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101.
102.
The simultaneous analysis of mixed discrete and continuous outcomes using nonlinear threshold models
Todd Coffey Chris Gennings 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(1):55-77
Mixed discrete and continuous outcomes are commonly measured on each experimental unit in dose-response studies in toxicology.
The dose-response relationships for these outcomes often have dose thresholds and nonlinear patterns. In addition, the endpoints
are typically correlated, and a statistical analysis that incorporates the association may result in improved precision. We
propose an extension of the generalized estimating equation (GEE) methodology to simultaneously analyze binary, count, and
continuous outcomes with nonlinear threshold models that incorporates the intra-subject correlation. The methodology uses
a quasi-likelihood framework and a working correlation matrix, and is appropriate when the marginal expectation of each outcome
is of primary interest and the correlation between endpoints is a nuisance parameter. Because the derivatives of threshold
models are not continuous at each point of the parameter space, we describe the necessary modifications that result in asymptotically
normal and consistent estimators. Using dose-response data from a neurotoxicity experiment, the methodology is illustrated
by analyzing five outcomes of mixed type with nonlinear threshold models. In this example, the incorporation of the intra-subject
correlation resulted in decreased standard errors for the threshold parameters. 相似文献
103.
This study examines factors affecting the rate and extent of biomass build-up among commercially important groupers, snappers, grunts, parrotfish and surgeonfish in a network of four marine reserves in southwest St. Lucia, Caribbean. Reserves constituted 35% of the total reef area originally available for fishing. Protection was instigated in 1995 after a baseline survey with annual or biennial censuses performed until 2002. Each survey consisted of 114 fifteen minute fish counts in reserves and 83 in fishing grounds, at depths of 5 m and 15 m in a 10 m diameter counting area. Estimates of number and size (cm) of target species were used to calculate fish family biomass. Data were analysed using three-way ANOVA in a before-after-control-impact pairs (BACIP) design. All families increased significantly in biomass over time at nearly all sites. Increases were greater in reserves than fishing grounds, except for grunts, and responses were strongest in parrotfish and surgeonfish. The combined biomass of families more than quadrupled in reserves and tripled in fishing grounds between 1995 and 2002. During this period coral cover declined by 46% in reserves and 35% in fishing grounds. Multiple regression showed that neither habitat characteristics nor habitat deterioration significantly affected rates of biomass build-up. The key factor was protection from fishing, which explained 44% of the variance in biomass growth. A further 28% of the variance was explained by sedimentation, a process known to stress reef invertebrates, significantly reducing the rate of biomass build-up. St. Lucia’s reserves succeeded in producing significant gains to fish stocks despite coral cover and structural complexity falling steeply over the period of the study. 相似文献
104.
Juan Valencia Chris Harwood Russell Washusen Andrew Morrow Matthew Wood Peter Volker 《Wood Science and Technology》2011,45(1):15-34
Peripheral longitudinal growth strain (LGS) was measured on a total of 81, 22-year-old pruned Eucalyptus nitens trees from five thinning treatments in a plantation thinning trial in Tasmania. Growth strain data were gathered at breast
height on each tree using the CIRAD-Forêt method. The effects of thinning treatment and tree diameter on LGS and its relationships
to solid-wood traits determined in a processing trial were examined. LGS was significantly higher in the direction of the
prevailing wind and was not significantly affected by either thinning treatment or tree diameter. An index of log end splitting
was positively related to tree diameter, sawlog position (upper/lower), and LGS, which explained up to an additional 20% of
the variance after tree diameter and log position were accounted for in a linear regression model. High LGS and log end splitting
were significant indicators of increased board end splitting. 相似文献
105.
Chris J. PengellyRalph V. Cartar 《Forest Ecology and Management》2011,261(11):2068-2074
The rate of floral nectar production, an important trait for supporting populations of pollinators, is known to be affected by local environment. This study examines the effect, 9 years after an anthropogenic disturbance—variable retention logging,—on the rate of daily nectar production of four common herbs (Chamerion angustifolium, Delphinium glaucum, Mertensia paniculata and Vicia americana) in the boreal forest of Alberta, Canada. We measured daily nectar production rates 9 years after experimental logging in three treatments: clear cuts (all merchantable trees removed), 50% of trees retained, and unlogged controls. Per-flower nectar production rate was unaffected by logging for C. angusifolium, D. glaucum, and M. paniculata, and lower in unlogged controls than in the 50% logging treatment for V. americana. Overall, we found a surprising robustness of nectar production of these herbaceous plants to the physical and environmental changes following logging. While other research has shown boreal forest logging to affect the densities of understory herbs, the per-flower nectar production in boreal forest understory herbs appears to be relatively insensitive to this disturbance. 相似文献
106.
Managing small remnants of native forest to increase biodiversity within plantation landscapes in the south west of Western Australia 总被引:1,自引:0,他引:1
Robert D. Archibald Michael D. Craig Katarzyna Bialkowski Chris Howe Treena I. Burgess Giles E.St.J. Hardy 《Forest Ecology and Management》2011,261(7):1254-1264
Recent expansion of industrial-scale plantations into agricultural areas in Australia has the potential to assist biodiversity conservation by rehabilitating ecosystems at the landscape scale and reducing edge effects, isolation and disturbance within remnants of native vegetation. However, the efficient management of remnants for biodiversity within a plantation estate requires knowledge of both the causes and the consequences of remnant degradation. With this in mind, we examined key ecosystem features and processes relating to soil chemistry, decomposition, native tree health and regeneration and vertebrate abundance, within small forest remnants (1-4 ha) embedded within Eucalyptus globulus plantations in south west Western Australia. Soil nutrient enrichment was significantly associated with a scale of vegetation modification in order from: (1) intact forest, (2) remnants with native understorey (UDN), (3) remnants with exotic understorey (UDE), (4) plantation and (5) pasture. We propose that, in this region, UDE remnants will often remain in a degraded state even after plantation establishment and the cessation of stock grazing. This is due, in part, to more rapid rates of nutrient turnover sustaining higher nutrient availability in the soil following the replacement of ligneous understorey plants with annual ones. Cotton strips placed in surface soils were often disintegrated in UDE remnants and largely intact in UDN remnants, indicating that decomposition was accelerated in the former. Continued tree decline and regeneration failure within UDE remnants will also reinforce the UDE condition. There was less canopy seed set and little or no seedling establishment in UDE remnants compared to UDN remnants. Therefore, management interventions to assist native tree regeneration in UDE remnants are needed. Fauna trapping highlighted the desirability of retaining, and preferably restoring, small remnants in plantations. Small native mammals were found exclusively in remnants and fewer introduced Mus musculus were present in UDN remnants. Higher numbers of native lizards were found in UDN remnants, but this result was not significant (marginally). 相似文献
107.
Chris Klok Jack Faber Guido Heijmans Jos Bodt Annemariet van der Hout 《Biology and Fertility of Soils》2007,43(5):549-556
In this paper we report on the influence of clay content and acidity of soil on growth and reproduction of the epigeic earthworm
species Lumbricus rubellus (Hoffm.), which is common in most temperate soils and abundant in grasslands. Growth, cocoon production and survival of L. rubellus were tested in 12 Dutch soils which differed in soil properties. A matrix model was used to assess the population-level consequences
of changes in growth and reproduction. Soil acidity had a strong negative effect on earthworm survival, and the maturation
weight decreased with clay content. Individual weight gain in L. rubellus decreased with both acidity and clay content. The acidity of soils had a larger influence on population growth rate than
the clay content. The acidity of the soil also changed the population composition towards younger age classes, whereas in
soils rich in clay, the population composition did not change. The average individual weight of L. rubellus in clayey soils, however, was lower compared with that in soils low in clay, a result that agrees with literature data. 相似文献
108.
Stephanie L. Meadows-Shropshire Chris Gennings W. Hans Carter 《Journal of Agricultural, Biological & Environmental Statistics》2005,10(1):104-117
In the analysis of mixtures of drugs/chemicals it is often of interest to test for the presence of interaction. If the hypothesis
of no interaction (additivity) is not rejected, then the analyst may reasonably claim additivity if and only if the study
is powered to a desired (e.g., biologically meaningful) level. The objective of this article is to address the sample size
and power issues related to testing the hypothesis of additivity at specified mixture points. The study of disinfectant by-products
(DBPs) found in drinking water, described in earlier literature, is used to illustrate the procedures for estimating power
and sample sizes for detecting interactions at specified mixtures. The four trihalomethanes used in the study are bromodichloromethane
(BDCM), chlorodibromomethane (CDBM), chloroform (CHCl3), and bromoform (CHBr3) 相似文献
109.
Bartels RA Paul A Green H Kapteyn HC Murnane MM Backus S Christov IP Liu Y Attwood D Jacobsen C 《Science (New York, N.Y.)》2002,297(5580):376-378
We present spatial coherence measurements of extreme ultraviolet (EUV) light generated through the process of high-harmonic up-conversion of a femtosecond laser. With a phase-matched hollow-fiber geometry, the generated beam was found to exhibit essentially full spatial coherence. The coherence of this laser-like EUV source was shown by recording Gabor holograms of small objects. This work demonstrates the capability to perform EUV holography with a tabletop experimental setup. Such an EUV source, with low divergence and high spatial coherence, can be used for experiments involving high-precision metrology, inspection of optical components for EUV lithography, and microscopy and holography with nanometer resolution. Furthermore, the short time duration of the EUV radiation (a few femtoseconds) will enable EUV microscopy and holography to be performed with ultrahigh time resolution. 相似文献
110.
Organ CL Schweitzer MH Zheng W Freimark LM Cantley LC Asara JM 《Science (New York, N.Y.)》2008,320(5875):499
We report a molecular phylogeny for a nonavian dinosaur, extending our knowledge of trait evolution within nonavian dinosaurs into the macromolecular level of biological organization. Fragments of collagen alpha1(I) and alpha2(I) proteins extracted from fossil bones of Tyrannosaurus rex and Mammut americanum (mastodon) were analyzed with a variety of phylogenetic methods. Despite missing sequence data, the mastodon groups with elephant and the T. rex groups with birds, consistent with predictions based on genetic and morphological data for mastodon and on morphological data for T. rex. Our findings suggest that molecular data from long-extinct organisms may have the potential for resolving relationships at critical areas in the vertebrate evolutionary tree that have, so far, been phylogenetically intractable. 相似文献