bluuebunny/crossref_metadata_2025 huggingface dataset:
A 3D multilayered model to simulate sterilized knee prostheses under walking conditions: analysis of gamma irradiation effects in UHWMPE
Authors: Plumet S. , Dubourg M.‐C.
Published: March 2003

Ultra High Molecular Weight PolyEthylene (UHMWPE) is the polymer bearing material most commonly used for the design of the tibial insert in total knee joint replacement (TKJR). Sterilization by gamma irradiation in air causes oxidative changes in the polymer component that leads to mechanical property variations through its thickness. This may be one of the causes of the component failure. A general three‐dimensional elastic contact model has been therefore developed to predict both the contact stresses and the resulting internal stresses accounting for the plastic component finite thickness and its multilayered structure (constitutive layers of different thickness and mechanical properties) resulting from oxidation. Predictions of the maximum contact pressure and internal stresses have demonstrated that sterilization induces higher contact pressure, stress discontinuities at layer interfaces, possible bending effects and delamination risks.
Stochastic modelling of low‐cycle fatigue damage in 316L stainless steel under variable multiaxial loading
Authors: Lee B.‐H. , Lee S.‐B.
Published: March 2003

In the present study, a stochastic model is developed for the low‐cycle fatigue life prediction and reliability assessment of 316L stainless steel under variable multiaxial loading. In the proposed model, fatigue phenomenon is considered as a Markov process, and damage vector and reliability are defined on every plane. Any low‐cycle fatigue damage evaluating method can be included in the proposed model. The model enables calculation of statistical reliability and crack initiation direction under variable multiaxial loading, which are generally not available. In the present study, a critical plane method proposed by Kandilet al. (Metals Soc., London280,203–210, 1982) maximum tensile strain range, and von Mises equivalent strain range are used to calculate fatigue damage. When the critical plane method is chosen, the effect of multiple critical planes is also included in the proposed model. Maximum tensile strain and von Mises strain methods are used for the demonstration of the generality of the proposed model. The material properties and the stochastic model parameters are obtained from uniaxial tests only. The stochastic model made of the parameters obtained from the uniaxial tests is applied to the life prediction and reliability assessment of 316L stainless steel under variable multiaxial loading. The predicted results show good aggreement with experimental results.
Mode I stress intensity factors for curved cracks in gears by a weight functions method
Authors: Guagliano M. , Vergani L.
Published: March 2003

The most recent trend in power transmission design considers the damage‐tolerant approach as one of the methods to obtain safe, reliable and light systems. This means that components containing cracks must be considered and analysed to understand the conditions that cause critical cracks and defects and their dimensions.

In this paper a cracked tooth of an automotive gearwheel is considered. A numerical procedure (based on the slice synthesis weight function method) to calculate the stress intensity factors of curved cracks due to bending loads is illustrated. The results are compared with those obtained by expensive finite element calculations. The agreement is satisfactory and the proposed technique proves to be very attractive from the point of view of time saving.

One example of an application to fatigue design practice is provided, namely the analysis of fatigue crack propagation in surface‐treated gears. The results show the role played by residual stresses induced by carburizing and shot peening.
Prediction of fatigue resistance of a hot‐dip galvanized steel
Authors: Vogt J.‐B. , Boussac O. , Foct J.
Published: March 2003

The paper clarifies the effect of a galvanizing coating on the fatigue strength of a ferritic steel. Depending on experimental conditions and on the microstructure of the coating, a reduction in fatigue strength is observed especially when the coating is thick. Cracks in the galvanizing coating rapidly form under cyclic loading and then propagate into the steel substrate. This completely modifies the distribution of crack lengths. Very short cracks are not observed in the steel when galvanized. It is shown that the propagation of a crack in the substrate from the coating is only possible when the crack completely crosses the coating. By assimilating the coating thickness to a crack in the steel substrate, the fatigue resistance of hot‐dip galvanized steel can be predicted using the Kitagawa–Takahashi diagram.
Enhanced reference stress‐based J and crack opening displacement estimation method for leak‐before‐break analysis and comparison with GE.EPRI method
Authors: Kim Y.‐J. , Huh N.‐S. , Kim Y.‐J.
Published: March 2003

Three‐dimensional, elastic–plastic finite element analyses for circumferential through‐wall cracked pipes are performed using actual tensile data of stainless steels, for two purposes. The first one is to validate the recently proposed enhanced reference stress (ERS) method to estimate the J‐integral and rack‐opening displacement (COD) for circumferential through‐wall cracked pipes. The second one is to compare the J and COD results from the ERS method with those based on the GE/EPRI method, together with the finite element results. It is found that the GE/EPRI method generally provides poor estimations of J and COD, and such poor estimation results from the Ramberg–Osgood (R–O) fit. Moreover, the GE/EPRI results can be very sensitive to how the stress–strain data are fitted using the R–O relation. The most appropriate fitting range for the R–O fit depends on the material, and thus no specific guideline can be given. On the contrary, the J and COD estimations based on the ERS method give more accurate and robust results than the GE/EPRI estimation. The present results, together with the experimental validation presented in the previous paper, provide sufficient confidence in using the proposed method in the leak‐before‐break analysis.
A computerized procedure for long‐life fatigue assessment under complex multiaxial loading
Authors: Li B. , Santos J. L. T. , Freitas M. DE
Published: March 2003

A computerized procedure is presented and evaluated for application examples of long‐life fatigue analyses of metallic materials under complex multiaxial loading. The method is based on the stress invariants and uses the minimum circumscribed ellipse approach for evaluating the effective shear stress amplitude under complex multiaxial loading. The applicability of the procedure for handling non‐proportional loading is examined through typical examples such as combined normal/shear stresses and combined bi‐axial normal stresses with complex stress time histories. The effects of phase shift angles, frequency ratios and waveforms on fatigue endurance were re‐analysed and compared with available experimental results from the literature. The comparison shows that the presented procedure based on stress invariants is a potential conservative engineering approach, very suitable for fast fatigue evaluation in the integrated computer aided fatigue design.
Assessment of welded structures by a local multiaxial fatigue approach
Authors: Van Dang K. , Bignonnet A. , Fayard J. L. , Janosch J. J.
Published: March 2003

A new local multiaxial computing method for the fatigue assessment of welded structures is presented. This approach is based on the use of an equivalent stress derived from the multiaxial fatigue criterion proposed by Dang Van for plain machined specimens. Application to fillet welds allows a better description of the influence of local geometries and residual stress states. These factors are related to the weld process and the quality of the weld. This approach, associated with definite shell finite element meshing rules in order to take account of the increase of local rigidity introduced by the weld, is also successfully used to assess the fatigue resistance of welded automotive structures.
Plastic η factor solutions of homogeneous and bi‐material SE(T) specimens for toughness and creep crack growth testing
Authors: Kim Y.‐J. , Budden P. J.
Published: March 2003

Based on slip line field analysis and finite element analysis of elastic‐perfectly plastic materials, plastic η factor solutions for single edge‐cracked specimens in tension (SE(T)) with a wide range of crack lengths are proposed, both for homogeneous specimens and for bi‐material specimens with interface cracks. Moreover, two different plastic η factor solutions are given: one based on experimental load–load line displacement records, ηVLLp , and the other based on experimental load–crack mouth opening displacement (CMOD) records, ηCMODp . Comparison with existing finite element results shows good agreement. For deep cracks (a/w > ∼0.45), the ηVLLp solutions are insensitive to the strain hardening, to the specimen length and to the specimen thickness. However, for shallower cracks (a/w < ∼0.45), the ηVLLp solutions are sensitive to the specimen thickness, to the strain hardening and to the specimen length, suggesting difficulties associated with a robust determination of J and C * integrals from experimental data. On the other hand, the ηCMODp solution is not sensitive to the crack length, to the specimen thickness, to strain hardening and to the specimen length, even for shallow cracked specimens. This suggests that the use of CMOD can provide robust J and C * estimation schemes even for shallow crack testing.
Study of crack growth in solid propellants
Authors: Gdoutos E. E. , Papakaliatakis G.
Published: March 2003

The stress and displacement fields in an edge‐cracked sheet specimen made of a solid propellant and subjected to a uniform displacement along its upper and lower faces was studied. The solid propellant was simulated as a hyperelastic material with constitutive behaviour described by the Ogden strain energy potential. A non‐linear finite deformation analysis was performed based on the finite element code ABAQUS. A detailed analysis of the stress field in the vicinity of the crack tip was undertaken. The deformed profiles of the crack faces near the crack tip were determined. The results of stress analysis were coupled with the strain energy density theory to predict the crack growth behaviour including crack initiation, stable crack growth and final termination for two specimens with different dimensions. Crack growth resistance curves representing the variation of crack growth increment versus applied displacement were drawn.
A ball‐indentation method to evaluate the critical stress for dislocation generation in a silicon substrate
Authors: Ohta H. , Miura H. , Kitano M.
Published: March 2003

Although silicon substrates for semiconductor devices do not contain any dislocations initially, the thin‐film stresses sometimes produce dislocations during fabricating processes. Since the dislocations degrade electrical characteristics, it is important to estimate the critical stress for dislocation generation in order to maintain the integrity and reliability of the device. Therefore, we have developed a method that uses a ball‐press test for evaluating the critical stress for dislocation generation. The critical stress is calculated from the critical press load for dislocation generation. We used this test method to determine the temperature dependence of the critical stress, and found that the change of the critical stress with temperature is not so large when compared with that of the reference tensile test data. This is because, since the tensile specimen contains some dislocations or initial damage, the tensile test data probably includes the influence of the movement of these dislocations. Furthermore, the effect of ion implantation was examined and this makes clear that the critical stress of implanted specimen is decreased.
A bi‐parametric Wöhler curve for high cycle multiaxial fatigue assessment
Authors: Susmel L. , Lazzarin P.
Published: March 2003

This paper presents a method for estimating high‐cycle fatigue strength under multiaxial loading conditions. The physical interpretation of the fatigue damage is based on the theory of cyclic deformation in single crystals. Such a theory is also used to single out those stress components which can be considered significant for crack nucleation and growth in the so‐called Stage I regime. Fatigue life estimates are carried out by means of a modified Wöhler curve which can be applied to both smooth and blunt notched components, subjected to either in‐phase or out‐of‐phase loads. The modified Wöhler curve plots the fatigue strength in terms of the maximum macroscopic shear stress amplitudes, the reference plane − where such amplitudes have to be evaluated − being thought of as coincident with the fatigue microcrack initiation plane. The position of the fatigue strength curve also depends on the stress component normal to such a plane and the phase angle as well. About 450 experimental data taken from the literature are used to check the accuracy of the method under multiaxial fatigue conditions.
Local texture and fatigue crack initiation in a Ti‐6Al‐4V titanium alloy
Authors: Le Biavant K. , Pommier S. , Prioul C.
Published: March 2003

ABSTRACT Fatigue crack initiation was studied in a bimodal TA6V titanium alloy. A ghost structure inherited from the forging process, the scale of which is roughly 100 times the apparent grain size, was found to govern the initiation process. In these macrograins, that we have labelled macrozones, most of the primary alpha grains (αp) are found to display the same crystallographic orientation. Fatigue cracks are initiated on the basal plane or, if basal slip is difficult, on the prismatic plane. Thus in macrozones, where basal or prismatic slip is easy, numerous neighbouring tiny cracks appear over the whole macrozone, which have the size of the primary αp grains. In these macrozones the contribution of crack coalescence to crack growth is consequently very significant. On the contrary, if basal and prismatic slips are both difficult in the macrozone, no crack can be found in the corresponding macrozone. The crack initiation process is thus highly heterogeneous at the scale of the macrozone. Furthermore, this microstructure is found to induce a large scatter in the fatigue life of notched samples.
‘Arching’ effect in elastic polycrystals: implications for the variability of fatigue lives
Authors: Pommier S.
Published: March 2003

ABSTRACT The paper deals with a study of heterogeneous stress and strain distribution in polycrystals in relation with elastic anisotropy of grains. A similitude with the arching effect widely studied in granular materials is proposed and this concept is extended to heterogeneous polycrystals in which the load transfer is not binary in the way it is in granular media but may vary significantly and suddenly from one grain to another according to the crystal orientation to the load direction. Experiments and 3D finite element analyses show that though the individual orientation of grains is random, the strain and stress distribution is not. A network is formed inside the polycrystal whose scale is larger than the grain size. The load percolation network consists in heavily loaded links whose direction is coincident with the direction of the principal stresses. So, the typical scale for the variability of the local stresses is not the grain size but the size of the load percolation network. Since this scale is found to be rather large in particular for iron, zinc and copper, this effect should contribute significantly to the variability of the fatigue lives of notched vs. smooth components.
The effect of material behaviour on the analysis of single crystal turbine blades: Part II – Component analysis
Authors: Maclachlan D. W. , Knowles D. M.
Published: March 2003

This paper describes the analysis of a turbine blade component using a slip system model developed for modern single‐crystal superalloys. Structural elasto‐viscoplastic calculations are carried out for the component. The emphasis throughout is on the effect of micromechanisms of deformation, accounted for in the material model, on the predicted overall behaviour of the component. With the recent proliferation in detailed material models that are available, it is prudent to take a step back and investigate the implications of such models for component analysis and design. This effect is manifested through the determination of a stabilised and redistributed stress state throughout the component. While some components are creep‐limited in design, many are fatigue‐limited and it is stabilised stresses which control the cyclic life of these components. The accuracy of the material model, incorporating various micromechanisms as a function of stress and temperature, can significantly effect these stabilised stresses. The effect of the crystallographic orientation on blade behaviour is illustrated and the implications of shakedown simulations for fatigue lifing of turbine blades are discussed.
Crack growth behavior and fatigue‐life prediction based on worst‐case near‐threshold data of a large crack for 9310 steel
Authors: LIU J. Z. , WU X. R. , DING C. F. , HU B. R. , WANG L. F. , ANNIGERI B. , VESTERGAARD L. H. , ROMANOWSKI A. , SCHNEIDER G. J. , FORTH S. C.
Published: March 2003

ABSTRACT Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks, near‐threshold growth behavior of large cracks at constant R‐ratio/decreasing ΔK and constant Kmax/decreasing ΔK, respectively, for 9310 steel. The results showed that a pronounced small‐crack effect was not observed even at R = −1, small cracks initiated by a slip mechanism at strong slip sites. Worst‐case near‐threshold testing results for large cracks under several Kmax values showed that an effect of Kmax on the near‐threshold behavior does not exist in the present investigation. A worst‐case near‐threshold test for a large crack, i.e. constant Kmax/decreasing ΔK test, can give a conservative prediction of growth behavior of naturally initiated small cracks. Using the worst‐case near‐threshold data for a large crack and crack‐tip constraint factor equations defined in the paper, Newman's total fatigue‐life prediction method was improved. The fatigue lives predicted by the improved method were in reasonable agreement with the experiments. A three‐dimensional (3D) weight function method was used to calculate stress‐intensity factors for a surface crack at a notch of the present SENT specimen (with r/w = 1/8) by using a finite‐element reference solution. The results were verified by limited finite‐element solutions, and agreed well with those calculated by Newman's stress‐intensity factor equations when the stress concentration factor of the present specimen was used in the equations.
Very high cycle regime fatigue of thin walled tubes made from austenitic stainless steel
Authors: CARSTENSEN J. V. , MAYER H. , BRØNDSTED P.
Published: March 2003

ABSTRACT Fatigue life data of cold worked tubes (diameter 4 mm, wall thicknesses 0.25 and 0.30 mm) of an austenitic stainless steel, AISI 904 L, were measured in the regime ranging from 2 × 105 to 1010 cycles to failure. The influence of the loading frequency was investigated as data were obtained in conventional rotating bending at 160 and 200 Hz and in ultrasonic axial loading at 20 kHz. Above 5 × 106 cycles the fatigue lifetimes found with both methods were comparable. The results show that the slope of the S–N curve significantly decreases beyond 108 cycles. Fracture surfaces were examined using scanning electron microscopy. Fatigue cracks initiate at the surface and no significant influence from frequency or from loading modes on fatigue crack initiation and growth is visible.
Non‐propagation conditions for fatigue cracks and fatigue in the very high‐cycle regime
Authors: PIPPAN R. , TABERNIG B. , GACH E. , RIEMELMOSER F.
Published: March 2003

ABSTRACT The propagation and non‐propagation conditions of cracks with crack closure and without crack closure are investigated in various materials. It was found that there exist lower limits for the crack‐driving force at which cracks do not propagate (da/dN is smaller than 10−10 mm cycle−1) for constant and variable amplitude loading. Finally, possible reasons for the growth of cracks below such limits, which may occur in the very high‐cycle regime, are discussed.
High‐cycle rotating bending fatigue property in very long‐life regime of high‐strength steels
Authors: OCHI Y. , MATSUMURA T. , MASAKI K. , YOSHIDA S.
Published: March 2003

ABSTRACT In order to investigate the fatigue properties of high‐strength steels in the very long‐life regime up to over 109 cycles, cantilever‐type rotating bending fatigue tests were carried out for two kinds of high‐strength steels, SUJ2 and SNCM439, which were machined by grinding and finished by electropolishing after grinding. And also, the residual stress on the specimen surface of the ground specimen was examined by X‐ray diffractometer in order to investigate effects of the residual stress on the fatigue properties. From the investigations, the S–N curves clearly have a tendency to decrease again in the longer‐life range over 107 cycles for both types of specimen and for both steels. From observations of fracture surfaces, it was found that fatigue crack origins could be grouped into two types: (i) ‘surface crack origin type’ in the shorter‐life regime and (ii) ‘internal crack origin type’ in the longer‐life regime.
The role of air in fatigue load interaction
Authors: SUNDER R. , PORTER W. J. , ASHBAUGH N. E.
Published: March 2003

Natural fatigue crack formation and growth were studied in notched Al–Cu alloy coupons through high‐resolution SEM fractography. The experiments were conducted under programmed loading conditions designed to induce microscopic marking of the crack formation and growth process under varying stress ratio and closure‐free crack tip conditions. Control experiments were performed by switching between an air and vacuum environment. In air, varying the stress ratio from 0.74 down to 0.64 retards crack growth by up to a factor of five. This ‘closure‐free’ stress ratio history effect totally disappears in vacuum, suggesting a significant environmental influence on stress ratio and its history. Crack‐tip stress state appears to moderate environmental action, revealing a potential mechanism sensitive to residual stress. Consequently, crack closure, residual stress and crack front and plane orientation are identified as major load interaction mechanisms whose synergistic action controls fatigue under variable amplitude loading. The study also appears to suggest that as a consequence of the crack seeking the path of least resistance, load‐sequence sensitive crack plane and front orientation may only induce retardation effects.
On the explicit evaluation of stress intensity factors in the hybrid boundary element method
Authors: DUMONT N. A. , LOPES A. A. O.
Published: March 2003

The hybrid boundary element method, as based on the Hellinger‐Reissner potential for assumed singular stress fields, is suited for the explicit mathematical description of high gradients. In this paper, after an introductory overview of the method and an account of different attempts of application to fracture mechanics problems, one makes use of Westergaard complex functions for numerically handling general curved cracks, in which the exact gradient of the stress field at crack tip is considered, for any planar configurations. Modes I and II stress intensity factors are directly obtained as the primary unknowns of the problem, which eliminates the necessity of post‐processing results. Fairly good accuracy is achieved at low computational costs, as numerically assessed by means of several examples. The formulation seems to present no restrictions, for cases of both single and multiple internal cracks. At present, an extension to generally curved edge cracks as well as to kinked cracks is being investigated, as the method relies on the existence of analytical functions for the explicit description of local stress fields.
Analysis of cracked steel members reinforced by pre‐stress composite patch
Authors: COLOMBI P. , BASSETTI A. , NUSSBAUMER A.
Published: March 2003

Pre‐stress bonded composite patch is a promising technique to reinforce steel member damaged by fatigue. The effectiveness of this technique was verified by fatigue tests on notched steel plates. Results showed that the application of carbon fibre reinforced plastic (CFRP) strips and, eventually, the introduction of a compressive stress by pretension of the CFRP strips prior to bonding produced a significant increment of the remaining fatigue life. In this paper, the stress intensity factor in the notched plates is computed by a two‐dimensional finite element model in connection with the three‐layer technique in order to reduce the computational effort. Due to high stress concentration at the plate crack tip, debond is assumed at the adhesive–plate interface. The goal is to illustrate the influence of some reinforcement parameters such as the composite strip stiffness, the pre‐stress level, the adhesive layer thickness and the size of the debonded region on the effectiveness of the composite patch reinforcement.
Cities and villages: infection hierarchies in a measles metapopulation
Authors: Grenfell  , Bolker
Published: March 2003

An important issue in the dynamics of directly transmitted microparasites is the relationship between infection probability and host density. We use models and extensive spatio‐temporal data for the incidence of measles to examine evidence for spatial heterogeneity in transmission probability, in terms of urban–rural hierarchies in infection rate. Pre‐vaccination measles data for England and Wales show strong evidence for urban–rural heterogeneities in infection rate – the proportion of urban cases rises significantly before major epidemics. The model shows that this effect is consistent with a higher infection rate in large cities, though small towns have epidemic characteristics intermediate between town and country. Surprisingly, urban and rural areas of the same population size have a similar propensity for local extinction of infection. A spatial map of urban–rural correlations reveals complex regional patterns of synchronization of towns and cities. The hierarchical heterogeneities in infection persist into the vaccine era; their implications for disease persistence and control are discussed.
No foundation of a “3/4 power scaling law” for respiration in biology
Authors: Riisgård
Published: March 2003

Respiration rate (R ) as a function of body mass (W  ) is usually expressed as R = aW b. Empirically, the b value is often close to 3/4 when organisms covering a large span in body mass are compared. But recent years research on the energetic cost of growth demonstrate that young and fast growing stages show higher weight specific respiration rates than older and adult stages, and this implies that the b values tend to be higher: b∼ 1 in small (young) organisms falling to b = 0.6–0.7 in larger (older) stages. Thus, respiration and growth are integrated through the energetic costs of growth. This explains why the b value is not a “natural constant” and why a “3/4 power scaling law” cannot be deduced from the interplay between pure physical and geometric constraints of the transport of oxygen.
Mate choice copying as public information
Authors: Nordell  , Valone
Published: March 2003

We outline a framework that views mate choice copying as public information. In this framework, all individuals assess the quality of potential mates. Individuals can benefit by copying when they cannot discriminate differences in potential mates, as copying can result in better discrimination. This framework predicts that the discrimination ability of individuals will strongly determine the frequency of copying behaviour observed: individuals with poor discrimination ability should often copy, whereas individuals with superior discrimination ability should rarely copy. Copying behaviour may occur in a wide variety of taxa but only be observed when individuals are presented with difficult discrimination tasks.
Cheating in mutualism: defection ofYucca baccataagainst its yucca moths
Authors: Bao  , Addicott
Published: March 2003

Yucca baccatacheats in its obligate pollination/seed predation mutualism with yucca moths. Although all individuals use the pollination services of yucca moths, many individuals do not reciprocate in sustaining yucca moth larvae. Cheating is associated with the morphology ofY. baccatapistils. InY. baccata, the apex of the ovary contains only inviable ovules, and there are two distinct flower types, one of which has twice as many potentially viable ovules as the other. Because yucca moths oviposit at the apex ofY. baccataovaries, larvae in flowers with few viable ovules fail to encounter viable ovules and therefore perish. Inflorescences generally have just one flower type, implying that some individuals cheat whereas others maintain the yucca moth population. Our most surprising observation, however, is that although the proportion of cheaters should be low, over 70% ofY. baccataindividuals cheat. We hypothesize that both density‐ and frequency‐dependent processes maintain a balance of cheaters and noncheaters in this system.
Digital imaging for colour measurement in ecological research
Authors: Villafuerte  , Negro
Published: March 2003

Traditional methods for colour quantification are complicated by the fact that colours change depending on illumination, and that different observers often perceive colours differently. Here we describe a new affordable method, which improves methods relying on human observers, to quantify patterns and colour variations. The procedure combines customized software with the use of digital cameras and commercial photofinishing software. The computer routines correct unavoidable illumination changes during image capturing, making all images comparable. Colours are quantified in a continuous scale of the conventional colour models developed for the human vision system, such as HSB, RGB, CMYK, or Lab, amenable for statistical analyses. We illustrate the use of this technique showing a previously unknown sexual dimorphism in the red‐legged partridge,Alectoris rufa, undetectable with the unaided human eye. We also demonstrate that the digital system provides a finer discrimination than human observers for scoring the plumage of partridges belonging to two different subspecies. This method has potential applications in behavioural ecology, physiology, genetics, evolutionary biology, and taxonomy.
Can high tree species richness be explained by Hubbell’s null model?
Authors: Yu  , Terborgh  , Potts
Published: March 2003

We examine several features of Hubbell’s nonequilibrium, or “null”, model of tree dynamics, which holds that species‐rich tropical tree communities are maintained on a local scale by a balance of extinction and immigration, and on a global scale by a balance of extinction and speciation. All species are held to be ecologically equivalent, such that species having equal initial abundances have equal probabilities of extinction or fixation. We show here that the null model is not robust to relaxation of the assumption of ecological equivalence. Recently, Zhang & Lin (1997; J. Theor. Biol. 188: 361–367) showed that persistence times decrease when unequal colonization rates are allowed, but their results still permit very long persistence times in stands of hundreds of thousands of stems or more. We extend their work by allowing tree mortality rates to differ across species, as is seen in all natural tree communities. As a result, persistence times drop dramatically, and forest composition becomes highly deterministic, such that long‐lived species drop out of the community much more slowly than short‐lived species. We also note that the use of tree deaths (instead of years) as a measure of time inflates estimates of persistence times. In summary, calculated persistence times of tropical tree species, even those in very large stands, no longer reach time scales plausible for speciation.
Asymmetry as a predictor of growth, fecundity and survival
Authors: Møller
Published: March 2003

Measures of developmental stability such as fluctuating asymmetry have been assumed to predict individual performance because asymmetry reflects an inability to cope with stressful situations, and because asymmetry hampers locomotion. However, the magnitude of this relationship between important fitness components (growth, fecundity, survival) and asymmetry has never been assessed. Based on a literature survey, estimates of the correlation between asymmetry and the three fitness components are presented. Pearson’s correlation coefficients weighted for sample size between asymmetry and growth, fecundity, and survival, respectively, were –0.15, –0.35, and –0.25, respectively, with all three coefficients being highly significant. All three relationships were extremely robust given very large fail‐safe numbers. The results were independent of whether studies or species were used as units of analysis. Hence asymmetry is a robust predictor of performance in fitness domains such as growth, fecundity, and survival, although only accounting for 2.1%, 12.3%, and 6.0% of the variance. This may be of importance for studies of sexual selection, but also for ecological and conservation biological studies, where the performance of individuals or groups of individuals are assessed.
Can sympatric speciation by disruptive sexual selection explain rapid evolution of cichlid diversity in Lake Victoria?
Authors: Seehausen  , Van Alphen
Published: March 2003

Rapid speciation can occur on ecological time scales and interfere with ecological processes, resulting in species distribution patterns that are difficult to reconcile with ecological theory. The haplochromine cichlids in East African lakes are an extreme example of rapid speciation. We analyse the causes of their high speciation rates. Various studies have identified disruptive sexual selection acting on colour polymorphisms that might cause sympatric speciation. Using data on geographical distribution, colouration and relatedness from 41 species endemic to Lake Victoria, we test predictions from this hypothesis. Plotting numbers of pairs of closely related species against the amount of distributional overlap between the species reveals a bimodal distribution with modes on allopatric and sympatric. The proportion of sister species pairs that are heteromorphic for the traits under disruptive selection is higher in sympatry than in allopatry. These data support the hypothesis that disruptive sexual selection on colour polymorphisms has caused sympatric speciation and help to explain the rapid radiation of haplochromine species flocks.
Mean corpuscular volume (MCV) as a measure of condition in birds
Authors: Bearhop  , Griffiths  , Orr  , Furness
Published: March 2003

Haematological parameters, particularly haematocrit, are frequently used in assessing condition in birds. Mean corpuscular volume (MCV) overcomes some of the drawbacks associated with measuring haematocrits and therefore should be a better condition index. We investigated the relationship between these blood parameters and the breeding performances of great skuas Catharacta skua. As predicted from activity budgets of the sexes, male MCVs were significantly positively correlated with hatch date and the MCVs of both sexes were negatively correlated with the number of chicks fledged. No correlations existed between these breeding parameters and haematocrit. Since males provision females and chicks, they probably influence breeding performance via foraging abilities, whereas in females nutritional investment may be more important. This is the first study of a free‐living/nondomesticated vertebrate population in which these kinds of relationship have been demonstrated and we conclude that MCV is a useful measure of condition.
Variability in relative importance of determinants of reef fish recruitment
Authors: Shima J.S.
Published: March 2003

Multiple processes can act together to determine abundance of organisms and structure of communities. Recently, appreciation of this fact has motivated development of conceptual and statistical frameworks that quantitatively assess the relative importance of multiple causal factors. However, little consideration has been given to variability in the “importance” of processes through space and time (i.e. robustness), which represents another facet of a process’s importance. Here, I focused on populations of a coral reef fish (Thalassoma hardwicke) and used an existing analytical method to assess the relative importance of initial population inputs (larval supply) and subsequent juvenile mortality in determining the average abundances of juvenile fish populations in different locations and times. The relative importance of processes varied significantly both temporally and spatially across a range of scales, and indicate a need for future assessments of relative importance to incorporate this variability.
Determinants of Home Range Size for Polar Bears (Ursus maritimus)
Authors: Ferguson S.H. , Taylor M.K. , Born E.W. , Rosing‐Asvid A. , Messier F.
Published: March 2003

The mean home range size of female polar bears (Ursus maritimus; 125 100 km2 ± 11 800; n = 93) is substantially larger than the predicted value (514 km2) for a terrestrial carnivore of similar weight. To understand this difference, we correlated home range size and sea ice characteristics. Home range size was related to (i) the ratio of land vs. sea within a given home range (42% of explained variance), and (ii) seasonal variation in ice cover (24%). Thus, bears using land during the ice‐free season had larger home ranges and bears living in areas of great seasonal variation in ice cover also had larger home ranges. In another analysis we investigated how variation in a bear’s environment in space and time affects its choice of home range. We found that polar bears adjusted the size of their home range according to the amount of annual and seasonal variation within the centre of their home range. For example, polar bears experiencing unpredictable seasonal and annual ice tended to increase their home range size if increasing home range size resulted in reducing variation in seasonal and annual ice. Polar bears make trade‐offs between alternate space‐use strategies. Large home ranges occur when variable ice cover is associated with more seals but also a more unpredictable distribution of those seals.
Spatial dynamics of adaptive sex ratios
Authors: Ranta E. , Lummaa V. , Kaitala V. , Merilä J.
Published: March 2003

According to Fisherian sex allocation theory, parents that can adjust their offspring sex ratio in response to skews in population sex ratio will maximize their fitness over parents lacking this ability. There is good evidence that adaptive sex ratio adjustment occurs in many natural populations, but deviations from theoretical predictions have also been observed. These anomalies may be more apparent than real. When the spatial dimension of sex ratio variation is ignored, then a mismatch between empirical data and theoretical predictions based on panmictic mating is to be expected. We illustrate this with data on human sex ratio variation in 21 preindustrial populations, and with a cellular automaton model built to obey Fisherian sex allocation rules. The results from the model generally match with the data. When information about the ambient sex ratio is limited, then the sex allocation decisions may appear locally maladaptive. In general, the results indicate that Fisher’s sex‐ratio theory may have greater explanatory power than previously thought.
Antibiotic production in a spatially structured environment
Authors: Wiener Pamela
Published: March 2003

Ecological factors influencing the effects of antibiotic production were explored experimentally and theoretically. A spatially structured model was used to model the dynamics of antibiotic‐producing and nonproducing bacteria in which growth of the nonproducers was reduced by neighbouring antibiotic producers. Various factors affecting spatial interactions between the bacteria were examined for their impact on antibiotic producers. Spatial clustering had a positive impact on the effect of antibiotic production, as measured by the decline in growth of the nonproducing strain, while increasing the initial density of the nonproducing strain had a negative impact. Experiments examined the growth of antibiotic‐producing Streptomyces species and a nonproducing, antibiotic‐sensitive strain of Bacillus subtilis that were coinoculated on surface media. There was an effect of the Streptomyces on Bacillus growth in some experiments but not in others. In light of the predictions from the model, unintentional clustering of cells is a more likely explanation for this finding than different initial Bacillus densities. The importance of spatial structure seen in this study is consistent with a terrestrial rather than an aquatic distribution of antibiotic‐producing bacteria, and may have implications in the search for novel antibiotics.
Is there evidence for the post‐invasion evolution of increased size among invasive plant species?
Authors: Willis A.J. , Memmott J. , Forrester R.I.
Published: March 2003

Many plant species grow taller and have higher reproductive capacity where they are nonindigenous invaders than where they are native components of the flora. Traditionally, it has been accepted that this is a plastic response to a benign environment, though recently this assumption has been challenged and a genetic basis for increased plant size has been invoked. We tested the hypothesis that the increased size of certain weed species is genetically, rather than environmentally, based. A common environment growth experiment revealed no significant differences in the size of Carduus nutans, Digitalis purpurea, Echium vulgare or Senecio jacobaea sampled from alien (Australia and New Zealand) or native (Britain and continental Europe) habitats. We conclude that post‐invasion genetic changes associated with increased size may be unusual and that the phenomenon, where it occurs, generally reflects a plastic response to a novel environment.
Transport carbon costs do not negate the benefits of agricultural carbon mitigation options
Authors: Smith P. , Smith T.J.F.
Published: March 2003

It has been suggested that some agricultural carbon (C) mitigation options will yield no net C benefit under full carbon accounting (i.e. when costs are included alongside benefits). The largest likely C cost of implementing many options is the fuel cost associated with transporting resources from the place where they are produced to the place where they are used. In this article, we show that fuel C costs of transporting resources are much lower than the C benefits of agricultural mitigation options. These findings suggest that the agricultural C mitigation options examined here will yield a net C benefit, even under full carbon accounting.
The functional implications of active and passive pollination in dioecious figs
Authors: Jousselin  , Kjellberg
Published: March 2003

Fig‐pollinating wasps lay their eggs in fig flowers. Some species of fig‐pollinating wasps are active pollinators, while others passively transfer pollen. In dioecious fig species, the ovules of male figs produce wasps but no seeds. By observations and experiments on four dioecious Ficus species we show that (i) passive pollinators distribute pollen haphazardly within figs, but fertilization of female flowers in male figs is inhibited. Consequently, wasp larvae will develop in nonfertilized ovules: they cannot benefit from pollination; (ii) active pollinators efficiently fertilize flowers in which they oviposit. Lack of pollination increases larval mortality. Hence, fig pollinators are not obligate seed eaters but ovule gallers. Active pollination has probably evolved as a way to improve progeny nourishment.

Comparison of pollination and oviposition process in male and female figs, suggests that stigma shape and function have coevolved with pollination behaviour, in relation to constraints linked with dioecy.
Plant–herbivore interactions and ecological stoichiometry: when do herbivores determine plant nutrient limitation?
Authors: Daufresne Tanguy , Loreau Michel
Published: March 2003

Recent studies on plant–herbivore indirect interactions via nutrient recycling have led to the hypothesis that herbivores with a low nitrogen: phosphorus ratio, feeding on plants with a higher nitrogen: phosphorus ratio, recycle relatively more nitrogen, driving plants into phosphorus limitation. We demonstrate in this paper that such a hypothesis is valid only under restricted conditions, i.e. the nitrogen: phosphorus ratio of inorganic nutrients supplied to the system must be neither too high nor too low compared with the nitrogen: phosphorus ratio of the whole plant + herbivore biomass. If plants have a greater affinity for phosphorus than for nitrogen, low herbivore nitrogen: phosphorus ratio can even promote nitrogen limitation. These results are qualitatively robust, whether grazing functions are donor‐controlled or recipient‐controlled. We present a graphical analysis of these conditions based on the Zero Net Growth Isocline method.
Foraging energetics of arctic cormorants and the evolution of diving birds
Authors: Grémillet David , Wanless Sarah , Carss David N. , Linton Danielle , Harris Mike P. , Speakman John R. , Le Maho Yvon
Published: March 2003

Efficient body insulation is assumed to have enabled birds and mammals to colonize polar aquatic ecosystems. We challenge this concept by comparing the bioenergetics of cormorants (Phalacrocorax carbo) living in temperate and arctic conditions. We show that although these birds have limited insulation, they maintain high body temperature (42.3 °C) when diving in cold water (1–10 °C). Their energy demand at these times is extremely high (up to 60 W kg−1). Free‐living cormorants wintering in Greenland (water temperature −1 °C) profoundly alter their foraging activity, thus minimizing time spent in water and the associated high thermoregulatory costs. They then meet their daily food demand within a single intense dive bout (lasting 9 min on average). Their substantial energy requirements are balanced by the highest predatory efficiency so far recorded for aquatic predators. We postulate that similar behavioural patterns allowed early diving birds (Cretaceous) to colonize cold coastal areas before they evolved efficient insulation.
Does “supersaturated coexistence” resolve the “paradox of the plankton”?
Authors: Schippers Peter , Verschoor Antonie M. , Vos Matthijs , Mooij Wolf M.
Published: March 2003

In contradiction with field observations, theory predicts that the number of coexisting plankton species at equilibrium cannot exceed the number of limiting resources, which is called the “paradox of the plankton”. Recently, Huisman & Weissing (1999, 2000) showed, in a model study, that the number of coexisting species may exceed the number of limiting resources when internal system feedback induces oscillations or chaos. In this paper, we use the term “supersaturated coexistence” for this phenomenon. On the basis of these findings, they claimed that the paradox of the plankton is solved. We investigated the prerequisites for supersaturated coexistence in the same model. Our results indicate that supersaturated coexistence is a rare phenomenon in parameter space, requires a very precise parameterization of the community members and is sensitive to the introduction of new species and the removal of the present species. This raises the question of whether supersaturated coexistence is likely to occur in nature. We conclude that the claim by Huisman & Weissing (1999, 2000) is premature.