bluuebunny/crossref_metadata_2025 huggingface dataset:
A statistical model of fatigue damage evolution in particulate‐reinforced metal‐matrix‐composites
Authors: Ding H.‐Z. , Biermann H. , Mughrabi H.
Published: March 2003

A statistical model of fatigue damage evolution has been developed for particulate‐reinforced metal‐matrix‐composites (MMCs) by taking into considerations both the initial damage distribution and the effect of particulate reinforcement on fatigue damage development. The growth of microscopically fatigue‐damaged regions in particulate‐reinforced MMCs is considered as a stochastic process, and both the non‐equilibrium statistical method and minimum strength principle are used to establish the evolution equation of fatigue damage. The fatigue damage evolution equation developed in the present study characterizes not only the kinetic process of fatigue damage evolution but also sets up the relationship between the mechanism of fatigue damage growth of the microscopically damaged regions and the result of fatigue damage, i.e. degradation of mechanical properties of particulate‐reinforced MMCs. A new expression for calculating the cumulative fatigue damage and a new formula for predicting the average fatigue strength of the particulate‐reinforced MMCs are derived. Experimental data of 2080Al/SiCp composites are analysed and compared with results obtained with the present model. It is shown that the experimental results can be described well by the calculations.
A model for evaluating the effect of fatigue crack repair by the infiltration method
Authors: Shin C. S. , Cai C. Q.
Published: March 2003

Infiltration of foreign materials into a fatigue crack has previously been shown to be able to retard the crack and extend fatigue life. Most of the related studies were empirical and phenomenological in nature. To aid engineering decisions, it would be advantageous if the possible outcome of a repair can be evaluated beforehand. To this end, a crack closure model taking into account the additional closure effect of the infiltrant has been developed and verified against experimental results. With this model, the sensitivity of the repair effect to various parameters such as mechanical properties of the infiltrant, depth of penetration and infiltration load level can be assessed.
A statistical model for cleavage fracture in notched specimens of C–Mn steel
Authors: Wang G. Z. , Chen J. H.
Published: March 2003

A statistical model for cleavage fracture in notched specimens of C‐Mn steel has been proposed. This model is based on a recently suggested physical model. This statistical model satisfactorily describes the distributions of the cumulative failure probability and failure probability density of 36 notched specimens fractured at various loads at test temperature of −196 °C. The minimum notch toughness has also been discussed.
Micro‐mechanical modelling of ductile damage and tearing – results of a European numerical round robin
Authors: Bernauer G , Brocks W
Published: March 2003

ABSTRACT Results of a European numerical round robin on the application of constitutive equations for ductile damage to simulate tearing of a ferritic steel are presented. The analysis of deformation and failure of a standard smooth tensile specimen is used to characterize the material and identify critical damage parameters for ductile tearing. Ductile crack growth in a C(T) specimen is then numerically simulated by applying the constitutive models of Gurson or Rousselier in order to predict a JR‐curve. Although the tensile tests were satisfactorily described, two main problems arose. The first one is that the participants' results differ considerably from each other with respect to the critical damage parameter obtained in the first task and the predicted JR‐curves in the second. The reasons were partially found in the strategies for parameter identification and the non‐unique definitions for crack initiation and crack growth used in the simulations. Further reasons are expected in the individual realization of the damage model in the FE programmes or user subroutines. The second problem is a general discrepancy between the simulations and the experiment: most of the predicted JR‐curves are too flat compared with the experimental one. Manifold conclusions can be drawn for future work aiming at improving the reliability of the numerical models and promoting their application in engineering practice.
Evolution of surface deformation during fatigue of PH 13‐8 Mo stainless steel using atomic force microscopy
Authors: Cretegny L. , Saxena A.
Published: March 2003

The relationship between microstructure and nucleation of fatigue cracks in PH 13‐8 Mo stainless steel was explored with the use of atomic force microscopy (AFM) that allowed an accurate quantitative characterization of the surface features. Fully reversed strain‐ controlled fatigue tests were performed at 0.4 and 0.6% strain amplitudes, and the evolution of the surface deformation was observed at various fractions of life. At 0.4% strain amplitude, fatigue surface damage occurred first in the shape of streaks about 4 nm deep that formed at the interface between martensite laths and at prior austenite grain boundaries, and eventually coalesced to form crack nuclei. The increase in strain amplitude to 0.6% led to the formation of large extrusions, on average between 2 and 5 μm long with heights between 10 and 200 nm, which were the preferred crack nucleation sites.
Simulations and experiments of stochastic characteristics for collective short fatigue cracks in steels
Authors: Y. HONG , L. ZHENG , Y. QIAO
Published: March 2003

ABSTRACT Stochastic characteristics prevail in the process of short fatigue crack progression. This paper presents a method taking into account the balance of crack number density to describe the stochastic behaviour of short crack collective evolution. The results from the simulation illustrate the stochastic development of short cracks. The experiments on two types of steels show the random distribution for collective short cracks with the number of cracks and the maximum crack length as a function of different locations on specimen surface. The experiments also give the variation of total number of short cracks with fatigue cycles. The test results are consistent with numerical simulations.
Effects of microstructure on mixed‐mode, high‐cycle fatigue crack‐growth thresholds in Ti‐6Al‐4V alloy
Authors: Nalla R. K. , Campbell J. P. , Ritchie R. O.
Published: March 2003

Effect of microstructure on mixed‐mode (mode I + II), high‐cycle fatigue thresholds in a Ti‐6Al‐4V alloy is reported over a range of crack sizes from tens of micrometers to in excess of several millimeters. Specifically, two microstructural conditions were examined—a fine‐grained equiaxed bimodal structure (grain size ∼20 µm) and a coarser lamellar structure (colony size ∼500 µm). Studies were conducted over a range of mode‐mixities, from pure mode I (ΔKII/ΔKI = 0) to nearly pure mode II (ΔKII/ΔKI ∼ 7.1), at load ratios (minimum load/maximum load) between 0.1 and 0.8, with thresholds characterized in terms of the strain‐energy release rate (ΔG) incorporating both tensile and shear‐loading components. In the presence of through‐thickness cracks—large (> 4 mm) compared to microstructural dimensions—significant effects of mode‐mixity and load ratio were observed for both microstructures, with the lamellar alloy generally displaying the better resistance. However, these effects were substantially reduced if allowance was made for crack‐tip shielding. Additionally, when thresholds were measured in the presence of cracks comparable to microstructural dimensions, specifically short (∼200 µm) through‐thickness cracks and microstructurally small (< 50 µm) surface cracks, where the influence of crack‐tip shielding would be minimal, such effects were similarly markedly reduced. Moreover, small‐crack ΔGTH thresholds were some 50–90 times smaller than corresponding large crack values. Such effects are discussed in terms of the dominant role of mode I behaviour and the effects of microstructure (in relation to crack size) in promoting crack‐tip shielding that arises from significant changes in the crack path in the two structures.
Parameter C lifetime calculation for the high cycle regime
Authors: PÖTING S. , ZENNER H.
Published: March 2003

An analytical fatigue lifetime prediction using the nominal stress concept and the Palmgren‐Miner‐Rule is performed. A comparison of prediction and experiment shows a large uncertainty. Miner‐Rule modifications have been developed to improve the fatigue lifetime prediction. Stress amplitudes below the fatigue limit are assumed to contribute to the damage at different ratings.

The modifications have a significant influence on the predicted fatigue lifetime especially in the high cycle regime. When the maximum stress value is just above the fatigue limit, this influence is most significant. Because there are not many test results of variable amplitude loads available for the high cycle regime, a verification is difficult.

In the present paper a new modification for the lifetime prediction is applied. The modification was introduced in 1988 by Hück and suggests a variable slope of a reference S–N‐curve for the damage accumulation. A parameter C and the relation between the maximum stress value of the load spectrum distribution and the fatigue limit determine the slope of the reference S–N curve.

The aim is to characterize the influence factors of the parameter C using test results for the high cycle regime achieved on a new test facility and from published test data. This would lead to an improvement of the lifetime prediction in the high cycle regime.
The influence of surface roughness on fluid flow through cracks
Authors: CHIVERS T. C.
Published: March 2003

ABSTRACT Leak‐before‐break (l‐b‐b) safety cases depend on predictions of flow rate through postulated cracks. The calculated flow rates are dependent upon assumptions made about a number of features including fluid friction, and this in turn is influenced by surface roughness and flow regime. This paper considers the uncertainties associated with flow rate prediction in both the laminar and fully rough turbulent regimes as influenced by fluid friction. It shows how uncertainties can be bounded. In particular it discusses the maximum values for fluid friction that might arise in practice. The use of computational fluid dynamics in future analyses could significantly reduce the uncertainties associated with fluid friction in cracks.
A notch stress intensity approach applied to fatigue life predictions of welded joints with different local toe geometry
Authors: LAZZARIN P. , LASSEN T. , LIVIERI P.
Published: March 2003

In the Notch Stress Intensity Factor (N‐SIF) approach the weld toe region is modelled as a sharp V‐shaped corner and local stress distributions in planar problems can be expressed in closed form on the basis of the relevant mode I and mode II N‐SIFs. Initially thought of as parameters suitable for quantifying only the crack initiation life, N‐SIFs were shown able to predict also the total fatigue life, at least when a large part of the life is spent as in the propagation of small cracks in the highly stressed region close to the notch tip. While the assumption of a welded toe radius equal to zero seems to be reasonable in many cases of practical interest, it is well known that some welding procedures are able to assure the presence of a mean value of the weld toe radius substantially different from zero. Under such conditions any N‐SIF‐based prediction is expected to underestimate the fatigue life. In order to investigate the degree of conservatism, a total of 128 fillet welded specimens are re‐analysed in the present work by using an energy‐based N‐SIF approach. The local weld toe geometry, characterised by its angle and radius, has been measured with accuracy for the actual test series. The aim of the work is to determine if the N‐SIF‐based model is capable of taking into account the large variability of the toe angle, and to quantify the inaccuracy in the predictions due to the simplification of setting the toe radius equal to zero.
The relationship between sensitisation and small punch test behaviour in a high‐nitrogen austenitic stainless steel at cryogenic temperatures
Authors: KWON I. H. , LEE S. I. , YU H. S.
Published: March 2003

Austenitic stainless components used in nuclear fusion reactors must be capable of maintaining reasonable mechanical properties to thermal ageing caused by welding and in‐service. Recently, high‐nitrogen (High‐N) austenitic stainless steels (SS) are receiving increased attention because of their strength advantages, but they have been found to be susceptible to dichromium nitride (Cr2N) precipitation during thermal exposure at 823–1073 K. The susceptibility to sensitisation at thermal ageing temperature for high‐N austenitic SS is examined using the single‐loop electrochemical potentiokinetic reactivation (EPR) test. High‐N SS were found to be susceptible to sensitisation caused by grain boundary precipitation of Cr2N, with the degree of sensitisation increasing systematically with ageing time and temperature. In particular, it was found that the precipitates, which effected sensitisation, were changed from carbides (M23C6) to nitrides (Cr2N) with increasing ageing time and temperature. The deterioration of mechanical properties associated with thermal ageing in high‐N SS was investigated by a small punch (SP) test using miniature specimens at cryogenic temperatures. Results indicated that the degradation of mechanical properties in this alloy was caused by a decrease of cohesive strength resulting from carbides (Cr23C6) and nitrides (Cr2N) precipitated in grain boundaries.
Costs of jasmonate‐induced responses in plants competing for limited resources
Authors: Van Dam Nicole M. , Baldwin Ian T.
Published: March 2003

We present a design to quantify fitness consequences of jasmonate‐induced responses in plants that are competing for limited resources with a conspecific. Under both high and low nitrogen supply rates, uninduced (control) Nicotiana attenuata plants growing next to a plant induced with 250 μg methyl jasmonate (MJ) yielded more seed capsules than control plants competing with another control plant. We conclude that there is a opportunity benefit for control plants growing next to an induced plant. Initially, MJ‐induced plants grew more slowly, but by senescence they had produced the same number of seed capsules as control plants that had competed with another control plant. Replacement series showed that the fitness of MJ‐induced plants is not influenced by the competitive status of their neighbour plant. We argue that competitive designs are useful tools for evaluating the phenotypic costs of ecologically important traits.
Predators induce interspecific herbivore competition for food in refuge space
Authors: Pallini  , Janssen  , Sabelis
Published: March 2003

Resource competition among herbivorous arthropods has long been viewed as unimportant because herbivore populations are controlled by predators. Although recently resurrected as an organizing force in arthropod communities on plants, there is still general agreement that resource competition among herbivores is reduced by predators. Here we show the reverse: predators induce interspecific resource competi‐tion among herbivores. We found that thrips larvae (Frankliniella occidentalis) use the web produced and inhabited by the spider mite Tetranychus urticae as a refuge from predation by the phytoseiid mite Neoseiulus cucumeris. Thrips larvae prefer clean plant parts, but move into the web upon perceiving volatile cues associated with thrips and predators. This behaviour leads to lower predation risk, but also to reduced developmental rate and lower production of thrips larvae due to competition with spider mites. In addition, thrips larvae consume spider‐mite eggs. Thus, predators induce interspecific competition and intraguild predation among herbivores within refuge space, even when host plants have an overall green appearance.
Indirect population interaction between two aphid species
Authors: Rott  , Müller  , Godfray
Published: March 2003

We designed an experiment to test whether two species of aphid feeding on different species of host plant influence each others population dynamics via shared parasitoids (apparent competition) or other indirect processes. Pea aphid (Acyrthosiphon pisum) colonies declined faster towards mid‐summer when there were nearby colonies of nettle aphid (Microlophium carnosum), though the significance of the difference (P = 0.06) was just short of the traditional 0.05 level. Observations suggested that parasitoids were not responsible for this difference, however, and that it was highly likely to be caused by aphid‐specific predators.
Carbon:phosphorus stoichiometry and food chain production
Authors: Sterner  , Clasen  , Lampert  , Weisse
Published: March 2003

Incident light was manipulated in large plankton towers containing algae, microbes, and herbivores. Paradoxically, food chain production was lower with greater light energy input. This apparent paradox is resolved by recognizing stoichiometric constraints to food chain production. At high light, elevated algal biomass was achieved mainly by increases in cellular carbon. Consumers have a high phosphorus demand for growth, and thus a large excess of carbon inhibited, rather than stimulated, their growth. These experiments may help us predict the consequences of anthropogenic perturbations in nutrients, carbon, and solar energy. They also may help us to understand the wide range of consumer biomass and production at a given level of primary productivity in ecosystems.
Sediment as a possible source of food for corals
Authors: Rosenfeld  , Bresler  , Abelson
Published: March 2003

Reef building corals are generally believed to require clear, nutrient‐poor water. Yet, paradoxically, coral reefs are among the most productive marine ecosystems in the world. This paradox is commonly explained by the coral’s ability to utilize a number of food resources, including zooplankton, phytoplankton, dissolved organic matter and photosynthetic products derived from their endosymbiotic algae, the zooxanthellae. Sedimentation is a known stressor for corals, inhibiting most feeding modes in various ways. However, evidence for enhanced sediment deposition on certain corals, induced by their morphologies, as well as for sediment ingestion by some corals, has led us to examine the use of sediment as a possible source of food for corals in addition to the other, known food sources. Our experiments with fluorescently labelled sediment show transfer of labelled organic matter from the sediment into the cells of the solitary coral Fungia horrida Dana 1846. The results provide the first direct evidence for the ability of a coral to digest the sediment’s organic fraction. These results may indicate a positive role of sediment, which up until now was considered to have only deleterious effects on corals.
Zonation of vegetation on lacustrine coastal dunes: effects of burial by sand
Authors: Maun M.A. , Perumal J.
Published: March 2003

More than 50 years ago it was proposed that zonation of major plant species on coastal dunes was determined by salt spray. Here, we argue against this hypothesis because (i) salt concentrations rarely exceed toxic levels; (ii) high precipitation in temperate latitudes washes the salt off the plants; (iii) major salt spray events occur in late autumn and winter when plants are dormant; and (iv) zonation also occurs on lacustrine dunes. Instead, we show evidence that zonation may be caused by burial because plant distribution was correlated with sand deposition and species were eliminated when burial exceeded their limits of tolerance, thus creating zones of different plant species. We conclude that in temperate regions (i) burial by sand may be among the most important factors in zonation, while salt spray may play a secondary role; and (ii) single environmental factors cannot be the determinants of a phenomenon as complex as species zonation.
Nitrogen fixation in seagrass meadows: Regulation, plant–bacteria interactions and significance to primary productivity
Authors: Welsh D.T.
Published: March 2003

The rhizosphere sediments of seagrasses are generally a site of intense nitrogen fixation activity and this can provide a significant source of “new” nitrogen for the growth of the plants. In this paper, I review the data concerning nitrogen fixation in seagrass ecosystems, the transfer of the fixed nitrogen from the bacteria to the plants and its contribution to the overall productivity of seagrasses in different climatic zones.

The relationship between the plants and diazotrophic heterotrophic bacteria in the rhizosphere is discussed, particularly focusing on the potentially important role of nitrogen‐fixing, sulphate‐reducing bacteria. The regulation of nitrogen fixation rates in the rhizosphere by photosynthetically driven oxygen and fixed carbon release by the plant roots and rhizomes, and the availability of ammonium in the porewater, is assessed. Finally, the hypothesis that a mutualistic or symbiotic association exists between the seagrasses and heterotrophic nitrogen fixers in the rhizosphere, based on the mutual exchange of fixed carbon and nitrogen, is discussed.
Discrimination within and between host species by a butterfly: implications for design of preference experiments
Authors: Singer Michael C. , Lee Joy R.
Published: March 2003

Experiments designed to reveal variation among individual parasites in preference for different host species may generate misleading results. Apparent variation in the order of preference among host species can be generated solely from variation in the strength of discriminations made within host species. We illustrate this with a study of oviposition preference in the butterfly Melitaea cinxia. All butterflies were tested on the same six individual plants, three Plantago lanceolata (P) and three Veronica spicata (V). Some insects repeatedly preferred all individual P over all individual V or vice versa. We designated these as “pure” species ranks. Other insects repeatedly produced “mixed” ranks, preferring some individual V over some P, and some individual P over some V. We show how a “mixed” rank butterfly could differ from a “pure” rank insect by discriminating either more within plant species and/or less between them. Therefore, discrimination within host species can mask or confound discrimination among species. We discuss implications for the design of preference experiments.
Bifurcations and chaos in ecology: lynx returns revisited
Authors: Javier G.P. Gamarra Javier G.P. , Solé Ricard V
Published: March 2003

One of the most popular data sets in ecology, that of lynx fur returns, is analysed in order to look for evidence for a bifurcation process. This bifurcation seems to be present from the observation of sudden shifts in the amplitude of oscillations of the lynx time series. Schaffer first proposed the possibility for such a bifurcation in 1985, and suggested that a possible source for the qualitative change of lynx’s fluctuations was an increased trapping effort, which eventually lead to high‐amplitude, chaotic dynamics. By studying the available information from the Hudson Bay Company records, we have found evidence for such an increased trapping pressure that (i) rapidly rose close to the shift from low‐amplitude to large‐amplitude fluctuations around 1820, and (ii) decreased around 1910, when there is another shift again to damped small oscillations. Although an increase in the top‐predator mortality in a three‐species food web typically leads to simpler dynamics and eventual predator extinction, here we show that a recent model involving a minimum bound in the lynx population, due to the presence of alternative prey in the lynx diet, consistently supports the presence of a bifurcation phenomenon.
Population fluctuations, power laws and mixtures of lognormal distributions
Authors: Allen A.P. , Li B.‐L. , Charnov E.L.
Published: March 2003

A number of investigators have invoked a cascading local interaction model to account for power‐law‐distributed fluctuations in ecological variables. Invoking such a model requires that species be tightly coupled, and that local interactions among species influence ecosystem dynamics over a broad range of scales. Here we reanalyse bird population data used by Keitt & Stanley (1998, Dynamics of North American breeding bird populations. Nature, 393, 257–260) to support a cascading local interaction model. We find that the power law they report can be attributed to mixing of lognormal distributions. More tentatively, we propose that mixing of distributions accounts for other empirical power laws reported in the ecological literature.
Foraging trade‐offs and resource patchiness: theory and experiments with a freshwater snail community
Authors: Chase Jonathan M. , Wilson Will G. , Richards Shane A.
Published: March 2003

Empirical results concerning a freshwater snail community are interpreted using a two‐species consumer model that incorporates resource structure. Behavioural‐scale measurements on a guild of five species of freshwater pond snails (Mollusca: Pulmonata) indicate a trade‐off between the ability to utilize a patch’s resource and the ability to quickly find new resource patches. Community‐level experiments demonstrate that both species richness and composition are affected by the patchiness of the environment. In particular, treatments with low patchiness are dominated by species best at exploiting local resources (diggers) whereas treatments with high patchiness are dominated by species best at finding new patches (grazers). Results from a controlled mesocosm experiment with two of the most common of these species, Helisoma trivolvis (a relative digger) and Physella gyrina (Physidae) (a relative grazer) show that the patchiness of the environment strongly influences the outcomes of interspecific competition among these two species: the digger performed much better in less patchy habitats, whereas the grazer performed better in more patchy habitats. A two‐species model of diggers and grazers modified to incorporate behavioural aspects of patchiness produces this same pattern of competitive outcomes.
Can floral repellents pre‐empt potential ant–plant conflicts?
Authors: Ghazoul Jaboury
Published: March 2003

Ants, by consuming floral nectar, are potential parasites of plant–pollinator mutualisms, the persistence of which depends on mechanisms preventing ants from visiting flowers. Here I report the existence of such a mechanism which, uniquely, appears general in its effects. I show that two acacia–ant mutualists are repelled by floral tissue chemicals from their own host‐plants as well as those from 13 other plant genera, only one of which associates symbiotically with ants. Furthermore, 18 of 25 ant species, from several subfamilies representing degrees of ant–plant interaction, are repelled by acacia floral chemicals. Thus floral ant repellents are widespread among plants, repel most ant species, and can prevent ants from parasitizing plant–pollinator mutualisms.
Climate change, species range limits and body size in marine bivalves
Authors: Kaustuv Roy , Jablonski David , Valentine James W.
Published: March 2003

We use data on the Pleistocene and modern range limits of Californian marine bivalves to show that species that shifted their geographical ranges in response to Pleistocene climatic fluctuations were preferentially drawn from the large end of the regional body size–frequency distributions. This difference is not due to phylogenetic effects (i.e. dominance of extralimital species by a few large‐bodied clades), differences among major ecological categories (burrowing versus surface‐dwelling, or suspension feeding versus non‐suspension feeding), or differences in modes of reproduction and larval development. In addition, we show that successful invasive species of bivalves in present‐day marine habitats also tend to be large‐bodied, despite the difference in mechanisms between present‐day and Pleistocene range expansions. These results indicate that range limits of large‐bodied bivalve species are more unstable than small‐bodied ones, and that body size and its correlates need to be considered when attempting to predict the responses of marine communities to climate change, biotic interchanges and human‐mediated invasions.
Possible effects of habitat fragmentation and climate change on the range of forest plant species
Authors: Honnay Olivier , Verheyen Kris , Butaye Jan , Jacquemyn Hans , Bossuyt Beatrijs , Hermy Martin
Published: March 2003

Global circulation models predict an increase in mean annual temperature between 2.1 and 4.6 °C by 2080 in the northern temperate zone. The associated changes in the ratio of extinctions and colonizations at the boundaries of species ranges are expected to result in northward range shifts for a lot of species. However, net species colonization at northern boundary ranges, necessary for a northward shift and for range conservation, may be hampered because of habitat fragmentation. We report the results of two forest plant colonization studies in two fragmented landscapes in central Belgium. Almost all forest plant species (85%) had an extremely low success of colonizing spatially segregated new suitable forest habitats after c. 40 years. In a landscape with higher forest connectivity, colonization success was higher but still insufficient to ensure large‐scale colonization. Under the hypothesis of net extinction at southern range boundaries, forest plant species dispersal limitation will prevent net colonization at northern range boundaries required for range conservation.
Extrinsic factors and the population sizes of threatened birds
Authors: Blackburn Tim M. , Gaston Kevin. J.
Published: March 2003

Attempts to explain the orders‐of‐magnitude variation observed in animal population sizes have principally focused on intrinsic differences between the taxa compared, but with limited success: most variation remains unexplained by such studies. However, animal population sizes may also vary in response to extrinsic factors, such as the environment occupied or the influence of human activities. Here, we use new estimates of the global population sizes of threatened bird species to examine extrinsic correlates of variation in their numbers, using general linear modelling and methods to control for phylogenetic relatedness. Threatened bird population sizes varied significantly with several extrinsic factors, including altitude, biogeographical region inhabited, type of extinction threat faced, and habitat used. They also vary with geographical range size, which was included in the analysis to control for its potentially confounding effects on the results. Details of the observed relationships, which vary with analytical method, are discussed. However, apart from geographical range size, none of the extrinsic variables analysed here explain more than a small percentage of the variation in threatened bird population sizes. Thus, it seems likely that a comprehensive explanation for why some species are common while others are rare will not be dominated by a single factor.
Rethinking ecological inference: density dependence in reef fishes
Authors: Osenberg Craig W. , St. Mary Colette M. , Schmitt Russell J. , Holbrook Sally J. , Chesson Peter , Byrne Barbara
Published: March 2003

We use a meta‐analysis of density dependence in reef fishes to evaluate how ecologists approach detection, inference, and estimation. We compared two groups of studies: those that detected effects of density on survival and those that did not. Distinctions between these groups have spawned heated debate about the processes that affect fish dynamics. Per capita effects of density were similar between the two groups, although total effects (and hence ambient density) were greater in studies that detected density effects. The majority of the variation in effects of density was not resolved by the classification of studies based on the authors' conclusions. These results suggest (1) that standard inferences based on null hypothesis tests may miss important sources of variation in effects and give rise to unnecessary debate; and (2) that estimation of effect sizes and model parameters (including their uncertainty) is a powerful alternative to detection of ecological processes.
Heteromyopia and the spatial coexistence of similar competitors
Authors: Murrell David J. , Law Richard
Published: March 2003

Most spatial models of competing species assume symmetries in the spatial scales of dispersal and interactions. This makes analysis tractable, and has led to the conclusion that segregation of species in space does not promote coexistence. However, these symmetries leave parts of the parameter space uninvestigated. Using a moment‐approximation method, we present a spatial version of the Lotka–Volterra competition equations to investigate effects of removing symmetries in the distances over which individuals disperse and interact. Some spatial segregation of the species always comes about due to competition, and such segregation does not necessarily lead to coexistence. But, if interspecific competition occurs over shorter distances than intraspecific competition (heteromyopia), spatial segregation becomes strong enough to promote coexistence. Such coexistence is most likely when the species have similar dynamics, in contrast to the competition–colonization trade‐off that requires large competitive differences between species.
No evidence that sexual selection is an ‘engine of speciation’ in birds
Authors: Morrow Edward H. , Pitcher Trevor E. , Arnqvist Göran
Published: March 2003

AbstractSexual selection has been implicated as having a role in promoting speciation, as it should increase the rate of evolution of reproductive isolation, and there is some comparative evidence that sexual selection may be related to imbalances in clade size seen in resolved phylogenies. By employing a new comparative method we are able to investigate the role of sexual selection in explaining the patterns of species richness across birds. We used data for testes size as an index of post‐mating sexual selection, and sexual size dimorphism and sexual dichromatism as indices of pre‐mating sexual selection. These measures were obtained for 1031 species representing 467 genera. None of the variables investigated explained the patterns of species richness. As sexual selection may also increase extinction rates, the net effect on species richness in any given clade will depend on the balancing effects of sexual selection upon speciation and extinction rates. We suggest that variance across clades in this balance may have resulted in the lack of a relationship between species richness and sexual selection seen in birds.
Inhibition of Escherichia coli precursor‐16S rRNA processing by mouse intestinal contents
Authors: Licht Tine Rask , Tolker‐Nielsen Tim , Holmstrøm Kim , Krogfelt Karen Angeliki , Molin Søren
Published: March 2003

The correlation between ribosome content and growth rate found in many bacterial species has proved useful for estimating the growth activity of individual cells by quantitative in situ rRNA hybridization. However, in dynamic environments, the stability of mature ribosomal RNA causes problems in using cellular rRNA contents for direct monitoring of bacterial growth activity in situ. In a recent paper, Cangelosi and Brabant suggested monitoring the content of precursors in rRNA synthesis (pre‐rRNAs) as an alternative approach. These are rapidly broken down after the cessation of bacterial growth. We have applied fluorescence in situ hybridization of pre‐16S rRNA to Escherichia coli cells growing in vitro in extracts from two different compartments of the mouse intestine: the caecal mucus layer, where E. coli grew rapidly, and the contents of the caecum, which supported much slower bacterial growth. The amounts of 23S rRNA and pre‐16S rRNA measured for E. coli growing in intestinal mucus corresponded to that expected for bacteria with the observed growth rate. In contrast, the slow‐growing E. coli cells present in intestinal contents turned out to have an approximately ninefold higher content of pre‐16S rRNA than cultures of the same strain growing rapidly in rich media. We present results suggesting that the mouse intestinal contents contain an agent that inhibits the growth of E. coli by disturbing its ability to process pre‐16S rRNA.
Electrokinetic movement of Escherichia coli in capillaries
Authors: Liu Zewen , Chen Wei , Papadopoulos Kyriakos D.
Published: March 2003

The movement of Escherichia coli K‐12 has been studied in capillaries with an 8 μm inside diameter under a DC electric field. At low applied voltages (< 0.2 V cm−1), bacterial motility is the predominant mode of motion, with bacteria swimming, tumbling and moving unidirectionally towards the anode and the cathode, although a significant difference between the mean moving speeds towards the two electrodes was observed. When the electric field was removed, the mean swimming speeds were the same for both directions. At higher voltages, all the cells were swept by electro‐osmotic flow exclusively towards the cathode.
Towards elucidation of microbial community metabolic pathways: unravelling the network of carbon sharing in a pollutant‐degrading bacterial consortium by immunocapture and isotopic ratio mass spectrometry
Authors: Pelz Oliver , Tesar Michael , Wittich Rolf‐Michael , Moore Edward R. B. , Timmis Kenneth N. , Abraham Wolf‐Rainer
Published: March 2003

Although much information on metabolic pathways within individual organisms is available, little is known about the pathways operating in natural communities in which extensive sharing of nutritional resources is the rule. In order to analyse such a consortium pathway, we have investigated the flow of 4‐chlorosalicylate as carbon substrate within a simple chemostat microbial community using 13C‐labelled metabolites and isotopic ratio mass spectrometric analysis of label enrichment in immunocaptured member populations of the community. A complex pathway network of carbon sharing was thereby revealed, involving two different metabolic routes, one of which is completely novel and involves the toxic metabolite protoanemonin. The high stability of the community results, at least in part, from interdependencies based on carbon sharing and the rapid removal of toxic metabolites.
Microbial characterization of a JP‐4 fuel‐contaminated site using a combined lipid biomarker/polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE)‐based approach
Authors: Stephen John R. , Chang Yun‐Juan , Gan Ying Dong , Peacock Aaron , Pfiffner Susan M. , Barcelona Michael J. , White David C. , Macnaughton Sarah J.
Published: March 2003

The impact of pollution on soil microbial communities and subsequent bioremediation can be measured quantitatively in situ using direct, non‐culture‐ dependent techniques. Such techniques have advantages over culture‐based methods, which often account for less than 1% of the extant microbial community. In 1988, a JP‐4 fuel spill contaminated the glacio‐fluvial aquifer at Wurtsmith Air Force Base, Michigan, USA. In this study, lipid biomarker characterization of the bacterial and eukaryotic communities was combined with polymerase chain reaction– denaturing gradient gel electrophoresis (PCR–DGGE) analysis of the eubacterial community to evaluate correlation between contaminant (JP‐4 fuel) concentration and community structure shifts. Vadose, capillary fringe and saturated zone samples were taken from cores within and up‐ and down‐gradient from the contaminant plume. Lipid biomarker analysis indicated that samples from within the plume contained increased biomass, with large proportions of typically Gram‐negative bacteria. Outside the plume, lipid profiles indicated low‐biomass microbial communities compared with those within the initial spill site. 16S rDNA sequences derived from DGGE profiles from within the initial spill site suggested dominance of the eubacterial community by a limited number of phylogenetically diverse organisms. Used in tandem with pollutant quantification, these molecular techniques should facilitate significant improvements over current assessment procedures for the determination of remediation end‐points.
Adaptation of Pseudomonas fluorescens to the plant rhizosphere
Authors: Rainey Paul B.
Published: March 2003

Saprophytic Pseudomonas are common root‐colonizing bacteria that can improve plant health. Efficient exploitation of these bacteria in agriculture requires knowledge of traits that enhance ecological performance in the rhizosphere. Here, I describe the development and application of a promoter‐trapping technology (IVET) that enables the isolation of Pseudomonas fluorescens genes that show elevated levels of expression in the rhizosphere. Using IVET, 20 P. fluorescens genes were identified that are induced during rhizosphere colonization, and their patterns of expression were analysed in laboratory media and in the rhizosphere. Fourteen genes showed significant homology to sequences in GenBank that are involved in nutrient acquisition, stress response, or secretion; six showed no homology. Seven of the rhizosphere‐induced (rhi) genes have homology to known non‐Pseudomonas genes. One of the rhi genes (hrcC) is a component of a type III secretion pathway, not previously known in non‐parasitic bacteria. Together, these genes provide a view of the rhizosphere environment as perceived by a rhizosphere colonist, and suggest that the nature of the association between P. fluorescens and the plant root may be more complex and intimate than previously thought.
Autotrophy of green non‐sulphur bacteria in hot spring microbial mats: biological explanations for isotopically heavy organic carbon in the geological record
Authors: Van Der Meer Marcel T. J. , Schouten Stefan , De Leeuw Jan W. , Ward David M.
Published: March 2003

Inferences about the evidence of life recorded in organic compounds within the Earth's ancient rocks have depended on 13C contents low enough to be characteristic of biological debris produced by the well‐known CO2 fixation pathway, the Calvin cycle. ‘Atypically’ high values have been attributed to isotopic alteration of sedimentary organic carbon by thermal metamorphism. We examined the possibility that organic carbon characterized by a relatively high 13C content could have arisen biologically from recently discovered autotrophic pathways. We focused on the green non‐sulphur bacterium Chloroflexus aurantiacus that uses the 3‐hydroxypropionate pathway for inorganic carbon fixation and is geologically significant as it forms modern mat communities analogous to stromatolites. Organic matter in mats constructed by Chloroflexus spp. alone had relatively high 13C contents (−14.9‰) and lipids diagnostic of Chloroflexus that were also isotopically heavy (−8.9‰ to −18.5‰). Organic matter in mats constructed by Chloroflexus in conjunction with cyanobacteria had a more typical Calvin cycle signature (−23.5‰). However, lipids diagnostic of Chloroflexus were isotopically enriched (−15.1‰ to −24.1‰) relative to lipids typical of cyanobacteria (−33.9‰ to −36.3‰). This suggests that, in mats formed by both cyanobacteria and Chloroflexus, autotrophy must have a greater effect on Chloroflexus carbon metabolism than the photoheterotrophic consumption of cyanobacterial photosynthate. Chloroflexus cell components were also selectively preserved. Hence, Chloroflexus autotrophy and selective preservation of its products constitute one purely biological mechanism by which isotopically heavy organic carbon could have been introduced into important Precambrian geological features.
Monitoring of activity dynamics of an anaerobic digester bacterial community using 16S rRNA polymerase chain reaction–single‐strand conformation polymorphism analysis
Authors: Delbès Céline , Moletta René , Godon Jean‐Jacques
Published: March 2003

The influence of parameter changes on the bacterial community of a laboratory‐scale anaerobic digester fed with glucose was investigated using a culture‐independent approach based on single‐strand conformation polymorphism (SSCP) analysis of total 16S rDNA and 16S rRNA amplification products. With the digester operating at steady state, the 16S rDNA SSCP patterns of the bacterial community showed eight peaks, whereas the 16S rRNA patterns showed six peaks with a very prominent one corresponding to a Spirochaetes‐related bacterium. An acidic shock at pH 6 caused an increase in the 16S rRNA level of two Clostridium‐related bacteria. After a 1 week starvation period, the major bacteria present reverted to a basal 16S rRNA level proportional to their 16S rDNA level. Starvation revealed the presence of a previously undetected peak whose corresponding sequence was deeply branched into the low G+C Gram‐positive bacteria phylum. Twenty‐four hours after a spiked addition to the starved digester community of starch, glucose, lactate or sulphate, an upsurge in several new 16S rRNA‐derived peaks was observed. Thus, the perturbation approach combined with 16S rRNA analysis revealed bacteria that had not been detected through 16S rDNA analysis.
Variations in antibiotic resistance profile in Enterobacteriaceae isolated from wild Australian mammals
Authors: Sherley Miranda , Gordon David M. , Collignon Peter J.
Published: March 2003

We carried out a retrospective analysis of 946 strains of Enterobacteriaceae isolated from wild Australian mammals between 1993 and 1997. The prevalence of resistance to fixed concentrations of 32 antimicrobial agents was determined, and the respective roles that taxonomic family of the host, state of origin and bacterial species play in defining prevalence and range of resistance were investigated. Our results demonstrated a low but widespread prevalence of antimicrobial resistance in wild isolates. Only amikacin, ciprofloxacin, meropenem and gentamicin inhibited growth in all 946 samples. There was extensive variation in the combination of antibiotics to which isolates were resistant, and multiple antibiotic resistance was common. Geographical location and host group significantly influenced the antibiotic resistance profile of an isolate, whereas bacterial species influenced both the resistance profile of an isolate and the number of antibiotics it was resistant to. The role of these factors in determining observed antibiotic resistance profiles suggests that any study measuring resistance in wild isolates should include the broadest possible range of bacterial species, host species and sampling locations. As such, this study provides an important new baseline for future measurements of antibiotic resistance in the Australian environment.