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Search Results: 1 - 10 of 35092 matches for " Wopke van der Werf "
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Heterogeneous Host Susceptibility Enhances Prevalence of Mixed-Genotype Micro-Parasite Infections
Wopke van der Werf ,Lia Hemerik ,Just M. Vlak,Mark P. Zwart
PLOS Computational Biology , 2011, DOI: 10.1371/journal.pcbi.1002097
Abstract: Dose response in micro-parasite infections is usually shallower than predicted by the independent action model, which assumes that each infectious unit has a probability of infection that is independent of the presence of other infectious units. Moreover, the prevalence of mixed-genotype infections was greater than predicted by this model. No probabilistic infection model has been proposed to account for the higher prevalence of mixed-genotype infections. We use model selection within a set of four alternative models to explain high prevalence of mixed-genotype infections in combination with a shallow dose response. These models contrast dependent versus independent action of micro-parasite infectious units, and homogeneous versus heterogeneous host susceptibility. We specifically consider a situation in which genome differences between genotypes are minimal, and highly unlikely to result in genotype-genotype interactions. Data on dose response and mixed-genotype infection prevalence were collected by challenging fifth instar Spodoptera exigua larvae with two genotypes of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV), differing only in a 100 bp PCR marker sequence. We show that an independent action model that includes heterogeneity in host susceptibility can explain both the shallow dose response and the high prevalence of mixed-genotype infections. Theoretical results indicate that variation in host susceptibility is inextricably linked to increased prevalence of mixed-genotype infections. We have shown, to our knowledge for the first time, how heterogeneity in host susceptibility affects mixed-genotype infection prevalence. No evidence was found that virions operate dependently. While it has been recognized that heterogeneity in host susceptibility must be included in models of micro-parasite transmission and epidemiology to account for dose response, here we show that heterogeneity in susceptibility is also a fundamental principle explaining patterns of pathogen genetic diversity among hosts in a population. This principle has potentially wide implications for the monitoring, modeling and management of infectious diseases.
Framework for Modelling Economic Impacts of Invasive Species, Applied to Pine Wood Nematode in Europe
Tarek Soliman, Monique C. M. Mourits, Wopke van der Werf, Geerten M. Hengeveld, Christelle Robinet, Alfons G. J. M. Oude Lansink
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0045505
Abstract: Background Economic impact assessment of invasive species requires integration of information on pest entry, establishment and spread, valuation of assets at risk and market consequences at large spatial scales. Here we develop such a framework and demonstrate its application to the pinewood nematode, Bursaphelenchus xylophilus, which threatens the European forestry industry. The effect of spatial resolution on the assessment result is analysed. Methodology/Principal Findings Direct economic impacts resulting from wood loss are computed using partial budgeting at regional scale, while impacts on social welfare are computed by a partial equilibrium analysis of the round wood market at EU scale. Substantial impacts in terms of infested stock are expected in Portugal, Spain, Southern France, and North West Italy but not elsewhere in EU in the near future. The cumulative value of lost forestry stock over a period of 22 years (2008–2030), assuming no regulatory control measures, is estimated at €22 billion. The greatest yearly loss of stock is expected to occur in the period 2014–2019, with a peak of three billion euros in 2016, but stabilizing afterwards at 300–800 million euros/year. The reduction in social welfare follows the loss of stock with considerable delay because the yearly harvest from the forest is only 1.8%. The reduction in social welfare for the downstream round wood market is estimated at €218 million in 2030, whereby consumers incur a welfare loss of €357 million, while producers experience a €139 million increase, due to higher wood prices. The societal impact is expected to extend to well beyond the time horizon of the analysis, and long after the invasion has stopped. Conclusions/Significance Pinewood nematode has large economic consequences for the conifer forestry industry in the EU. A change in spatial resolution affected the calculated directed losses by 24%, but did not critically affect conclusions.
A Suite of Models to Support the Quantitative Assessment of Spread in Pest Risk Analysis
Christelle Robinet, Hella Kehlenbeck, Darren J. Kriticos, Richard H. A. Baker, Andrea Battisti, Sarah Brunel, Maxime Dupin, Dominic Eyre, Massimo Faccoli, Zhenya Ilieva, Marc Kenis, Jon Knight, Philippe Reynaud, Annie Yart, Wopke van der Werf
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0043366
Abstract: Pest Risk Analyses (PRAs) are conducted worldwide to decide whether and how exotic plant pests should be regulated to prevent invasion. There is an increasing demand for science-based risk mapping in PRA. Spread plays a key role in determining the potential distribution of pests, but there is no suitable spread modelling tool available for pest risk analysts. Existing models are species specific, biologically and technically complex, and data hungry. Here we present a set of four simple and generic spread models that can be parameterised with limited data. Simulations with these models generate maps of the potential expansion of an invasive species at continental scale. The models have one to three biological parameters. They differ in whether they treat spatial processes implicitly or explicitly, and in whether they consider pest density or pest presence/absence only. The four models represent four complementary perspectives on the process of invasion and, because they have different initial conditions, they can be considered as alternative scenarios. All models take into account habitat distribution and climate. We present an application of each of the four models to the western corn rootworm, Diabrotica virgifera virgifera, using historic data on its spread in Europe. Further tests as proof of concept were conducted with a broad range of taxa (insects, nematodes, plants, and plant pathogens). Pest risk analysts, the intended model users, found the model outputs to be generally credible and useful. The estimation of parameters from data requires insights into population dynamics theory, and this requires guidance. If used appropriately, these generic spread models provide a transparent and objective tool for evaluating the potential spread of pests in PRAs. Further work is needed to validate models, build familiarity in the user community and create a database of species parameters to help realize their potential in PRA practice.
H2 emission as a diagnostic of physical processes in starforming galaxies
Paul P. van der Werf
Physics , 2000,
Abstract: Observations and interpretation of extragalactic rotational and rovibrational H2 emission are reviewed. Direct observations of H2 lines do not trace bulk H2 mass, but excitation rate. As such, the H2 lines are unique diagnostics, if the excitation mechanism can be determined, which generally requires high-quality spectroscopy and suitable additional data. The diagnostic power of the H2 lines is illustrated by two cases studies: H2 purely rotational line emission from the disk of the nearby spiral galaxy NGC891 and high resolution imaging and spectroscopy of H2 vibrational line emision from the luminous merger NGC6240.
Star formation at high redshift as traced by near-infrared Halpha surveys
Paul P. van der Werf
Physics , 1997,
Abstract: Surveys of redshifted emission lines in the near-infrared will provide unique information on the cosmic star formation history. Near-infrared Halpha surveys will probe the cosmic star formation density and the evolution of the luminosity function of star forming galaxies out to z\sim2.5, including the important z=1.5-2.0 epoch, where according to current knowledge the cosmic star formation density peaks. Halpha and other hydrogen recombination lines (except Lyman alpha) and [O II] are all useful tracers of star formation provided the luminosity function is probed to sufficiently deep levels. However, shallow surveys or surveys of Lyman alpha or [O III] emission are not suitable for measuring or even constraining the cosmic star formation density. A first estimate of the star formation density at z\approx2.25 is derived, based on a new and sensitive large-area ESO near-infrared survey for Halpha emission at this redshift. Future observational directions, at near-infrared and longer wavelengths, for reading the cosmic star formation history are outlined.
Starbursts in ultraluminous infrared galaxies - fueling and properties
Paul P. van der Werf
Physics , 2001,
Abstract: The properties of starbursts in ultraluminous infrared galaxies are discussed, with particular emphasis on the fueling, the amount of extinction and the intrinsic properties of the nuclear starbursts. It is shown by the example of NGC6240 that the H_2 vibrational lines can be used to measure the rate of gas inflow into the potential well, which is sufficient to fuel a nuclear starburst of the intensity required to account for the far-infrared emission. It is shown that in Arp220 the faintness of all tracers of ionized gas can be accounted for by Lyman continuum absorption by dust within the ionized regions, combined with significant (but not extreme) extinction; there is no reason to invoke the presence of extreme extinction, an old starburst, or an additional non-stellar power source in Arp220.
Observations and diagnostic use of highly redshifted fine structure lines
Paul P. van der Werf
Physics , 1998,
Abstract: The diagnostic use and detectability of luminous fine structure lines from high redshift galaxies is reviewed in the light of results from COBE concerning the Milky Way and from ISO on low redshift galaxies. At the highest luminosities (L>10^{12}\Lsun) the [CII] 158 micron line is somewhat less luminous with respect to the bolometric luminosity than for lower luminosity objects. Thus, surveys for this line must emphasize depth. The [CII] line will be the principle spectroscopic probe of the deep universe for the MMA and FIRST. A deep search for [CII] 158 micron emission from the dusty z=4.693 quasar BR1202-0725 is presented. The resulting 3\sigma upper limit implies that for this object L[CII]}/LFIR<0.0006%, a highly significant result indicating that distant luminous objects may represent a natural extension towards higher luminosities of the ultraluminous infrared galaxies at low redshift.
Dust and molecular gas in high redshift radio galaxies
Paul P. van der Werf
Physics , 1998,
Abstract: This review discusses the current status of our knowledge of emission by dust and molecular gas in high redshift radio galaxies, and the uncertainties in the derivation of physical parameters from these data. The similarity of far-infrared luminous, gas-rich high redshift radio galaxies and local ultraluminous infrared galaxies (ULIGs) is discussed. Given that local ULIGs rapidly convert most of their gas reservoir into stars, far-infrared luminous high-z radio galaxies are likely undergoing immense bursts of star formation, possibly accounting for a large fraction of the final stellar populations in these systems. These results are discussed in the context of formation scenarios of massive galaxies.
Cooling lines as probes of the formation and buildup of galaxies and black holes
P. P. van der Werf,M. Spaans
Physics , 2009, DOI: 10.1051/spica/200903009
Abstract: We discuss the use of SPICA to study the cosmic history of star formation and accretion by supermassive black holes. The cooling lines, in particular the high-J rotational lines of CO, provide a clear-cut and unique diagnostic for separating the contributions of star formation and AGN accretion to the total infrared luminosity of active, gas-rich galaxies. We briefly review existing efforts for studying high-J CO emission from galaxies at low and high redshift. We finally comment on the detectability of cooling radiation from primordial (very low metallicity) galaxies containing an accreting supermassive black hole with SPICA/SAFARI.
Genesis of the dusty Universe: modeling submillimetre source counts
Alireza Rahmati,Paul P. van der Werf
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19475.x
Abstract: We model the evolution of IR galaxies using a phenomenological approach to match the observed source counts at different IR wavelengths. We introduce a new algorithm for reproducing source counts based on direct integration of probability distributions rather than Monte-Carlo sampling. We construct a simple model for the evolution of the luminosity function and the colour distribution of IR galaxies which utilizes a minimum number of free parameters. Moreover we analyze how each of these parameters is constrained by observational data. The model is based on pure luminosity evolution and adopts the Dale & Helou SED templates. We find that the 850um source counts and their redshift distribution depend strongly on the shape of the luminosity evolution function, but only weakly on the details of the SEDs. We derive the best-fit evolutionary model using the 850um counts and redshift distribution as constraints. Moreover our best-fit shows a flattening of the faint end of the luminosity function towards high redshifts and requires a colour evolution which implies the typical dust temperatures of objects with the same luminosities to decrease with redshift. We compare our best-fit model to observed source counts at shorter and longer wavelengths which indicates our model reproduces the 70um and 1100um source counts remarkably well, but under-produces the counts at intermediate wavelengths. Analysis reveals that the discrepancy arises at low redshifts, indicating that revision of the adopted SED library towards lower dust temperatures (at a fixed infrared luminosity) is required. This modification is equivalent to a population of cold galaxies existing at low redshifts, as also indicated by recent Herschel results, which are underrepresented in IRAS sample. We show that the modified model successfully reproduces the source counts in a wide range of IR and submm wavelengths.
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