bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of notch geometry on the local cleavage fracture stress σf
Authors: Wang G. Z. , Wang H. J. , Chen J. H.
Published: March 2003

An elastic–plastic finite element method (FEM) is used to analyse the stress and strain distributions ahead of notches with various depths and flank angles in four‐point bending (4PB) specimens of a C–Mn steel. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σf is measured. By increasing the notch depth and notch flank angle from 2.25 to 8.25 mm and 10 to 90°, respectively, the distributions of high stress and strain at the moment of fracture show considerable variations. However, the value of σf stays relatively constant. The critical fracture event is thus shown to be identical, i.e. the propagation of a ferrite grain‐sized crack into the neighbouring matrix. It is concluded that σf is mainly determined by the length of the critical microcrack, while the notch geometry and its associated stress volume have little effect on the value of σf . The cleavage site ahead of a notch is determined by the stress distributions and the positions of the weakest grains.
Numerical investigation of creep crack growth in cross‐weld CT specimens. Part I: influence of mismatch in creep deformation properties and notch tip location
Authors: Segle P. , Andersson P. , Samuelson L. Å.
Published: March 2003

Creep crack growth (CCG) in cross‐weld CT specimens is investigated using two‐dimensional finite element simulations. A creep ductility‐based damage model describes the accumulation of creep damage ahead of the crack tip where a constraint parameter and the creep strain rate perpendicular to the crack plane are used as characterizing parameters.

The numerical results reveal that, not only the material properties of the region in which the crack is propagating, but also the deformation properties of the surrounding material influence the CCG behaviour. For the specimen configurations investigated, the location of the starter notch in the HAZ of the cross‐weld CT specimen has, however, a minor influence on the CCG rate and the value of C *. This applies as long as the crack is propagating within a sufficiently narrow region that has material properties which can be regarded as homogeneous.
Short fatigue cracks in austempered ductile cast iron (ADI)
Authors: Marrow  , Çetinel
Published: March 2003

The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 107 cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus decelerated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit.
Stress intensity factors for a surface‐breaking crack in a half‐plane subject to contact loading
Authors: Seed G. M.
Published: March 2003

Modes I and II stress intensity factors are derived for a crack breaking the surface of a half‐plane which is subject to various forms of contact loading. The method used is that of replacing the crack by a continuous distribution of edge dislocations and assume the crack to be traction‐free over its entire length. A traction free crack is achieved by cancelling the tractions along the crack site that would be present if the half‐plane was uncracked. The stress distribution for an elastic uncracked half‐plane subject to an indenter of arbitrary profile in the presence of friction is derived in terms of a single Muskhelishvili complex stress function from which the stresses and displacements in either the half‐plane or indenter can be determined. The problem of a cracked half‐plane reduces to the numerical solution of a singular integral equation for the determination of the dislocation density distribution from which the modes I and II stress intensity factors can be obtained. Although the method of representing a crack by a continuous distribution of edge dislocations is now a well established procedure, the application of this method to fracture mechanics problems involving contact loading is relatively new. This paper demonstrates that the method of distributed dislocations is well suited to surface‐breaking cracks subject to contact loading and presents new stress intensity factor results for a variety of loading and crack configurations.
A model to assess the fatigue behaviour of ageing aircraft fuselage
Authors: Diamantakos I. D. , Labeas G. N. , Pantelakis SP. G. , Kermanidis TH. B.
Published: March 2003

A computer efficient model for assessing the fatigue behaviour of ageing aircraft components in the presence of multiple site damage is introduced. The model involves the computation of crack initiation scenarios on the basis of a probabilistic approach and the estimation of the fatigue behaviour of complex, highly loaded aircraft structures using a deterministic concept. To reduce the computing effort because of the model size and mesh difficulties in crack propagation calculations, a sub‐structuring procedure under the FE method is utilized. An incremental approach for calculating crack initiation and crack propagation has been involved; it leads to a further essential reduction of the computing effort. Computer simulation is compared with experimental results for characteristic multiple site damage problems of aluminium 2024 lap and butt joints.
Fracture behaviour of PC/ABS resin under mixed‐mode loading
Authors: Husaini  , Kishimoto K. , Notomi M. , Shibuya T.
Published: March 2003

Fracture behaviour of polycarbonate (PC)/acrylonitrile‐butadiene‐styrene (ABS) under mixed‐mode loading conditions was studied for several weight fractions of PC and ABS. Mode I and mixed‐mode fracture tests were carried out by using compact–tension–shear specimens. At a certain value of mixed‐mode loading ratio KII /KI  a crack of the shear type will initiates at the initial crack tip. Fracture toughness increases under mixed‐mode loading with an increase in the mode II component, whereas it reduces with the appearance of a shear‐type fracture. Fracture toughness and the appearance of a shear‐type fracture depends on the blending ratio of PC and ABS. The transition to shear‐type fracture occurs at lower value of KII /KI for resins with higher fracture toughness.
Critical linkages of coalescing microcracks under stress loading
Authors: LI T. , YANG W.
Published: March 2003

ABSTRACT The fracture process of brittle materials with randomly orientated microcracks critically depends on strong interactions among microcracks and the coalescence path that leads to a fatal crack. In this paper, a model based on the coalescence process for planar orientated microcracks is presented. An energy ratio is defined as the competition between the potential energy release and the new crack surface energy in each coalescence step, which is a token of the excessive driving force for microcrack propagation. A critical linkage dictates the coalescence of microcracks under stress loading. Probabilities of microcrack coalescence dominated by the first linkage and subsequent linkages are analysed for collinear and wavy microcrack arrays in detail.
On ‘multi‐stage’ fatigue life diagrams and the relevant life‐controlling mechanisms in ultrahigh‐cycle fatigue
Authors: MUGHRABI H.
Published: March 2003

ABSTRACT In recent years, several fatigue studies on different metallic materials have indicated that fatigue failures can occur even at amplitudes below the conventional high‐cycle fatigue (HCF) fatigue limit in the gigacycle or ultrahigh‐cycle fatigue (UHCF) range (number of cycles to failure in excess of ca. 107−108). In the latter case, fatigue failures were observed to originate from internal defects (non‐metallic inclusions). The S–N curves displayed a multi‐stage shape, sometimes with a second lower fatigue limit in the UHCF range.

The present paper specifies the conditions under which internal and/or surface fatigue failure can occur at low load levels below the conventional HCF fatigue limit. The relevant fatigue thresholds are discussed for two classes of materials, namely pure single‐phase metallic materials (type I) without internal defects and materials such as steels or cast materials (type II) with internal defects (inclusions or pores). In the case of type II materials, the probability of surface versus internal fatigue is discussed in terms of the volume density, size and location of the inclusions and the relevant cracking mechanisms. It is argued that, in both type I and type II materials, multi‐stage S–N curves or Manson–Coffin plots can be expected under certain conditions.
Specific features of high‐cycle and ultra‐high‐cycle fatigue
Authors: LUKÁŠ P. , KUNZ L.
Published: March 2003

ABSTRACT Several specific features of high‐cycle fatigue (HCF) and ultra‐high‐cycle fatigue (UHCF) are discussed both on the basis of direct experimental results and on the basis of speculations extrapolating HCF data to the UHCF regions. The following points are dealt with: (i) Extent and distribution of cyclic plastic deformation. (ii) Sensitivity of the fatigue behaviour in the HCF and UHCF regions to the start‐up procedure of the test and to the preceding stress–strain history. (iii) Role of persistent slip bands (PSBs) and the microcrack initiation in the HCF and UHCF regions. (iv) Dependence of the notch sensitivity on the amplitude of cycling. (v) Effect of high number of very small amplitude cycles on creep behaviour of superalloy single crystals.
Long‐term corrosion fatigue behaviour of structural materials
Authors: EBARA R.
Published: March 2003

ABSTRACT In this paper, long‐term corrosion fatigue behaviour of structural materials such as 12Cr stainless steel and Ti−6Al−4V alloy for steam turbine moving blade and high‐strength steels for ship and offshore structures up to the ultrasonic frequencies is reviewed mainly on the basis of the author's experimental results. The emphasis is focused upon the corrosion fatigue behaviour of 12Cr stainless steel in NaCl and NaOH aqueous solutions. It is shown that the corrosion fatigue strength of 12Cr stainless steel is strongly dependent on NaCl content in aqueous solution. It is also exemplified that the ultrasonic corrosion fatigue strength of 12Cr stainless steel decreases remarkably if an aggressive environment like 40% NaOH aqueous solution should be formed at the crack bottom even in a dilute NaOH aqueous solution.
Durability assessment of automotive aluminium parts
Authors: HEULER P. , BIRK O.
Published: March 2003

Automotive chassis components made from aluminium alloys are often used without any paint or corrosion protection system. Therefore, the fatigue performance under corrosive environments has to be assessed as part of the design process. Literature data reveal a diffuse picture with regard to the extent of fatigue strength reduction due to corrosive environments. On the other hand, there is no standard or well‐established guideline on relevant details of corrosion fatigue tests. Thus the definition of such tests has to be based on individual judgement. After a brief review of corrosion fatigue data and trends collected from the literature, test results are reported for components tested under simplified spectrum load cases in 3.5% NaCl including spectrum variations via omission and truncation. These data backed up by a numerical fatigue life prediction exercise have been used to support decisions on test conditions to be adopted for proof testing.
Fatigue design of structures under thermomechanical loadings
Authors: Charkaluk E. , Bignonnet A. , Constantinescu A. , Dang Van K.
Published: March 2003

This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.
Evaluating the linear normalization technique for deriving J‐resistance curves
Authors: TARPANI J. R. , COLAFEMEA A. A. , BOSE FILHO W. W. , SPINELLI D.
Published: March 2003

In this paper, results from the linear normalization (LN) technique of Reese and Schwalbe for deriving J‐crack resistance (J–R) curves have been compared, related to J–Δa (J‐integral–ductile crack growth) data points, to those obtained from traditional elastic compliance technique. Research results regarding a nuclear grade steel exhibiting a wide range of elastic–plastic fracture resistance agree quite well for both techniques until a certain level of toughness of the material. Below this critical level, LN produces inconsistent results for the sub‐sized compact tension specimens (0.4T C[T]). The evidence suggests that the loss of applicability of the LN technique can be determined on the basis of the η plastic factor (ηpl) for the best linear correlation achieved for ΔPN–Δa (normalised load gradient–ductile crack growth) data.
Micromechanics modelling of ductile fracture in tensile specimens using computational cells
Authors: DOS SANTOS F. F. , RUGGIERI C.
Published: March 2003

This study extends the computational cell framework to model ductile fracture behaviour in tensile specimens. In the computational cell model, ductile damage occurs through void growth and coalescence (by cell extinction) within a thin layer of material located well inside the fracture process zone for the ductile process. Laboratory testing of a high strength structural steel provides the experimental stress–strain data for round bar and circumferentially notched tensile specimens to calibrate the cell model parameters for the material. Numerical simulations employing the micromechanics model reproduce the essential features of the ductile behaviour for the tensile specimens, including the development of intense necking and void growth in the centre of the specimen cross section. The resulting methodology enables the detailed study of ductile failure in small‐scale tensile specimens.
A shakedown model for high‐cycle fatigue
Authors: CRUZ I. , ZOUAIN N.
Published: March 2003

The phenomenological description of the elastic accommodation occurring in a representative volume is implemented in a high‐cycle fatigue criterion. A model for fatigue is directly formulated in the usual continuum mechanics elastic formulation; there, internal variables are introduced to represent the changes in the mesoscopic level of the material. These internal variables do not interact with the macroscopic elastic analysis. The condition for infinite life is the existence of internal variables, independent of time, and rendering admissible stress variations with respect to a fatigue admissibility criterion. The model is applied to experimental data and compared to other criteria.
On the relationship and the short fatigue crack threshold
Authors: McEVILY A. J. , ENDO M. , MURAKAMI Y.
Published: March 2003

Murakami and Endo have used the parameter to successfully predict the endurance limits and the threshold levels for components, which contain small defects or cracks. The present paper uses a modified linear–elastic fracture mechanics (LEFM) approach to examine the mechanistic basis for these correlations. The modifications include consideration of the endurance limit rather than the threshold level as a factor controlling fatigue crack growth in the very short crack growth range, consideration of elastic–plastic behaviour, and consideration of the role of crack closure in the wake of a newly formed crack. Predictions based upon the modified LEFM behaviour are found to be consistent with the earlier predictions of Murakami and Endo.
The anatomy of the interspecific abundance–range size relationship for the British avifauna: I. Spatial patterns
Authors: Gaston Kevin J. , Blackburn Tim M. , Gregory Richard D. , Greenwood Jeremy J.D.
Published: March 2003

Data from the British Trust for Ornithology Common Birds Census and two atlases of breeding birds were used to examine the form of the interspecific abundance–range size relationship for the British avifauna. The relationship is positive for both farmland and woodland habitats and over two different periods, with some evidence of curvilinearity, using either proportion of occupied sites or numbers of occupied 10 × 10 km squares as measures of range size, and mean density at occupied sites as a measure of abundance. A log‐linear plot gives the highest correlation. The relationship is stronger if based on maximum local densities than if based on average densities, but there is no relationship using minimum local densities. Relationships based on abundances at individual sites are uniformly positive for all sites, although the relationships for many sites also show evidence of curvilinearity, especially when range size is measured as the proportion of occupied sites. Species show significant concordance in their rank abundances across sites. We discuss some implications of these results.
Competition and facilitation in multispecies plant‐herbivore systems of productive environments
Authors: Huisman Jef , Olff Han
Published: March 2003

We develop a multispecies plant‐herbivore model to explore how plant competition for light and the selectivity of herbivores affect abundance patterns of plants and herbivores along productivity gradients. The model considers a small and a tall plant species, a generalist herbivore, and a selective herbivore. The selective herbivore feeds only on the small plant species. In the absence of the generalist herbivore, the tall plant species becomes increasingly dominant with increasing productivity, and the small plant and its selective herbivore disappear. The model shows that generalist herbivores can facilitate selective herbivores by suppressing competition for light. This favours the small plant species, and thereby the selective herbivores. The model predictions are qualitatively consistent with field studies of multispecies plant‐herbivore systems.
Environmental stress increases the prevalence and intensity of blood parasite infection in the common lizard Lacerta vivipara
Authors: Oppliger  , Clobert  , Lecomte  , Lorenzon  , Boudjemadi  , John‐Alder
Published: March 2003

Parasites affect the life‐histories and fitness of their hosts. It has been demonstrated that the ability of the immune system to cope with parasites partly depends on environmental conditions. In particular, stressful conditions have an immunosuppressive effect and may affect disease resistance. The relationship between environmental stress and parasitism was investigated using a blood parasite of the common lizardLacerta vivipara.In laboratory cages, density and additional stressors had a significant effect on the intensity of both natural parasitaemia and parasitaemia induced by experimental infection. Four weeks after infection, crowded lizards had three times more parasites than noncrowded lizards. After 1 month of stress treatment, naturally infected lizards had a significantly higher level of plasma corticosterone and a higher parasite load than nonstressed individuals. In seminatural enclosures, stress induced by the habitat quality affected both the natural blood parasite prevalence and the intensity of parasitaemia of the host.
The “school trap”: a mechanism promoting large‐amplitude out‐of‐phase population oscillations of small pelagic fish species
Authors: Bakun  , Cury
Published: March 2003

Small coastal pelagic marine fishes often form mixed schools containing two or more species. Individuals of a less abundant species, which are entrained in schools that are dominated numerically by a more abundant species, may be adversely impacted. This may help to account for an observed pattern of strong alternations in dominance between sardines and anchovies in a number of regions of the world’s oceans.
Amphipod gigantism dictated by oxygen availability?
Authors: Spicer  , Gaston
Published: March 2003

A recent study of gigantism in amphipods proposes a novel mechanism which could be used to explain Bergmann’s rule, namely differences in environmental O2 concentration. Consideration of the respiratory biology of this group indicates significant flaws in this proposal. Nonetheless, it is not unreasonable to predict a relationship between ectotherm body size and O2 partial pressure (pO2) in some systems, where the partial pressure gradient is steep, either in time (comparison of Carboniferous and Permian atmospheres) or space (animals inhabiting oceanic O2 minimum layers, and, particularly in freshwaters, living at high altitude). Animal gigantism can be correlated with periods of very high environmental pO2 over geological time. Unfortunately, data for present day spatial patterns are scant.
Red–blue plots for detecting clusters in count data
Authors: Perry J.N. , Winder L. , Holland J.M. , Alston R.D.
Published: March 2003

A new index and four new graphical displays, termed “red–blue” plots, are presented to study and measure clustering in spatially referenced count data. The index can detect clusters in the form of patches, comprising several nearby large counts, and in the form of gaps, comprising several nearby small counts. The new methods quantify the degree to which the count for each sample unit contributes towards the overall degree of clustering, either as part of a patch or as a gap; provide tests of nonrandomness to detect clustering; and facilitate a comprehensive definition of the size and dimension of a cluster. The methods are illustrated using aphid field data.
Spatial explicit demography: The effects habitat patch isolation have on vole matrilines
Authors: Johannesen E. , Andreassen H.P. , Ims R.A.
Published: March 2003

Populations of many species are spatially structured in matrilines, and their dynamics may be determined by matriline specific demographic processes. We examined whether the isolation of habitat patches (i.e. interpatch distance) affected the demography of matrilines in 14 experimentally fragmented populations of the root vole. Matrilines inhabiting the most isolated patches decreased in size over the breeding season, while matrilines in less isolated patches increased. The survival rate of adult females was the main factor underlying the variation in growth rates among matrilines. Low survival when patches were isolated seemed to be due to long‐distance interpatch movements exposing females to increased predation rate.

The differential success of matrilines in patchy populations with variable interpatch distances acted to decrease the matrilineal diversity at the population level. Furthermore, isolated patches may function as sinks. Thus spatially explicit landscape features may affect both population demography and genetics.
Differential effects of tropical arbuscular mycorrhizal fungal inocula on root colonization and tree seedling growth: implications for tropical forest diversity
Authors: Kiers Erica T. , Lovelock Catherine E. , Krueger Eileen L. , Herre Edward A.
Published: March 2003

The potential for mycorrhizae to influence the diversity and structuring of plant communities depends on whether their affinities and effects differ across a suite of potential host species. In order to assess this potential for a tropical forest community in Panama, we conducted three reciprocal inoculation experiments using seedlings from six native tree species. Seeds were germinated in sterile soil and then exposed to arbuscular mycorrhizal fungi in current association with naturally infected roots from adults of either the same or different species growing in intact forest. The tree species represent a range of life histories, including early successional pioneers, a persistent understory species, and emergent species, typical of mature forest. Collectively, these experiments show: (i) the seedlings of small‐seeded pioneer species were more dependent on mycorrhizal inocula for initial survival and growth; (ii) although mycorrhizal fungi from all inocula were able to colonize the roots of all host species, the inoculum potential (the infectivity of an inoculum of a given concentration) and root colonization varied depending on the identity of the host seedling and the source of the inoculum; and (iii) different mycorrhizal fungal inocula also produced differences in growth depending on the host species. These differences indicate that host–mycorrhizal fungal interactions in tropical forests are characterized by greater complexity than has previously been demonstrated, and suggest that tropical mycorrhizal fungal communities have the potential to differentially influence seedling recruitment among host species and thereby affect community composition.
Can plants use entomopathogens as bodyguards?
Authors: Elliot  , Sabelis  , Janssen  , Van Der Geest  , Beerling  , Fransen
Published: March 2003

For 20 years, ecologists have been gathering evidence in support of the hypothesis that plants can use insect natural enemies such as predators and parasitoids as bodyguards to protect themselves from herbivory, but entomopathogens have escaped this consideration. We extend the bodyguard hypothesis to ask whether plants can use entomopathogens as bodyguards. We first discuss the evolutionary context of such tritrophic interactions and then categorize possible mechanisms as: (1) maintaining a population of bodyguards on the plant surface, (2) increasing contact rates between insect host and pathogen and (3) increasing the susceptibility of the host. We explore these mechanisms further, examining published studies for evidence for the hypothesis. We then discuss potential costs to the plant of promoting pathogens as bodyguards which may include a reduction in the efficiency of other “bodyguard” species, the incidental promotion of plant pathogens and the risk of entomopathogens developing phytopathogenicity. Aside from our intention to stimulate the testing of the bodyguard hypothesis with entompathogens and to provide a conceptual framework for this, we hope to bring evolutionary ecology and insect pathology closer together.
Optimal time to patency in parasitic nematodes: host mortality matters
Authors: Morand  , Poulin
Published: March 2003

We develop an optimality model based on classical epidemiological models to investigate the optimal time to patency in parasitic nematodes in relation to host mortality and parasite mortality. We found that the optimal time to patency depends on both host longevity and prepatent mortality of nematodes. We tested our models using a comparative analysis of the relationships between nematode time to patency, nematode mortality and host mortality. Although we confirmed the importance of prepatent mortality, we also found a significant positive influence of host mortality. Host mortality rate affects parasite survivorship and life history strategies in the same way that habitat‐specific mortality regimes drive the evolution of life histories in free‐living organisms.
Plant community diversity and invasibility by exotics: invasion of Mediterranean old fields by Conyza bonariensis and Conyza canadensis
Authors: Prieur‐Richard A.‐H. , Lavorel S. , Grigulis K. , Dos Santos A.
Published: March 2003

A series of communities were established in situ to differentiate the effects of species richness, functional richness and functional group identity on invasibility of Mediterranean annual old fields. We monitored the demographic and vegetative parameters of two exotic annuals introduced as seedlings, Conyza bonariensis and C. canadensis. Community species richness and functional composition determined resistance to invasion by Conyza.Conyza bonariensis biomass decreased with increasing species richness. Legumes increased the biomass and consequently the net fecundity of both Conyza, while survival was favoured by Asteraceae. Communities with fewer Asteraceae and grasses increased the reproductive effort of C. bonariensis. A separate glasshouse experiment using the same species mixes revealed that establishment of Conyza decreased with increasing species richness or when grasses were present. Patterns of Conyza performance are interpreted in the light of measurements of ecosystem functional parameters, making it possible to formulate hypotheses about mechanisms limiting community invasibility.
Phenotypic plasticity and the possible role of genetic assimilation: Hypoxia‐induced trade‐offs in the morphological traits of an African cichlid
Authors: Chapman L.G. , Galis F. , Shinn J.
Published: March 2003

In this study we investigate the possible role of phenotypic plasticity and genetic assimilation in the process of adaptation and evolutionary change in the cichlid Pseudocrenilabrus multicolor victoriae. In the field we compared a population of a stable hypoxic habitat with one of a stable well‐oxygenated habitat. In the laboratory, we compared individuals from the same mother raised under hypoxic or well‐oxygenated conditions to examine phenotypic plasticity. Morphological parameters of three categories were measured: (a) the gill apparatus, (b) the surrounding structural elements, and (c) the outer shape of the fish. Swamp‐dwelling fish had a 29% greater total gill surface area than fish from the well‐oxygenated habitat due to their larger gill filament length and greater lamellar area. In the plasticity experiment, total gill surface area was 18% greater in the hypoxia group due to a larger number of longer filaments. Surrounding elements and outer shape also differed between the field populations and between fish grown under hypoxic and well‐oxygenated conditions, but there was disparity between the field results and the plasticity experiment. The disparity between field and experimental fish may be due to: (a) differences in selection pressures between populations, (b) different constraints for genetic and plasticity changes, or (c) selection against plastic responses to hypoxia. Our results suggest that both (a) and (c) are involved.
Experimental evidence for testis size evolution via sperm competition
Authors: Hosken D.J. , Ward P.I.
Published: March 2003

Sperm competition theory predicts increased spermatogenic investment with increased sperm competition risk when competition is numerical. There is ample correlational evidence for this relationship in a wide range of taxa. However, as with all correlations, this does not establish cause and effect. Nevertheless, there are no published experimental studies of the evolutionary influence of sperm competition on testis size. We report here on evolutionary responses of testis size to variation in sperm competition intensity in the yellow dung fly. Experimental flies were divided across two treatments, polyandrous or monogamous, with four replicates of each. There was a rapid evolutionary response in testis size resulting from selection via sperm competition, with larger testes found when sperm competition intensity was greatest. These results provide direct experimental evidence of evolutionary change consistent with macro‐evolutionary patterns found across a wide range of taxa.
Fisherian and “good genes” benefits of mate choice: how (not) to distinguish between them
Authors: Kokko H.
Published: March 2003

“Good genes” models of mate choice are commonly tested by examining whether attractive males sire offspring with improved survival. If offspring do not survive better (or indeed survive less well), but instead inherit the attractiveness of their father, results are typically interpreted to support the Fisherian process, which allows the evolution of preferences for arbitrary traits. Here, I show that the above view is mistaken. Because of life‐history trade‐offs, an attractive male may perform less well in other components of fitness. A female obtains a “good genes” benefit whenever males show heritable variation in quality, even if high‐quality males invest so much in sexual advertisement that attractiveness has no positive correlation with any other life‐history trait than male mating success itself. Therefore, a negative correlation between attractiveness and viability does not falsify good genes, if mating with a high‐quality male results on average in superior offspring performance (mating success of sons included). The heritable “good genes” benefit can be sustained even if sexually antagonistic genes cause female offspring sired by high‐quality males to survive and reproduce less well. Neglecting the component of male mating success from measurements of fitness returns from sons and daughters will bias the advantage of mating with a high‐quality male downwards. This result may partly account for the rather weak “good genes” effects found in a recent meta‐analysis.
The ghost of granivory past
Authors: Howe H.F. , Brown J.S.
Published: March 2003

We report a 36‐month echo of vole (Microtus pennsylvanicus) seed‐selection in the composition of synthetic forb communities in Wisconsin. An initial study showed direct suppression of an exceptionally large‐seeded (seed > 15 mg) species (Silphium integrifolium, Asteraceae) by seed‐size predation by rodents during the winter, resulting in indirect release of small‐seeded (seed ≤ 1.5 mg) species. Twelve months after planting, plant diversities (Simpson’s D) were 33% higher in plots exposed to winter seed selection by rodents. Thirty‐six months after planting, Silphium integrifolium had almost equalized densities in rodent access (20.8 ± 4.1 m–2) and exclusion (29.0 ± 5.2 m–2) plots, but still suppressed both large‐seeded (≥ 3.5 mg) and small‐seeded (≤ 1.5 mg) species (F1,16=11.84 and F1,16=10.42, P ≤ 0.005, respectively). A multivariate analysis of covariance (MANCOVA) distinguished effects of Silphium integrifolium (Wilk’s lambda P=0.029) from echoes of earlier winter granivory (Wilk’s lambda P=0.014). Thirty months after rodent exclusion, diversity (D) remained 27% higher in plots once opened to winter granivory (adjusted mean 4.70 ± 0.37 SE) as compared with closed plots (3.70 ± 0.26; F1,16=5.12, P < 0.05). Echoes of earlier granivory remained after rodent‐induced imbalances in the abundances of the competitive driver of this system, Silphium integrifolium, all but disappeared.
Attractive sinks, or how individual behavioural decisions determine source–sink dynamics
Authors: Delibes Miguel , Ferreras Pablo , Gaona Pilar
Published: March 2003

Gundersen et al. (2001, Source‐sink dynamics: how sinks affect demography of sources. Ecol. Lett., 4, 14–21.) suggested that sinks can severely affect the demography of populations in source habitats. We propose this is a common result when animals lack cues associated with reduced fitness inside sinks and consequently select habitat inappropriately. These attractive sinks can result either from undetected risks of mortality (as in the experiment of Gundersen et al. 2001) or from undetected poor breeding probabilities (due to bioaccumulation of pesticides, for instance). Thus, individual habitat choice is a key process underlying source–sink dynamics.
How large do reserve networks need to be?
Authors: Rodrigues Ana S. L. , Gaston Kevin J.
Published: March 2003

Reserve networks are essential for the long‐term persistence of biodiversity. To fulfil this goal, they need not only to represent all species to be conserved but also to be sufficiently large to ensure species’ persistence over time. An extensive literature exists on the required size of individual reserves, but to date there has been little investigation regarding the appropriate size of entire networks. The IUCN’s proposal that 10% of each nation be reserved is often presented as a desirable target, but concerns have been raised that this is insufficient and is dictated primarily by considerations of feasibility and politics.

We found that the minimum percentage of area needed to represent all species within a region increases with the number of targeted species, the size of selection units, and the level of species’ endemism. This has important implications for conservation planning. First, no single universal target is appropriate, as ecosystems or nations with higher diversity and/or higher levels of endemism require substantially larger fractions of their areas to be protected. Second, a minimum conservation network sufficient to capture the diversity of vertebrates is not expected to be effective for biodiversity in general. Third, the 10% target proposed by the IUCN is likely to be wholly insufficient, and much larger fractions of area are estimated to be needed, especially in tropical regions.
Against Bergmann’s rule: fly sperm size increases with temperature
Authors: Blanckenhorn Wolf U. , Hellriegel Barbara
Published: March 2003

A long‐standing school textbook biological rule, Bergmann’s rule, asserts that animals (and their constituent parts) grow bigger when it is colder. This seems to hold for many warm‐blooded animals, as well as for egg, cell and body size of most cold‐blooded animals. A unifying mechanism producing this pattern has not been found. We here provide the first experimental evidence that the size of an important type of cell, namely sperm, increases (rather than decreases) with temperature in a cold‐blooded animal, the yellow dung fly. By pointing to an exception, our work either questions the generality of one prominent category of explanation of Bergmann’s rule, that of a physiological constraint, or alternatively suggests that sperm differ fundamentally in their physiology from other cells.
Begging scrambles with unequal chicks: interactions between need and competitive ability
Authors: Parker Geoff A. , Royle Nick J. , Hartley Ian R.
Published: March 2003

When offspring compete for the attentions of provisioning parents, empirical and theoretical work has generally concluded that chicks honestly signal their “need” for resources and that parents control allocation. Here, we develop models to show that when allocation of food resources is determined by competitive begging scrambles between sibs, the offspring’s ESS begging levels, shares of food and personal fitness gained will be determined by an interaction between their competitive abilities and their true needs. Many of the predictions of this scramble competition model are qualitatively very similar to models of honest signalling of need, where parents, not offspring, control the allocation of food. Consequently it will be difficult to distinguish between the two mechanisms of food allocation based on empirical observations of the responses of chicks to feeding by parents.
Body size and invasion success in marine bivalves
Authors: Roy Kaustuv , Jablonski David , Valentine James W.
Published: March 2003

The role of body size in marine bivalve invasions has been the subject of debate. Roy et al. found that large‐bodied species of marine bivalves were more likely to be successful invaders, consistent with patterns seen during Pleistocene climatic change, but Miller et al. argued that such selectivity was largely driven by the inclusion of mariculture species in the analysis and that size‐selectivity was absent outside of mariculture introductions. Here we use data on non‐mariculture species from the north‐eastern Pacific coast and from a global species pool to test the original hypothesis of Roy et al. that range limits of larger bivalves are more fluid than those of smaller species. First, we test the hypothesis that larger bivalve species are more successful than small species in expanding their geographical ranges following introduction into new regions. Second, we compare body sizes of indigenous and non‐indigenous species for 299 of the 303 known intertidal and shelf species within the marine bivalve clade that contains the greater number of non‐mariculture invaders, the Mytilidae. The results from both tests provide additional support for the view that body size plays an important role in mediating invasion success in marine bivalves, in contrast to Miller et al. Thus range expansions in Recent bivalves are consistent with patterns seen in Pleistocene faunas despite the many differences in the mechanisms.
Tropical pasture carbon cycling: relationships between C source/sink strength, above‐ground biomass and grazing
Authors: Wilsey Brian J. , Parent Gabrielle , Roulet Nigel T. , Moore Tim R. , Potvin Catherine
Published: March 2003

We measured net ecosystem CO2 exchange (NEE) in Panamá over C4 pasture plots that varied in grazing intensity. After adjusting for variation in light, there were noticeable effects of grazing‐related variables on CO2 exchange that were largely dependent on the developmental stage of the plant canopy. Above‐ground productivity was positively related to grazing intensity (r2=0.30). Two experimentally grazed fields had significantly lower standing crop biomass but no significant difference in CO2 uptake (24.2 μmol/m2/s) compared with two ungrazed fields (20.3 μmol/m2/s). Grazed fields had significantly lower ecosystem respiration rates (10.3 μmol/m2/s) than did ungrazed fields (17.6 μmol/m2/s). These results suggest that, although these pastures were possible sources of CO2 during the time intervals sampled, the size of the sources tended to be dampened by cattle grazing through reductions in ecosystem respiration. Thus, it appears that disturbance caused by cattle grazing will not always result in an increase in CO2 release from tropical pastures to the atmosphere.
The alpha–beta–regional relationship: providing new insights into local–regional patterns of species richness and scale dependence of diversity components
Authors: Gering Jon C. , Crist Thomas O.
Published: March 2003

Ecologists frequently regress local species richness on regional species richness to draw inferences about the processes that structure local communities. A more promising approach is to quantify the contributions of alpha and beta diversity to regional diversity (the ABR approach) using additive partitioning. We applied this approach to four local–regional relationships based on data from 583 arboreal beetle species collected in a hierarchically nested sampling design. All four local–regional relationships exhibited proportional sampling, yet the ABR approach indicated that each was produced by a different combination of alpha and beta richness. Using the results of the ABR analysis, we also analysed the scale dependence of alpha and beta using a hierarchical linear model. Alpha diversity contributed less than expected to regional diversity at the finest spatial scale and more than expected at the broadest spatial scale. A switch in relative dominance from beta to alpha diversity with increasing spatial scale suggested scale transitions in ecological processes. Analysing the scale dependence of diversity components using the ABR approach furthers our understanding about the additivity of species diversity in biological communities.
Terrestrial vertebrates promote arbuscular mycorrhizal fungal diversity and inoculum potential in a rain forest soil
Authors: Gehring Catherine A. , Wolf Julie E. , Theimer Tad C.
Published: March 2003

We examined whether terrestrial vertebrates affected the arbuscular mycorrhizal fungal spore communities and mycorrhizal inoculum potential (MIP) of a tropical rain forest soil by comparing plots where terrestrial vertebrates had been excluded for 3 years to adjacent control plots. We extracted spores from soil using sucrose density gradient centrifugation and assayed MIP by growing seedlings of maize (Zea mays) and a rain forest tree (Flindersia brayleana) in intact soil cores from exclosure and control plots. Control plots had significantly higher spore abundance, species richness and diversity than exclosures. Spore community composition also differed significantly between exclosure and control plots. Seedlings of both plant species grown in control cores had significantly higher arbuscular‐mycorrhizal colonization than those grown in exclosure cores. This study suggests that loss of vertebrates could alter rates of mycorrhizal colonization with consequences for community and ecosystem properties.
Refuting a controversial case of a human‐mediated marine species introduction
Authors: Wares John P. , Goldwater Deena S. , Kong Bo Y. , Cunningham Clifford W.
Published: March 2003

Human activities have strongly impacted natural communities through the introduction of non‐native species in historical times. A frequently cited marine example is Littorina littorea, a common intertidal gastropod that was first reported in North America in 1840. The seemingly sudden appearance and rapid geographical spread of this species southward from Nova Scotia has led many researchers to consider L. littorea a human‐mediated species introduction. This is despite allozyme and subfossil evidence that the `European periwinkle' was in North America long before 1840. Our mitochondrial and nuclear DNA sequence data confirm that L. littorea has been in continuous residence in North America for at least 8000 years. It appears most likely that ecological interactions, rather than oceanographic or climatic forces, maintained the limited geographical distribution of L. littorea prior to the 19th century.
The 2002 European seal plague: epidemiology and population consequences
Authors: C. Harding Karin , Härkönen Tero , Caswell Hal
Published: March 2003

We present the first epidemiological data on the 2002 outbreak of phocine distemper virus (PDV) in European harbour seals (Phoca vitulina). The epizootic curve to date supports a mortality rate and probability of infection identical to that of the 1988 outbreak, which killed 58% of the population. Thus immunity is playing no significant role in the dynamics of the current outbreak. Because the timing of the outbreak is important in determining local mortality rates, we predict higher mortality rates on the European continent than in Great Britain or Ireland. A stochastic model is used to quantify how recurrent epizootics affect the long‐term growth, fluctuation, and persistence of the population. Recurrent PDV epizootics with the observed frequency and severity would reduce the long‐term stochastic growth rate of the harbour seal population by half, and significantly increase the risk of quasi‐extinction.