFP increased in number during culture and were not affected in viability or proliferation by co-culture with allogeneic hSpl. In this in vitro, primary exposure study, hSpl demonstrated a natural selection and adaptation to a short-term cell culture environment, and that neither allogeneic nor xenogeneic cell antigens incited a greater cellular immunoactivation than co-cultured hSpl alone.

Endemicity response timelines for Plasmodium falciparum elimination
David L Smith, Simon I Hay
Malaria Journal , 2009, DOI: 10.1186/1475-2875-8-87
Abstract: The epidemiological theory for the decline in the Plasmodium falciparum parasite rate (PfPR, the prevalence of infection) following intervention was critically reviewed and where necessary extended to consider superinfection, heterogeneous biting, and aging infections. Timelines for malaria control and elimination under different levels of intervention were then established using a wide range of candidate mathematical models. Analysis focused on the timelines from baseline to 1% and from 1% through the final stages of elimination.The Ross-Macdonald model, which ignores superinfection, was used for planning during the Global Malaria Eradication Programme (GMEP). In models that consider superinfection, PfPR takes two to three years longer to reach 1% starting from a hyperendemic baseline, consistent with one of the few large-scale malaria control trials conducted in an African population with hyperendemic malaria. The time to elimination depends fundamentally upon the extent to which malaria transmission is interrupted and the size of the human population modelled. When the PfPR drops below 1%, almost all models predict similar and proportional declines in PfPR in consecutive years from 1% through to elimination and that the waiting time to reduce PfPR from 10% to 1% and from 1% to 0.1% are approximately equal, but the decay rate can increase over time if infections senesce.The theory described herein provides simple "rules of thumb" and likely time horizons for the impact of interventions for control and elimination. Starting from a hyperendemic baseline, the GMEP planning timelines, which were based on the Ross-Macdonald model with completely interrupted transmission, were inappropriate for setting endemicity timelines and they represent the most optimistic scenario for places with lower endemicity. Basic timelines from PfPR of 1% through elimination depend on population size and low-level transmission. These models provide a theoretical basis that can be further ta
Statics and dynamics of malaria infection in Anopheles mosquitoes
David L Smith, F Ellis McKenzie
Malaria Journal , 2004, DOI: 10.1186/1475-2875-3-13
Abstract: Mathematical models have played an important role in understanding the epidemiology of malaria and other infectious diseases [1-3]. Models provide concise quantitative descriptions of complicated, non-linear processes, and a method for relating the process of infection in individuals to the incidence of infection or disease in a population over time. Important insights have come from dynamic and static analysis of these models [4,5]. Dynamic analysis focuses on changes in the incidence or prevalence of an infectious disease in a population over time. Statics involve populations at a steady state. Dynamic epidemic processes leave a signature in a population that can be examined statically, often from static analyses of age-specific patterns in epidemiological status, based on an appropriate cross-section of the population at a point in time – a snapshot of a population, stratified by age provides information about its historical dynamics [4,5]. The assumption that a population is at dynamic equilibrium and has a stationary age distribution is frequently violated in practice, but it provides a useful starting point for more sophisticated analysis. Combining mathematical modelling with statistical analysis allows the use of static patterns to understand dynamics; basic epidemiological parameters can be estimated in the absence of many years of time series data. The relationship between the statics and dynamics of Plasmodium infections in female Anopheles mosquitoes has been a focal point in classic studies of malaria transmission [2]. For mosquitoes, it is easier to measure parity, the reproductive age of the adult mosquito, than the chronological age of the mosquito, the time since adult emergence [6,7]. Saul et al. reformulated classic models for static age-infection relationships as cyclical feeding models; i.e. the models were formulated in terms of parity. They used the models to analyse mosquito statics, derive a formulae for the sporozoite rate, and define indiv
ROAD SURVEYS FOR TURTLES: CONSIDERATION OF POSSIBLE SAMPLING BIASES
David A. Steen,Lora L. Smith
Herpetological Conservation and Biology , 2006,
Abstract: .—Herpetofaunal surveys often rely on observations obtained via road cruising. The ease with which many speciesof amphibians and reptiles can be observed on roads makes this a useful technique. However, road surveys have inherentlimitations and biases, particularly for turtles. Observations of turtles along roads are likely biased towards large, adultfemale freshwater turtles on nesting forays and male terrestrial turtles that typically have a large home range. Turtles mayalso use roadsides as habitat and their presence on roads may not necessarily be reflective of their abundance in adjacentnatural habitats. Researchers who use road surveys to examine demographic parameters of a turtle population (e.g., sexratio or age class structure), or to describe a turtle community (e.g., species richness) should consider these biases in theirconclusions and explicitly note the role of road cruising in data collection.
Changes in Soil Composition and Floral Coverage on a Glacial Foreland Chronosequence in Southern Iceland  [PDF]
Lawrence H. Tanner, Ann E. Walker, Morgan Nivison, David L. Smith
Open Journal of Soil Science (OJSS) , 2013, DOI: 10.4236/ojss.2013.34022
Abstract: The land surface in front of the Skaftafellsj?kull in southern Iceland, exposed by ice recession commencing about the start of the twentieth century, constitutes a foreland with a maximum age of about 100 years and a more distal outwash plain. The ages of different surfaces within this sequence are constrained by moraines of known or estimated ages. Across this chronosequence, we measured at various sites the extent of floral coverage of the surface, the soil carbon and nitrogen contents of the substrate and the soil CO2 flux rate. All measured parameters exhibit values increasing with distance from the ice front, which correlates approximately with age. The strongest correlations are seen between distance and the carbon and nitrogen concentrations of the soil. Marked horizonation of the soil is observed only on the oldest surfaces (100+ years).
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