排序方式： 共有29条查询结果，搜索用时 31 毫秒
1.
Daniel G. Gavin W. Wyatt Oswald Eugene R. Wahl John W. Williams 《Quaternary Research》2003,60(3):356-367
The modern analog technique typically uses a distance metric to determine the dissimilarity between fossil and modern biological assemblages. Despite this quantitative approach, interpretation of distance metrics is usually qualitative and rules for selection of analogs tend to be ad hoc. We present a statistical tool, the receiver operating characteristic (ROC) curve, which provides a framework for identifying analogs from distance metrics. If modern assemblages are placed into groups (e.g., biomes), this method can (1) evaluate the ability of different distance metrics to distinguish among groups, (2) objectively identify thresholds of the distance metric for determining analogs, and (3) compute a likelihood ratio and a Bayesian probability that a modern group is an analog for an unknown (fossil) assemblage. Applied to a set of 1689 modern pollen assemblages from eastern North America classified into eight biomes, ROC analysis confirmed that the squared-chord distance (SCD) outperforms most other distance metrics. The optimal threshold increased when more dissimilar biomes were compared. The probability of an analog vs no-analog result (a likelihood ratio) increased sharply when SCD decreased below the optimal threshold, indicating a nonlinear relationship between SCD and the probability of analog. Probabilities of analog computed for a postglacial pollen record at Tannersville Bog (Pennsylvania, USA) identified transitions between biomes and periods of no analog. 相似文献
2.
M. Wahl 《The Photogrammetric Record》1983,11(62):195-201
The Renault Company has developed a system for the mathematical definition of complex surfaces known as the Unisurf method. In association with this method, close range photogrammetry is used to provide the digital data required for the analysis of a surface. Photogrammetric techniques are of value in various fields of application, including the styling of new models of cars, design modifications to existing models, car accident investigations and statistical studies on human bodies. 相似文献
3.
Walter Wahl 《International Journal of Earth Sciences》1942,32(4-5):550-562
Ohne Zusammenfassung 相似文献
4.
Stefanie D. Moorthi Helmut Hillebrand Martin Wahl Ulrike-G. Berninger 《Estuaries and Coasts》2008,31(1):152-162
Diversity within distinct trophic groups is proposed to increase ecosystem functions such as the productivity of this group and the efficiency of resource use. This proposition has mainly been tested with plant communities, consumer assemblages, and multitrophic microbial assemblages. Very few studies tested how this diversity–productivity relationship varies under different environmental regimes such as disturbances. Coastal benthic assemblages are strongly affected by temporal instability of abiotic conditions. Therefore, we manipulated benthic ciliate species richness in three laboratory experiments with three diversity levels each and analyzed biomass production over time in the presence or absence of a single application of a disturbance (ultraviolet-B [UVB] radiation). In two out of three experiments, a clear positive relationship between diversity and productivity was found, and also the remaining experiment showed a small but nonsignificant effect of diversity. Disturbance significantly reduced the total ciliate biomass, but did not alter the relation between species richness and biomass production. Significant overyielding (i.e., higher production at high diversity) was observed, and additive partitioning indicated that this was caused by niche complementarity between ciliate species. Species-specific contribution to the total biomass varied idiosyncratically with species richness, disturbance, and composition of the community. We thus present evidence for a significant effect of consumer diversity on consumer biomass in a coastal ciliate assemblage, which remained consistent at different disturbance regimes. 相似文献
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6.
Walter Wahl 《International Journal of Earth Sciences》1943,34(2-6):209-225
Ohne Zusammenfassung 相似文献
7.
Comparison of Traditional and Novel Polymeric Adsorbents with Regard to the Adsorption of 1,2-Dichloroethane from Water The adsorption of 1,2-dichloroethane from water onto traditional and novel polymeric adsorbents was studied in this publication. It was found that the novel (cross-linked) polymeric adsorbents have a higher loading than the traditional (not crosslinked) polymeric adsorbents. The main reason for that is the larger specific surface. There is no remarkable difference between the crosslinked and not crosslinked polymeric adsorbents due to the kinetics of adsorption. 相似文献
8.
João Canning-Clode Manfred Kaufmann Markus Molis Martin Wahl & Mark Lenz 《Marine Ecology》2008,29(1):115-124
Disturbance and productivity are often cited as the main factors determining temporal and spatial patterns in species distribution and the diversity of communities. A field experiment was conducted to test the role of these factors in the structuring of early successional fouling communities in a nutrient limited system at the south coast of Madeira Island. Macro‐benthic sessile communities, established on artificial settlement substrata, were manipulated and surveyed over a 9‐week period. We applied mechanical disturbances of four different frequencies crossed with three levels of inorganic nutrient enrichment. Fertilization enhanced community diversity by favouring the establishment and growth of macroalgae. Disturbance reduced diversity by eliminating species – but only at the highest nutrient level. This is explained by a multiple‐stressor model; species most sensitive to nutrient deficiency (only present in the highest enrichment treatment) were simultaneously the most sensitive to disturbance. 相似文献
9.
A portion of the debate about climate reconstructions of the past millennium, and in particular about the well-known Mann-Bradley-Hughes
(“MBH” 1998, 1999) reconstructions, has become disconnected from the goal of understanding natural climate variability. Here,
we reflect on what can be learned from recent scientific exchanges and identify important challenges that remain to be addressed
openly and productively by the community. One challenge arises from the real, underlying trend in temperatures during the
instrumental period. This trend can affect regression-based reconstruction performance in cases where the calibration period
does not appropriately cover the range of conditions encountered during the reconstruction. However, because it is tied to
a unique spatial pattern driven by change in radiative balance, the trend cannot simply be removed in the method of climate
field reconstruction used by MBH on the statistical argument of preserving degrees of freedom. More appropriately, the influence
from the trend can be taken into account in some methods of significance testing. We illustrate these considerations as they
apply to the MBH reconstruction and show that it remains robust back to AD 1450, and given other empirical information also
back to AD 1000. However, there is now a need to move beyond hemispheric average temperatures and to focus instead on resolving
climate variability at the socially more relevant regional scale. 相似文献
10.
The Mann et al. (1998) Northern Hemisphere annual temperature reconstruction over 1400–1980 is examined in light of recent
criticisms concerning the nature and processing of included climate proxy data. A systematic sequence of analyses is presented
that examine issues concerning the proxy evidence, utilizing both indirect analyses via exclusion of proxies and processing
steps subject to criticism, and direct analyses of principal component (PC) processing methods in question. Altogether new
reconstructions over 1400–1980 are developed in both the indirect and direct analyses, which demonstrate that the Mann et
al. reconstruction is robust against the proxy-based criticisms addressed. In particular, reconstructed hemispheric temperatures
are demonstrated to be largely unaffected by the use or non-use of PCs to summarize proxy evidence from the data-rich North
American region. When proxy PCs are employed, neither the time period used to “center” the data before PC calculation nor
the way the PC calculations are performed significantly affects the results, as long as the full extent of the climate information
actually in the proxy data is represented by the PC time series. Clear convergence of the resulting climate reconstructions
is a strong indicator for achieving this criterion. Also, recent “corrections” to the Mann et al. reconstruction that suggest
15th century temperatures could have been as high as those of the late-20th century are shown to be without statistical and
climatological merit. Our examination does suggest that a slight modification to the original Mann et al. reconstruction is
justifiable for the first half of the 15th century (∼+0.05∘), which leaves entirely unaltered the primary conclusion of Mann et al. (as well as many other reconstructions) that both
the 20th century upward trend and high late-20th century hemispheric surface temperatures are anomalous over at least the
last 600 years. Our results are also used to evaluate the separate criticism of reduced amplitude in the Mann et al. reconstructions
over significant portions of 1400–1900, in relation to some other climate reconstructions and model-based examinations. We
find that, from the perspective of the proxy data themselves, such losses probably exist, but they may be smaller than those
reported in other recent work.
The National Center for Atmospheric Research is sponsored by the National Science Foundation, USA.
The authors contributed equally to the development of the research presented. 相似文献