bluuebunny/crossref_metadata_2025 huggingface dataset:
An experimental and theoretical approach for an estimation of ΔKth
Authors: Herold H. , Streitenberger M. , Zinke M. , Orazi L. , Cammarota G. P.
Published: March 2003

The existence of a fatigue threshold value may affect the design process when a damage‐tolerant design is considered that uses non‐destructive techniques for evaluating the shape and dimensions of the defects inside materials. Obviously it should be possible to estimate the stress field surrounding these defects and this is not generally a problem with modern numerical methods.

Many factors are involved in determining the growth rate of a fatigue crack. Some of these are highly significant and it is possible to obtain the coefficients of a correlation function. Some others are not well defined and the only effect is to expand the scatter of experimental data.

Consider the sigmoidal curve we obtain when plotting the crack growth rate versus the applied ΔKI . A very difficult parameter to measure but very useful for fatigue design is the ΔKIthvalue, because below this value a crack may be forming, hence, here ΔKIthis defined by the transition between a normal (e.g. 10−10 m/cycle) and a very low range of crack growth rate (<10−10 m/cycle).

The ΔKIthvalue is very difficult to obtain by experimental methods because the growth rate is of the order or less than the atomic lattice span (3 × 10−10 m/cycle), but we can correlate the transition value with the cyclic crack tip plastic zone size and other structural parameters of metallic materials.

The aim of this work is to offer a contribution about the parameters which influence ΔKIthin stainless steels and welded joints based on the crack tip plastic zone radius.
An analysis of fracture under biaxial loading using the non‐singular T‐stress
Authors: Andrews R.M. , Garwood S.J.
Published: March 2003

Finite element analysis using a two‐dimensional modified‐boundary‐layer approach wasused to model the effects of biaxial loading on crack tip stress fields. Loadings wereapplied corresponding to an elastic KI field, non‐singular T‐stress and a biaxial stress.For through‐thickness cracks the T‐stress inherent in the specimen geometry isaugmented by the external biaxial stress. For surface‐notched specimens the biaxialstress acts out of the crack plane. This effect was modelled with generalized plane strainelements. Results were analysed using the Anderson‐Dodds approach for cleavage andthe Beremin model in the ductile regime. Biaxial loading is predicted to have a largeeffect on the toughness of a through‐thickness crack but little effect on a surface crack.Experimental results from a previous series of large‐scale biaxial fracture tests aregenerally consistent with these predictions.
Small crack growth in combined bending–torsion fatigue of A533B steel
Authors: Park J. , Nelson D. , Rostami A.
Published: March 2003

Crack growth rate data from bending, torsional and in‐plane and 90° out‐of‐phase combined bending–torsional fatigue tests of A533B steel are presented. Crack growth was monitored from initial sizes generally in the range of 50–300 μm to final sizes of several millimetres. Crack growth rate was found to vary linearly with crack size. Two approaches for correlating the A533B crack growth rate were evaluated, an effective strain‐based intensity factor range and a method based on total cyclic strain energy density. The approaches were also evaluated using small crack growth data from the literature for SAE 1045 steel and Inconel 718 specimens tested under axial–torsional loadings. Predicted crack growth lives using both approaches were found to agree within a factor of two of observed lives for nearly all of the data examined.
Fatigue strength of welded joints under multiaxial loading: experiments and calculations
Authors: Yousefi F. , Witt M. , Zenner H.
Published: March 2003

In reality most welded components are loaded with a combination of different variable forces and moments that often cause a state of multiaxial stress in the fatigue‐critical areas. If the multiaxial loading is non‐proportional, traditional deformation‐based hypotheses are not able to give a reliable lifetime prediction. This investigation is a cooperation between three German research institutes to build an experimental database for the verification of different concepts of lifetime prediction. In accordance with former investigations, a flange‐tube connection made of steel P460 was used. The test program was divided into constant amplitude and variable amplitude tests. The ratio between the nominal bending and shear stress is 1. For the variable amplitude tests, a Gaussian‐standard is used. A lifetime prediction software for multiaxial state of cyclic stress was developed. The software has a modular structure and allows calculations with different hypotheses and methods. The calculations are based on the local elastic stresses. This is an acceptable method for high‐cycle fatigue. In this work, two general types of calculation, the Integral Approach and Critical Plane Approach and a local stress‐based modification of the von Mises Criterion, the hypothesis of effective equivalent stress (EESH) are shown. The damage accumulation is performed with the elementary Miner’s rule (S–N curve without fatigue limit). The statistical distributions of the damage sums are also shown.
The continuous crack flexibility model for crack identification
Authors: Chondros T. G.
Published: March 2003

The presence of a crack in a structural member introduces a local flexibility that affects its dynamic response. Moreover, the crack will open and close in time depending on the loading conditions and vibration amplitude. The changes in dynamic characteristics can be measured and lead to an identification of the structural changes which eventually might lead to the detection of a structural flaw. The results of various independent evaluations of changes in the natural frequency of vibrations of cracked structural elements are reported. A crack model of a continuous flexibility, found with fracture mechanics methods using the displacement field in the vicinity of the crack developed recently is used here. The analytical results for the cracked elements behaviour based on the continuous crack flexibility vibration theory were correlated with numerical solutions, the lumped‐crack beam vibration analysis and experimental results obtained on aluminium and steel beams with open cracks.
Non‐destructive fatigue damage characterization of laminated thermosetting fibrous composites
Authors: Pantelakis* SP. G. , Kyriakakis EM. CH. , Papanikos P.
Published: March 2003

The fatigue behaviour of the laminated thermosetting Fiberdux 6376‐HTA composite material is studied experimentally for both constant and variable amplitude stress reversal loading. The fatigue‐induced material degradation is correlated to non‐destructive evaluation data obtained from C‐scan graphs through the concept of the damage severity factor (DSF ). TheDSFis able to account for the varying severity of damage at the different specimen locations and is used to quantify the fatigue‐induced damage. The concept of theDSF, introduced earlier by the authors for constant amplitude fatigue loading of thermoplastic fibrous composites, is applied to characterize fatigue damage of thermosetting fibrous composites and is extended to account for variable amplitude loading. Constant amplitude fatigue tests at various stress levels were performed to correlate fatigue damage to the change of mechanical properties, such as axial stiffness, residual tensile strength and interlaminar shear strength, and to develop expressions to relateDSFto the degradation of the mechanical properties with increasing fatigue damage. Correlation betweenDSFevolution and consumed fatigue life is made and fatigue damage functions involving stress amplitude dependency are formulated. These expressions together with a modified rainflow method are then used to assess fatigue life under variable amplitude fatigue loading; computed fatigue lives are compared against experimental results.
The effect of material behaviour on the analysis of single crystal turbine blades: Part I – Material model
Authors: Maclachlan D. W. , Knowles D. M.
Published: March 2003

ABSTRACT This paper describes a slip system model developed for the analysis of modern single crystal superalloys – specifically, the first generation alloys RR2000 and SRR99, and the second generation alloy CMSX‐4. The single crystal model is implemented as an ABAQUS User MATerial (UMAT) subroutine, the framework is based on the classical theory of single crystal plasticity. The constitutive equations used have different formulations based on the micromechanisms of deformation and experimental measurements. The emphasis is on the effect of incorporating the micromechanisms of material behaviour on predicted macroscopic results. Several important phenomena and mechanisms, which are required for explaining creep properties as a function of stress, temperature and orientation are identified and included in the model. These include: activation of < 101 > {111} and < 112 > {111} slip systems, rigid body rotation, continuum damage, slip system softening, dislocation interaction, threshold behaviour and rafting. Model simulations are compared with experimental data in various deformation regimes. In a later part of the paper, the model is used to analyse the performance of a single crystal turbine blade. This enables the effect of the micromechanisms of deformation on overall component behaviour to be quantified.
The Equivalent Strain Energy Density approach re‐formulated and applied to sharp V‐shaped notches under localized and generalized plasticity
Authors: LAZZARIN P. , ZAMBARDI R.
Published: March 2003

In the case of a rounded notch, the stress and strain at the notch tip can be determined by the traditional Neuber rule or by the Equivalent Strain Energy Density (ESED) approach, as formulated by Glinka and Molski. In the latter case the elastoplastic strain energy density at the notch tip is thought of as coincident with that determined under purely elastic conditions. For sharply V‐shaped notches this approach is not directly applicable, since the strain energy density at the notch tip tends toward infinity both for a material obeying an elastic law and a material obeying a power hardening law. By using the notch stress intensity factors, the present paper suggests a re‐formulation of the ESED approach which is applied no longer at the notch tip but to a finite size circular sector surrounding the notch tip. In particular we have adopted the hypothesis that, under plane strain conditions, the value of the energy concentration due to the notch is constant and independent of the two constitutive laws. When small scale yielding conditions are present, such a hypothesis immediately results in the constancy of the strain energy averaged over the process volume. As a consequence, plastic notch stress intensity factors valid for sharp V‐shaped notches can be predicted on the basis of the linear elastic stress distributions alone.
Environmental considerations for fatigue cracking
Authors: WEI R. P.
Published: March 2003

ABSTRACT An essential aim for the study of fatigue cracking is the development of science‐based methods for reliability and service life assessments as a part of the life‐cycle design and management of engineered systems. In this context, a critical comparison between a mechanistically based probability and the more traditional, empirically based statistical approaches for reliability analysis and life prediction is presented. The comparison serves to highlight the need for quantitative, mechanistic understanding and modeling and for the adoption of a multidisciplinary approach. Current understanding of the influences of gaseous and aqueous environments on fatigue cracking, particularly with respect to the effects of frequency and temperature, is briefly summarized, and special considerations for fatigue at very high frequencies are discussed. The efficacy of the mechanistically based probability approach is illustrated through a comparison of the estimated and observed distribution of corrosion and corrosion fatigue damage in Boeing 707 aircraft that had been in commercial service for 24 years.
Influence of atmospheric moisture on slow fatigue crack growth at ultrasonic frequency in aluminium and magnesium alloys
Authors: PAPAKYRIACOU M. , MAYER H. , FUCHS U. , STANZL‐TSCHEGG S. E. , WEI R. P.
Published: March 2003

ABSTRACT The deleterious influence of atmospheric moisture on the fatigue properties of an aluminium wrought alloy AlZnMgCu1.5‐T6, an aluminium cast alloy AlSi9Cu3 and magnesium cast alloys AM60 hp, AZ91 hp and AS21 hp has been studied at a cycling frequency of 20 kHz. Atmospheric moisture accelerates fatigue crack growth and decreases the threshold stress intensities to 55–75% of the respective values in vacuum. In ambient air, fatigue crack growth rates were up to two decades higher than those in vacuum. Accelerated crack growth was found at propagation rates below about 2 × 10−9 m cycle−1 in aluminium alloys and below about 3 × 10−8 m cycle−1 in magnesium alloys. As the threshold regime is approached, fatigue cracks in ambient air either propagate at a minimum mean growth rate on average of approximately one lattice spacing per cycle or they stop propagating, whereas mean growth rates of 10−12 m cycle−1 were found in vacuum. Crack initiation and slow fatigue crack growth mainly determine lifetimes in the high cycle regime, and endurance data obtained at ultrasonic frequency in ambient air of 40–60% relative humidity are similar to lifetimes measured at conventional frequencies.
Measurement and analysis of wheel loads for design and fatigue evaluation of vehicle chassis components
Authors: DECKER M. , SAVAIDIS G.
Published: March 2003

Using a multiaxial‐randomly loaded air‐suspension rear‐axle for commercial vehicles as an example, this paper gives an insight in the mechanics and interactions of wheel forces and moments, forces acting in the suspension components and the stress response of the axle casing. Taking load‐time and stress‐time data measured on a prototype vehicle on a test track as a basis, fatigue, frequency and correlation analyses for all relevant manoeuvres and straight‐ahead driving situations are performed. Special focus is given to the study and determination of appropriate correlations between the mentioned properties for manoeuvres and straight ahead driving on rough roads. The results point out all decisive load situations which may affect the fatigue behaviour of the axle under operational conditions and were taken into account for the derivation of optimized design solutions, as well as for the experimental verification of the final solution in the test rig.
Determination of load spectra for durability approval of car drive lines
Authors: OELMANN B.
Published: March 2003

Knowledge of the significan and representative load spectra is one important requirement for a high‐quality, quick durability life approval of vehicle components on a test rig or by calculation. The way that such load spectra can be derived from measurements is demonstrated in this paper by the example of a passenger car drive line. The paper includes a brief introduction followed by an outline of the objectives. The proposed approach will then be illustrated and the results will be presented. Examples of applications will be shown and an outlook for further activities presented.
A perturbation solution for a crack in a power‐law material under gross yielding
Authors: Chen G. X. , Wang C. H. , Rose L. R. F.
Published: March 2003

Abstract In order to develop an analytical method for quantifying the plastic‐blunting behaviour of a short crack embedded in a notch plastic zone, the perturbation solution of He and Hutchinson is extended to include the effect of strain gradient. An edge‐cracked plate subjected to a linearly varying remote strain is considered in this work to simulate the plastic deformation associated with a small crack at a notch root. The strain hardening of the material is assumed to obey a power‐law. Comparison with finite‐element (FE) computations demonstrates that this perturbation solution provides accurate values for the crack‐tip opening displacement (CTOD) under gross‐yielding conditions for a range of hardening parameters.
Fluctuating asymmetry in an insect host: a big role for big parasites?
Authors: Thomas  , Ward  , Poulin
Published: March 2003

Parasites are expected to be associated with host developmental instability because developmentally unstable hosts may be more susceptible to, or more frequently exposed to, parasitic infections compared with developmentally stable ones, or because parasites may directly disrupt host development. In this study, we analysed the relationship between developmental stability in the wetaPleioplectron simplex(Orthoptera) and infections by hairworm and mermithid parasites. These parasites have a long development in their host and the size of adult worms exceeds the length of the host by a considerable amount (15–20 cm). For one character (femora) we found that fluctuating asymmetry was significantly higher in parasitized individuals compared with unparasitized ones, in each of two samples collected in different years. Because no relationship was observed between the level of fluctuating asymmetry and the parasite size or number, however, infection by hairworm and mermithid parasites could be more a consequence of developmental instability than a cause. For other characters (tibia, external and internal spines of femora), the level of fluctuating asymmetry between parasitized and unparasitized individuals was not significantly different. Our results are discussed in relation to ecological constraints met by hairworm and mermithid parasites to complete their life cycle.
Inbreeding and outcrossing in Yucca whipplei: consequences for the reproductive success of plant and pollinator
Authors: Richter Kevin S. , Weis Arthur E.
Published: March 2003

Unlike most pollinators, yucca moths are active pollinators of their host plants. Females lay their eggs in the flowers they pollinate, and their larvae feed solely on the resulting seeds. Previous evidence suggests that the yucca moth Tegeticula maculata avoids self‐pollinating their host Yucca whipplei. Other yucca moths may self‐pollinate more frequently. When pollinating, yucca moths are also reported to fly large distances between plants, bypassing neighbouring plants in the process. We experimentally verify the suggestion of Pellmyr et al. that yucca is more likely to retain fruits from self‐pollination if overall fruit set is low. Thus, selection on moths to avoid self‐pollinating should be density dependent. We found no evidence that mating with close neighbours resulted in inbreeding depression, thus the moth's long‐distance flights between plants are yet to be explained.
Nestedness as a conservation tool: plants and birds of oak‐hazel woodland in Sweden
Authors: Hansson
Published: March 2003

Nestedness was examined for vascular plants and birds in the centres and edges of 26 sites of ancient oak‐hazel woodland in Sweden. Both taxa exhibited significant nestedness in site centres and for whole sites, but not at the edges for birds. Woodland ranks of nestedness differed between plants and birds. Rank of nestedness of birds, but not of plants, depended on area. Horizontal habitat structure affected nestedness of both plants and birds. Mobility appears decisive for creating rank differences between sites for various taxa. High mobility may also explain a greater edge effect in birds from allochtonous, more or less transient individuals. Nestedness in relation to mobility, particularly at edges, should be of theoretical interest. The possible use of nestedness patterns in conservation makes further analyses urgent for less mobile taxa.
Below‐ground herbivory promotes soil nutrient transfer and root growth in grassland
Authors: Bardgett  , Denton  , Cook
Published: March 2003

Extremely little is known about the ecosystem‐level implications of below‐ground herbivory, which often represents the dominant form of consumption of primary productivity. We provide the first empirical evidence that low levels of below‐ground herbivory may promote soil nutrient flux and root growth of both host plants and companion plants. Low levels of white clover (Trifolium repens L.) root infection by clover cyst nematodes (Heterodera trifolii Goffart) increased root growth by 141% and 219% in the host plant and the uninfected neighbouring grass (Lolium perenne L.), respectively. Root infection increased the size of the soil microbial biomass in the root zone and the transfer of 15N from the host plant to soil and the neighbouring grass. These data suggest that low amounts of below‐ground herbivory may increase the transfer of plant carbon and nitrogen below‐ground, leading to increases in root growth and soil nutrient recycling in grasslands. Presumably, such interactions will influence the competitive interactions between plant species, altering plant community structure in grasslands.
Perceptual scales of spatial heterogeneity of periphyton for freshwater snails
Authors: Kawata  , Agawa
Published: March 2003

The hypothesis that individuals recognize their environment as homogeneous when the scales of spatial heterogeneity of resources are smaller than a certain scale, but can distinguish them as patches when they are larger than that scale was tested using freshwater snails (Physa acuta) in various distributions in periphyton environments. In a pattern of periphyton distribution in which the size of algal cells was 47 mm, individuals moved significantly more slowly on algal cells than on nonalgal cells. However, in other patterns in which the sizes of algal cells were 23.5 mm and 15.7 mm, the speeds of individuals on algal and nonalgal cells were not significantly different. These results support the hypothesis that individuals use the environment homogeneously when the scales of spatial heterogeneity of resources are smaller than a certain scale, but they can distinguish between patches when the scales are larger.
The immunological function of allosuckling
Authors: Roulin A. , Heeb P.
Published: March 2003

Young mammals are unable to mount an efficient immune response against invading pathogens. Until their immune system is mature mothers transmit to their young immunological compounds during lactation. Given that genetic and foster mothers can assume this protective role, we propose that young mammals may gain immunological benefits by suckling more than one nursing female, a behaviour referred to as “allosuckling”. This hypothesis has so far not been considered as a potential explanation for the propensity of young mammals to suckle foster mothers. However, pathogen transmission through milk during allosuckling may reduce the immunological net benefit that young gain, and furthermore allosuckling may increase pathogen transmission between foster and genetic mothers implying costs of allosuckling for all participants. Here, we develop the immunological function of allosuckling hypothesis (IFA) as a potential explanation for intra‐and interspecific variation in allosuckling frequency. We present published experimental evidence for the assumption that immunological benefits of allosuckling depend on the immunological status of the offspring, the foster and the genetic mothers. Finally, we give predictions arising from the IFA hypothesis and propose that the IFA may provide a new explanation as to why neonates suckle various females and why foster females often refuse to nurse nonoffspring.
Bower‐building: coevolution of display traits in response to the costs of female choice?
Authors: Humphries  , Ruxton
Published: March 2003

Bower‐building is peculiar to polygynous species in the family Ptilonorhynchidae (Bowerbirds). Here we summarize past and current theories regarding the evolution of bower‐building, evaluate the evidence that can be used to differentiate between them and suggest future areas for research.
Mixture analysis of asymmetry: modelling directional asymmetry, antisymmetry and heterogeneity in fluctuating asymmetry
Authors: Van Dongen  , Lens  , Molenberghs
Published: March 2003

The occurrence of different forms of asymmetry complicates the analysis and interpretation of patterns in asymmetry. Furthermore, between‐individual heterogeneity in developmental stability (DS) and thus fluctuating asymmetry (FA), is required to find relationships between DS and other factors. Separating directional asymmetry (DA) and antisymmetry (AS) from real FA and understanding between‐individual heterogeneity in FA is therefore crucial in the analysis and interpretation of patterns in asymmetry. In this paper we introduce and explore mixture analysis to (i) identify FA, DA and AS from the distribution of the signed asymmetry, and (ii) to model and quantify between‐individual heterogeneity in developmental stability and FA. In addition, we expand mixtures to the estimation of the proportion of variation in the unsigned FA that can be attributed to between‐individual heterogeneity in the presumed underlying developmental stability (the so‐called hypothetical repeatability). Finally, we construct weighted normal probability plots to investigate the assumption of underlying normality of the different components. We specifically show that (i) model selection based on the likelihood ratio test has the potential to yield models that incorporate nearly all heterogeneity in FA; (ii) mixtures appear to be a powerful and sensitive statistical technique to identify the different forms of asymmetry; (iii) restricted measurement accuracy and the occurrence of many zero observations results in an overestimation of the hypothetical repeatability on the basis of the model parameters; and (iv) as judged from the high correlation coefficients of the normal probability plots, the underlying normality assumption appears to hold for the empirical data we analysed. In conclusion, mixtures provide a useful statistical tool to study patterns in asymmetry.
Defoliation and mycorrhizal symbiosis: a functional balance between carbon sources and below‐ground sinks
Authors: Saikkonen K. , Ahonen‐Jonnarth U. , Markkola A.M. , Helander M. , Tuomi J. , Roitto M. , Ranta H.
Published: March 2003

Herbivory is generally assumed to negatively influence mycorrhizal fungi because of reduced photosynthate to support mycorrhizae following defoliation. We examined effects of 60% and 100% defoliation (excluding current year needles) on tree growth and ectomycorrhizal associations of 10–15 year old Scots pines (Pinus sylvestris). Over 98% of short roots were colonized by mycorrhizal fungi, and contrary to expectation, defoliation did not decrease the proportion of living fungi in fine roots. Furthermore, defoliation did not alter the ratios of produced needle biomass to the biomass of fine roots or living fungi in fine roots. The composition of mycorrhizal morphotypes was changed, however, which suggests competition among different mycorrhizal growth forms owing to their carbon demands. We propose that these outcomes are a consequence of a functional balance between carbon sources in plant foliage and below‐ground sinks, i.e. growing roots and mycorrhizal associates.
Patterns of species interaction strength in assembled theoretical competition communities
Authors: Kokkoris G.D. , Troumbis A.Y. , Lawton J.H.
Published: March 2003

Strength of interactions between species may be an important tool in our effort to understand community structure. Recent theoretical and empirical findings suggest that despite the presence of some strong interactions, weak interactions prevail in communities. Here, we examine how mean interaction strengths change as theoretical competition communities assemble and what the distribution of interaction coefficients is in the communities that are formed during the assembly process. Our results show that the mean competition strengths fall as assembly progresses and that most interactions in the communities formed are weak. Communities that are invulnerable to further invasions are those where interspecific interactions are weaker than the average interaction strength between species in the pool. If these results can be generalized to more than one trophic level, implications for management and conservation of natural communities are substantial.
Prey size of single‐prey loaders as an indicator of prey abundance
Authors: Houston A.I.
Published: March 2003

In this paper I am concerned with the behaviour of seabirds that bring back just one prey item at a time to their young. I use a simple model from central place foraging theory to show that the size of fish that a parent bird brings back may increase or decrease with an increase in the abundance of fish. This means that it may not be possible to use the size of fish that is fed to the young as an indicator of prey abundance.
The influence of arbuscular mycorrhizae on the relationship between plant diversity and productivity
Authors: Klironomos John N , McCune Jenny , Hart Miranda , Neville John
Published: March 2003

Ecological theory predicts a positive and asymptotic relationship between plant diversity and ecosystem productivity based on the ability of more diverse plant communities to use limiting resources more fully. This is supported by recent empirical evidence. Additionally, in natural ecosystems, plant productivity is often a function of the presence and composition of mycorrhizal associations. Yet, the effect of mycorrhizal fungi on the relationship between plant diversity and productivity has not been investigated. We predict that in the presence of AMF, productivity will saturate at lower levels of species richness because AMF increase the ability of plant species to utilize nutrient resources. In this study we manipulated old‐field plant species richness in the presence and absence of two species of AMF. We found that in the absence of AMF, the relationship between plant species richness and productivity is positive and linear. However, in the presence of AMF, the relationship is positive but asymptotic, even though the maximum plant biomass was significantly different between the two AMF treatments. This is consistent with the hypothesis that AMF increase the redundancy of plant species in the productivity of plant communities, and indicates that these symbionts must be considered in future investigations of plant biodiversity and ecosystem function.
The interspecific biomass–density relationship for terrestrial plants: where do clonal red seaweeds stand and why?
Authors: Scrosati
Published: March 2003

For crowded stands of terrestrial plants, ranging from mosses to trees, plant (or ramet, for clonal plants) density is negatively related to stand biomass. Stand biomass and ramet density were determined for Mazzaella cornucopiae and for Pterocladiella capillacea, two morphologically distinct intertidal clonal red seaweeds, to compare them with terrestrial plants. For these seaweeds, ramet densities were similar to the highest values reported for terrestrial plants (mosses, specifically). Stand biomass was higher than average values expected from the terrestrial interspecific biomass–density relationship, but lower than the limits expected from the terrestrial ultimate biomass–density line. These seaweeds show unexpectedly low ramet slenderness and high biomass packing per unit of volume, compared with the trend observed for terrestrial plants. Possible explanations for these differences are related to the particular physiology and habitat of intertidal clonal seaweeds.
Progressive invasion and allopatric speciation can also explain distribution patterns of rock‐dwelling cichlids from southern Lake Victoria: a comment on Seehausen and van Alphen (1999)
Authors: Bouton
Published: March 2003

I show that distribution patterns of rock‐dwelling haplochromine cichlids in the south‐eastern part of Lake Victoria can be explained by allopatric speciation alone. Observed patterns are consistent with a progressive invasion of different lineages from older ( = deeper) parts of the lake, after which some lineages speciate more frequently than others. Based on a quantitative analysis of distribution data, Seehausen and van Alphen conclude that a high percentage of sympatric speciation (17.5%) is essential to explain the patterns. In my view this conclusion has its origins in an unjustifiable assumption. Seehausen and van Alphen assume that the unresolved phylogeny is balanced. However, allopatric speciation would typically result in imbalanced phylogenies.
Causes and consequences of incest avoidance in the cooperatively breeding mole‐rat, Cryptomys darlingi (Bathyergidae)
Authors: Greeff J.M. , Bennett N.C.
Published: March 2003

A number of social mole‐rat species maintain a strong reproductive skew (only one breeding pair in the group) solely through incest avoidance. Incest avoidance probably evolved for one of two reasons, namely for actually maintaining a reproductive skew or, alternatively, to avoid high inbreeding depression. In the latter case a strong reproductive skew would result as a fortuitous by‐product of the combination of a cloistral family life style of mole‐rats and incest avoidance. We undertook breeding experiments in which the fertility of pairs of unrelated individuals were compared with that of pairs of double first cousins. Inbreeding depression was remarkably high and an accompanying model suggests that it may be sufficient to support the idea that strong incest avoidance evolved primarily to eliminate the costs of inbreeding and subsequently facilitated the evolution of reproductive skew.
The evolution of female‐limited polymorphisms in damselflies: a signal detection model
Authors: Sherratt T.N.
Published: March 2003

Female‐limited intraspecific colour variation is a widely distributed trait within damselflies. Typically, one morph resembles the male (the andromorph) whereas one, or sometimes more, do not [the heteromorph(s)]. While several selective explanations have been offered, such as decreased harassment by males balanced by predation or lack of mating success, field data indicate that andromorphs and heteromorphs mate at equal frequencies in the field, and survive equally well. In this paper, I use a signal detection model to characterize the properties of a new male‐mimicry hypothesis, in which andromorphs are not only more similar to males, but are also encountered more by males. I show that this combination of frequency‐dependent and frequency‐independent factors readily combine to generate a balanced polymorphism. The model explains why morphs have similar mean mating frequencies, why the experimentally observed mating preferences of males vary between ponds, and why the frequency of andromorphs tends to rise with sex ratio.
Plant‐mediated indirect effects and the persistence of parasitoid–herbivore communities
Authors: Vos M. , Berrocal S.M. , Karamaouna F. , Hemerik L. , Vet L.E.M.
Published: March 2003

We have examined the effects of herbivore diversity on parasitoid community persistence and stability, mediated by nonspecific information from herbivore‐infested plants.

First, we investigated host location and patch time allocation in the parasitoid Cotesia glomerata in environments where host and/or nonhost herbivores were present on Brassica oleracea leaves. Parasitoids were attracted by infochemicals from leaves containing nonhost herbivores. They spent considerable amounts of time on such leaves. Thus, when information from the plant is indistinct, herbivore diversity is likely to weaken interaction strengths between parasitoids and hosts. In four B. oleracea fields, all plants contained herbivores, often two or more species. We modelled parasitoid–herbivore communities increasing in complexity, based on our experiments and field data. Increasing herbivore diversity promoted the persistence of parasitoid communities. However, at a higher threshold of herbivore diversity, parasitoids became extinct due to insufficient parasitism rates. Thus, diversity can potentially drive both persistence and extinctions.
Habitat destruction and extinction in competitive and mutualistic metacommunities
Authors: Klausmeier C.A.
Published: March 2003

Because habitat loss is a leading cause of extinction, it is important to identify what kind of species is most vulnerable. Here, I use algebraic and graphical techniques to study metacommunity models of weak competition or locally facultative mutualism in which species may coexist within patches. Because a competition–colonization trade‐off is not required for regional coexistence of competitors, poor competitors are often regionally rare and most prone to extinction, in contrast to results from previous models of strongly competitive metapopulations. Metacommunities of mutualists can suffer the abrupt extinction of both species as habitat destruction is increased. These highlight the importance of identifying the mechanisms by which species coexist to predict their response to habitat loss.
Characterizing population vulnerability for 758 species
Authors: Fagan  , Meir  , Prendergast  , Folarin  , Karieva
Published: March 2003

We investigate relationships between life history traits and the character of population dynamics as revealed by time series data. Our classification of time series is according to ‘extinction category,’ where we identify three classes of populations: (i) weakly varying populations with such high growth rates that long‐term persistence is likely (unless some extreme catastrophe occurs); (ii) populations with such low growth rates that average population size must be large to buffer them against extinction in a variable environment; and (iii) highly variable populations that fluctuate so dramatically that dispersal or some other refuge mechanism is likely to be key to their avoidance of extinction. Using 1941 time series representing 758 species from the Global Population Dynamics Database, we find that, depending on the form of density dependence one assumes, between 46 and 90% of species exhibit dynamics that are so variable that even large carrying capacities could not buffer them against extinction on a 100‐year time horizon. The fact that such a large proportion of population dynamics are so locally variable vindicates the growing realization that dispersal, habitat connectedness, and large‐scale processes are key to local persistence. Furthermore, for mammals, simply by knowing body size, age at first reproduction, and average number of offspring we could correctly predict extinction categories for 83% of species (60 of 72).
Predicting the direction of sexual selection
Authors: Kokko  , Monaghan
Published: March 2003

Our current understanding of the operation of sexual selection is predicated on a sex difference in parental investment, which favours one sex becoming limiting and choosy over mates, the other competitive and nonchoosy. This difference is reflected in the operational sex ratio (OSR), the ratio of sexually receptive males to females, considered to be of fundamental importance in predicting the direction of sexual selection. Difficulties in measuring OSR directly have led to the use of the potential reproductive rates (PRR) as a measure of the level of investment in offspring of males and females. Several recent studies have emphasized that other factors, such as variation in mate quality and sex differences in mortality patterns, also influence the direction of sexual selection. However, as yet there has been no attempt to form a comprehensive theory of sex roles. Here we show that neither OSR nor PRR is the most fundamentally important determinant of sex roles, and that they are not interchangeable. Instead, the cost of a single breeding attempt has a strong direct effect on competition and choosiness as well as consistent relationships to both OSR and PRR. Our life history based approach to mate choice also yields simple, testable predictions on lack of choice in either sex and on mutual mate choice.
Restoration of biodiversity enhances agricultural production
Authors: Bullock James M. , Pywell Richard F. , Burke Mike J.W. , Walker Kevin J.
Published: March 2003

Restoration of ecological communities is important to counteract global losses in biodiversity. However, restoration on agricultural land is perceived as being costly because of losses in agricultural production. We suggest the reported positive relationship between diversity and productivity means biodiversity could be used to enhance agricultural production. We examined this in hay meadow restoration experiments at seven sites across southern Britain. At each site two seed mixes (“species‐poor” with 6–17 species and “species‐rich” with 25–41 species) were applied in a randomised block experiment. Hay yield was higher in the species‐rich treatment from the second year onward, by up to 60%. Comparing the two treatments in all sites, there was a simple linear relationship between the difference in species number and the amount of increase in hay production. Fodder quality was the same in both treatments. This suggests farmers can maximize high quality herbage production in re‐sown grasslands by maximizing biodiversity.
Transient dynamics and persistence of ecological systems
Authors: Hastings Alan
Published: March 2003

Using spatially coupled predator–prey systems as an example of a cyclic ecological system where coexistence depends on oscillations, transient dynamics of models where there are no stable persistent solutions are shown to be a reasonable explanation of persistence over ecological time scales. The parameter values leading to transients within the context of a particular model may be far from parameter values that lead to stable solutions, so transients will need to be explicitly considered in model analysis. Since natural systems with many coupled oscillating species are common, and natural communities are often reset by disturbances or seasonality, transients should play a central role in understanding natural systems.
Stoichiometric food quality and herbivore dynamics
Authors: Muller Erik B. , Nisbet Roger M. , Kooijman Sebastiaan A. L. M. , Elser James J. , McCauley Edward
Published: March 2003

Herbivores may grow with nutrient or energy limitation, depending on food abundance and the chemical composition of their food. We present a model that describes herbivore growth as a continuous function of two limiting factors. This function uses the synthesizing unit concept, has the hyperbolic Monod model as a limiting case, and has the same number of parameters as the Monod model coupled to Liebig’s discontinuous minimum rule. We use the model to explore nutrient‐limited herbivore growth in a closed system with algae, Daphnia and phosphorus as the limiting nutrient. Phosphorus in algae may substantially influence Daphnia growth. This influence changes over time and is most pronounced when algae and Daphnia populations fluctuate strongly. Relative to classic models that only consider food quantity as a determinant of Daphnia growth, our model shows richer dynamical behaviour. In addition to the standard positive equilibrium, which may be stable or unstable depending on nutrient availability, a new positive equilibrium may arise in our model when mortality rates are relatively high. This equilibrium is unstable and reduces the likelihood of long‐term persistence of Daphnia in the system.
The light : nutrient ratio in lakes: a test of hypothesized trends in bacterial nutrient limitation
Authors: Chrzanowski Thomas H. , Grover James P.
Published: March 2003

The light‐to‐nutrient hypothesis explores how the balance between energy (as light energy) and nutrients (as total phosphorus) shapes aquatic ecosystem structure and process. The balance of energy and nutrients is thought to regulate ecosystem structure and process such that, in a “high” light‐to‐nutrient environment, bacteria would probably be driven towards phosphorus (P) limitation, whereas, in a “low” light‐to‐nutrient environment, bacteria would be driven towards carbon (C) limitation. We assessed the growth limitation of bacteria in two reservoirs of the southern U.S.A. using a mortality‐corrected dilution‐growth approach. We compared the frequency of P and C growth limitation with the intralake variation in the light‐to‐nutrient environment. As a metric of the light‐to‐nutrient environment, we used the ratio of the mean light in the surface mixed layer (Im) to the total phosphorus concentration (TP ). In each lake, bacterial growth was more often P‐limited when the Im : TP ratio was above the median ratio than below. We believe our data provide the first evidence supporting this aspect of the light‐to‐nutrient hypothesis.
Complexity can enhance stability in competitive systems
Authors: Rozdilsky Ian D. , Stone Lewi
Published: March 2003

Empirical observations often indicate that complexity enhances stability, while most theoretical studies, such as May’s (1972) classic paper, point to the opposite. Despite the wide generality of these latter theoretical analyses, our examination of the well‐known competitive Lotka–Volterra system reveals that increasing complexity (measured in terms of connectance) can enhance species coexistence and persistence in model communities (measured in terms of their feasibility and stability). The high feasibility and stability found for tightly interconnected competitive subsystems might provide an explanation for the clumped structure in food webs.
Food web structure and habitat loss
Authors: Melián Carlos J. , Bascompte Jordi
Published: March 2003

In this paper we explore simple food web models to study how metacommunity structure affects species response to habitat loss. We find that patch abundances and extinction thresholds vary according to the kind of food web. Second, for intermediate species, a slight decrease in the exploration cost of the better competitor has a strong effect on the extinction threshold of the poorer competitor. When predicting extinction risk one should consider not only the amount of habitat destroyed, but also the structure of the food web in which species are embedded. Both direct and indirect interactions are critical for predicting the consequences of habitat destruction.
When random sampling does not work: standard design falsely indicates maladaptive host preferences in a butterfly
Authors: Singer Michael C. , Stefanescu Constantí , Pen Ido
Published: March 2003

In experiments that investigate species’ interactions, individuals are often chosen at random to represent their populations. However, this practice can generate misleading results when individuals of different species do not interact at random. We illustrate this effect by examining oviposition preferences of Euphydryas aurinia butterflies from three populations using three different plant genera. We first offered each insect a randomly chosen member of its own host population and a foreign host (Succisa pratensis) not present in the insect’s habitat. The butterflies uniformly preferred the foreign Succisa over their own hosts. Preferences were apparently maladaptive because insects wasted time searching for a nonexistent plant. We repeated the experiment using individual hosts that had naturally received eggs in the field. The overall preference for Succisa and the appearance of maladaptation both disappeared. In the original experiments, our random choice of experimental host individuals had combined with strong within‐species discrimination by the butterflies and with overlap of acceptability between host species to obscure the true nature of host preference.