bluuebunny/crossref_metadata_2025 huggingface dataset:
MODAL APPROACH FOR PROCESSING ONE‐ AND THREE‐POINT BEND TEST DATA FOR DSIF–TIME DIAGRAM DETERMINATION. PART II—CALCULATIONS AND RESULTS
Authors: Rokach IV
Published: March 2003

In Part I of this paper, using the modal superposition method, equations for dynamic SIF calculations are derived for an arbitrary linear model of an impact bend specimen. In this paper (Part II), modal parameters and other data which are necessary for the DSIF determination have been calculated for three types of specimen model: the Euler–Bernoulli beam model, and two‐ (2D) and three‐dimensional (3D) solid models. For the latter two cases, calculations were performed using the finite element program ADINA. Results for the 2D model of the specimen were fitted by polynomials for a wide range of specimen geometry parameters and Poisson’s ratio values. Considerable differences were observed between the beam model parameters and the 2D or 3D ones. The differences in results for the 2D and 3D models are small and mainly connected with non‐uniformity of the SIF distribution along the front of a through‐crack in the 3D solid. Results of processing one‐ and three‐point bend test data reported in the literature are presented. Numerical DSIF values are compared with the experimental ones.
EFFECT OF GRAIN SIZE ON COLLECTIVE DAMAGE OF SHORT CRACKS AND FATIGUE LIFE ESTIMATION FOR A STAINLESS STEEL
Authors: Hong  , Qiao  , Liu  , Zheng
Published: March 2003

In the short crack regime of the fatigue process, grain boundaries in steels are barriers against crack growth. In this paper, we use: (1) a method involving crack density; and (2) a method of dimensional analysis, to evaluate the effects of grain size and grain‐boundary resistance on short crack behaviour and fatigue life. The results show that the fatigue life increases with a decrease in grain size and an enlargement in the obstacle effect of a grain boundary. An experimental investigation is consequently performed and four groups of stainless steel specimens are used with different grain sizes. The experimental measurements show the dependence of fatigue properties on grain size, which are in good agreement with the theoretical results.
CREEP BEHAVIOUR OF FULL SIZE WELDED JOINTS: DESIGN IMPROVEMENTS
Authors: Allais L , Dessalas JP , Forgeron T , Fran D , Huthmann H , Escaravage C , Poette C , Sassoulas H
Published: March 2003

In order to ascertain the validity of creep strength reduction factors for welds in real components, creep tests were performed at 600 °C on large specimens made of welded 316 L (N) stainless steel plates. Manual Metal Arc (MMA) welds as well as Tungsten Inert Gas (TIG) welds were investigated. The overall, and in some cases, local deformations of the weld were recorded. The experimental results were compared with two‐dimensional, and in some cases, three‐dimensional finite elements computations using the creep behaviour of the materials as determined by testing small standard specimens. The times to rupture were compared with predictions from design codes and other simplified engineering methods.
CRITICAL PLANE APPROACHES IN HIGH‐CYCLE FATIGUE: ON THE DEFINITION OF THE AMPLITUDE AND MEAN VALUE OF THE SHEAR STRESS ACTING ON THE CRITICAL PLANE
Authors: Papadopoulos IV
Published: March 2003

Critical plane approaches are useful methods when designing against long‐term fatigue of machine components made from metals. Somewhat surprisingly, the very basic problem of the evaluation of the amplitude and mean value of the shear stress acting on the critical plane is still not resolved satisfactorily for non‐proportional cyclic loading conditions. In the present paper, existing proposals for solving this problem are briefly reviewed and their weaknesses highlighted. Then it is shown, through particular examples, that application of these proposals can lead to ambiguous results. Therefore, new definitions of the amplitude and mean value of the shear stress acting on the critical plane are formulated here. These new definitions are free from any ambiguity because they are based on the construction of the unique minimum‐circumscribed circle to the path described by the shear stress on the critical plane. The centre of this circle defines the mean shear stress, whereas its radius provides the corresponding shear stress amplitude. The algorithm yielding this minimum‐circumscribed circle is presented in some detail.
Numerical evaluation of the stress distribution ahead of a crack tip for a finite‐width centre‐cracked specimen loaded by stresses along a portion of the crack
Authors: Liu W. N. , Mang H. A. , Liu C. H.
Published: March 2003

Recently, the force balance method (FBM) has been proposed to calculate geometric correction factors for the stress distribution ahead of a crack tip. In this paper, the effectiveness of the FBM is proved by means of a numerical evaluation of the stress distribution ahead of a crack tip in finite‐width centre‐cracked specimens loaded by stresses along a portion of the crack. The finite element method is used for this purpose.
Modelling damage accumulation in filled polymers
Authors: Bykov
Published: March 2003

A variant of the non‐linear endochronic viscoelasticity theory is developed wherein the reduced‐time function depends on a damage parameter, equivalent strains or stresses, as well as on some functions enabling one to distinguish between loading and unloading processes. Some combinations of the first and second invariants of strain and stress tensors are used as the equivalent strains or stresses.

The existence of flaws is taken into account by the introduction of special equivalent stresses which are dependent on principal tensile stresses and the characteristics of the material.

As a criterion of damage equivalence in non‐linear viscoelastic materials, the condition of a specific dissipation equality is introduced. Experiments for the determination of material functions entering the constitutive equations are outlined.

The proposed variant of the theory has made it possible to realistically explain the results of creep and constant loading rate tensile tests and creep tests to rupture under conditions of two‐step loading sequences acting on filled polymeric materials.
Spectral methods to estimate local multiaxial fatigue failure for structures undergoing random vibrations
Authors: Pitoiset X. , Rychlik I. , Preumont A.
Published: March 2003

This paper proposes computationally efficient frequency domain formulations for two well‐known multiaxial fatigue failure criteria, namely Matake’s critical plane criterion and Crossland’s criterion. For that purpose, it is shown how fatigue‐related variables involved in both criteria can be estimated from the power spectral density matrix of the local stress vector. The finite element model of an example structure is then used to illustrate the application of the proposed frequency domain approaches. It is observed that both frequency domain formulations produce consistent results when compared with those obtained in the time domain from Monte‐Carlo simulations of local stress vectors while offering tremendous computer savings. A frequency domain tool indicating whether the principal stress directions do rotate with time or not during the loading at a given location in the structure is also presented.
Some fracture mechanics properties in high‐velocity impact‐damaged aluminium plates
Authors: Lanciotti A. , Ottaviano S.
Published: March 2003

The paper describes the results of an experimental programme aimed at the evaluation of the fracture mechanics properties of high velocity, about 6 km s−1 , impact‐damaged aluminium plates, 2219‐T851. The energy of the impacts was the ballistic limit, i.e. large bumps were produced but the plates were not perforated. Test were carried out to evaluate the residual stress field in the impact‐damaged area, the static resistance of the plates in the presence of a through crack and the crack propagation rate of a through crack located at the centre of the impact‐damaged area. The results of the tests clearly show that the mechanical properties of the material remain practically unchanged after the high‐velocity impact. Furthermore, by taking into account the geometry of the bumps, the mean crack propagation rate of a defect can be predicted quite accurately.
Modelling cracks in finite bodies by distributed dislocation dipoles
Authors: Dai D. N.
Published: March 2003

The method of continuous distribution of dislocations is extended here to model cracks in finite geometries. The cracks themselves are still modelled by distributed dislocations, whereas the finite boundaries are represented by a continuous distribution of dislocation dipoles. The use of dislocation dipoles, instead of dislocations, provides a unified formulation to treat both simple and arbitrary boundaries in a numerical solution. The method gives a set of singular integral equations with Cauchy kernels, which can be readily solved using Gauss–Chebyshev quadratures for finite bodies of simple shapes. When applied to arbitrary geometries, the continuous distribution of infinitesimal dislocation dipoles is approximated by a discrete distribution of finite dislocation dipoles. Both the stress intensity factor and the T‐stress are evaluated for some well‐known crack problems, in an attempt to assess the performance of the methods and to provide some new engineering data.
Fatigue crack nuclei in austempered ductile cast iron
Authors: MARROW T. J. , ÇETINEL H. , AL‐ZALMAH M. , MACDONALD S. , WITHERS P. J. , WALTON J.
Published: March 2003

ABSTRACT Short fatigue crack nuclei in austempered ductile cast iron have been studied using optical microscopy, scanning electron microscopy, atomic force microscopy and X‐ray microtomography and by electron backscatter diffraction analysis. Fatigue cracks nucleate at graphite nodules and shrinkage microporosity. The crack nuclei are arrested and retarded by barriers in the microstructure, by either blocking of slip at boundaries or owing to the requirement for tilt and twist of the stage I crystallographic crack at grain boundaries. These observations indicate that both the size of the defects, such as graphite nodules and microporosity, and the size of the prior austenite grains control the largest crack nucleus that can develop, and hence determine the component fatigue limit.
Variable amplitude loading in the very high‐cycle fatigue regime
Authors: STANZL‐TSCHEGG S. E. , MAYER H. , STICH A.
Published: March 2003

ABSTRACT Ultrasonic fatigue testing is appropriate to perform random loading tests in the regime of very high numbers of cycles. It has been shown that neither an endurance limit nor a threshold stress intensity exists under loading with randomly varying amplitudes even for materials that do show these limits under constant amplitude loading conditions. The technical features of the ultrasonic testing technique in order to perform random fatigue tests are shortly described. Endurance tests were performed on smooth specimens of AlSi7Mg (A356.0) aluminium alloy and on notched AISI 4142 and C45 steel specimens. The previous studies of crack propagation and threshold behaviour on AISI 420 ferritic chromium steel and GGG 100‐B cast iron are included. Experimental results on lifetime and fatigue crack growth measurements under randomly varying amplitudes, as well as lifetime predictions, based on constant amplitude measurements and damage accumulation calculations are reported.
Some effects of martensitic transformation on fatigue resistance
Authors: HORNBOGEN E.
Published: March 2003

ABSTRACT Some fatigue phenomena are discussed, which are because of diffusionless transformation of metastable metallic phases. The following aspects are relevant to high‐cycle fatigue resistance: (i) crystallographic reversibility (shape memory alloys, SMA), or irreversibility, with change in volume (austenitic steels) of the martensitic transformation; (ii) mechanical, thermal, thermomechanical cycling; (iii) effects on crack nucleation or growth because of the structural instability; and (iv) fatigue of functional properties (shape memory, transformation temperatures). Micromechanical models are proposed in order to define the conditions for considerable retardation of crack initiation and early stages of crack growth. Special aspects as the role of heat flow (SMA) or localized formation of a ferromagnetic phase at the crack tip (steels) are discussed briefly.
Strain rate effects for aluminum and magnesium alloys in finite element simulations of steering wheel armature impact tests
Authors: ALTENHOF W. , AMES W.
Published: March 2003

This paper addresses the strain rate effects for aluminum and magnesium steering wheel armatures when they are subjected to dynamic impact tests. Two geometrically different steering wheel armatures, a three spoke proprietary aluminum alloy armature and a four spoke magnesium alloy (AM50A) armature, underwent experimental impact testing. The testing conditions for each armature were different; testing with the aluminum alloy armature involved impacts with a deformable chestform and the magnesium armature experienced impact tests with a rigid plate. Finite element models of all testing apparatuses were developed for both testing conditions and numerical simulations were conducted based on the experimental method employed. Strain rate effects for the aluminum alloy were considered using the Cowper–Symonds constitutive relation and a Johnson–Cook material law was utilized for the magnesium alloy. Simulations were conducted with and without strain rate effects considered. The comparison between the experimental and numerical methods illustrate that there is only a minor change in the numerical testing results with the inclusion of strain rate effects, however, a better correlation between experimental and numerical methods occurs.
Towards a functional understanding of “good genes”
Authors: Siva‐Jothy  , Skarstein
Published: March 2003

The Hamilton & Zuk hypothesis (1982) underpins our understanding of the relationship between secondary sexual characters, parasites, and immunological function. However, despite the wealth of empirical studies that attempt to address issues raised by the Hamilton & Zuk hypothesis, there have been no overt attempts to identify the “good genes” that females select or how those good genes influence the host’s immune system. Behavioural ecologists have generally viewed this aspect of immunity as a black box. In this review we propose candidate good genes in vertebrates, discuss their role in immune function and parasite resistance, and discuss several aspects of the assumptions that pervade studies of parasite mediated sexual selection in vertebrates. We also examine invertebrates (specifically insects) where our current knowledge of these systems suggests the patterns apparent in vertebrates are not underpinned by the same genetic mechanisms.
Coping with multiple enemies – the evolution of resistance and host‐parasitoid community structure
Authors: Fellowes M. D. E. , Kraaijeveld A. R.
Published: March 2003

Recent work has shown that the evolution of Drosophila melanogaster resistance to attack by the parasitoid Asobara tabida is constrained by a trade‐off with larval competitive ability. However, there are two very important questions that need to be answered. First, is this a general cost, or is it parasitoid specific? Second, does a selected increase in immune response against one parasitoid species result in a correlated change in resistance to other parasitoid species? The answers to both questions will influence the coevolutionary dynamics of these species, and also may have a previously unconsidered, yet important, influence on community structure.
Male “macho” mammals exploiting females versus male “Don Juan” birds exploited by females: the opposite‐sex exploitation (OSEX) theory
Authors: Caizergues  , Lambrechts
Published: March 2003

We hypothesize that the general differences in breeding systems between birds and mammals results from a different outcome of the between‐sex arms race. The arms race has been won by male mammals that can impose their mating decision over females, but has been lost by male birds because of the extreme specialization of forearms that do not allow males to trap and hold females during copulation. Male birds compensate for their inability to force copulation by developing “Don Juan” behaviour to persuade females to accept copulation.
Top‐predator abundance and chaos in tritrophic food chains
Authors: Rinaldi S. , De Feo O.
Published: March 2003

Nonlinear models and laboratory experiments suggest that populations can be chaotic, whereas field data show that a fair proportion of observed populations are not too far from being chaotic. Thus, a natural question arises: do ecosystems enjoy special properties at the edge of chaos? By limiting the analysis to three classes of tritrophic food chains and to the role played by the nutrient available to the bottom of the ecosystem, we show that top‐predator mean abundance is maximum at the edge of chaos. The geographical variability of species abundance along nutrient gradients and the dynamic complexity of observed natural populations are consistent with our findings. Effective guidelines for the sustainability of exploited ecosystems are also derived.
Dynamics within mutualism and the maintenance of diversity: inference from a model of interguild frequency dependence
Authors: Bever J.D.
Published: March 2003

Numerical models have suggested that the dynamics within mutualisms are not important for the maintenance of diversity. In this study it is demonstrated that the dynamics within mutualism can contribute to the maintenance of diversity within its participants, using a general model of frequency dependence between two mutualistically interacting guilds. Specifically, it is demonstrated that while mutualisms may exhibit positive feedback in density, there may be a negative feedback within a mutualism as a result of the change in composition within the interacting guild. Such a negative feedback results from an asymmetry in the delivery of benefit between participants of the mutualism that generates a negative interguild frequency dependence. This dynamic contributes to the maintenance of diversity within the interacting guilds. Conditions are identified for the maintenance of diversity and the maximization of benefit from mutualism within the context of the model. The utility of these conditions for testing hypotheses using data from the mutualistic interaction between plants and mycorrhizal fungi is then demonstrated.
On the fraction of habitat allocated to marine reserves
Authors: Mangel M.
Published: March 2003

The case for marine reserves is strengthening, and both deterministic and stochastic calculations show that fisheries management using reserves may achieve harvests comparable with management without reserves. Thus, depending upon the metric used, reserves need not disadvantage harvest. Reserves provide a buffer that increases the chances of sustainability of the stock, and thus the fishery. In this paper, I develop methods (deterministic and stochastic) that allow one to determine how much habitat needs to be set aside as reserve, once societal decisions concerning the goals of reserves are made. The answer to the question: “how much habitat needs to be allocated to reserves” is not a simple single number. Rather, it is a procedure that can be employed once biological, operational and social information are provided. The methods also apply to reserves used to aid stock recovery.
Truncated power laws: a tool for understanding aggregation patterns in animals?
Authors: Sjöberg Mikael , Albrectsen Benedicte , Hjältén Joakim
Published: March 2003

Statistical distributions like the negative binomial distribution are commonly used to describe aggregation patterns in animals. However, recently it has been suggested that truncated power laws (TPLs) may also be used for this kind of analysis. A TPL consists of two power functions separated by a cut‐off size (C*). The cut‐off size and the slope of power function one (β1) for the smallest group sizes have been suggested to have a biological explanatory value.

We applied TPLs to aggregation data of tephritid seed predators on a composite plant, aphids on willows and grey seals on a haulout site. β1 varied between 0.60 and and −0.72, which is higher than predicted. In addition, resource distribution and animal density influenced β1 and C*. This indicates that environmental dimensionality suggested to affect β1 is masked by ecological factors. We conclude that TPLs are useful due to their simplicity and, in comparison with traditional methods, provide additional biologically relevant information. Truncated power laws can therefore prove to be useful in studies of animal behaviour and population dynamics.
Edge effects on the prevalence and mortality factors of Phytomyza ilicis (Diptera, Agromyzidae) in a suburban woodland
Authors: McGeoch M.A. , Gaston K.J.
Published: March 2003

Although the effects of edges on the biotas of habitat patches have been widely discussed, there have been few empirical studies of the mechanistic basis of population and community differences between patch edges and interiors. This is particularly true of differential effects of edges on species’ mortalities, and on interactions in insect populations and communities. Here we examine edge‐associated differences in the prevalence and mortality factors of the holly leaf‐miner Phytomyza ilicis, in a suburban woodland in Sheffield, U.K. Leaf miner prevalence was higher and survivorship to adulthood lower at the woodland edge. Natural enemies and other mortality factors contributed differently to total mortality at the edge and interior. Mechanisms underlying edge effects on P. ilicis arose from the interaction between microclimate, adult movement and host‐plant quality. The differential induction of species mortality found, confirms the complexity of species’ responses to habitat edges and the importance of understanding the effects of edges on species interactions.
Reducing ambiguity in describing plant–insect interactions: “preference”, “acceptability” and “electivity”
Authors: Singer
Published: March 2003

Ecologists and evolutionary biologists have a common interest in plant–insect interactions. Ecologists develop terminology describing patterns of association between plants and insects, while evolutionary biologists use the same words to denote potentially heritable traits of individuals. Use of the same terms to describe both traits of the interaction and traits of the organisms hinders communication. An example is “preference”, often used by ecologists to denote properties of the plant–insect interaction and by behavioural or evolutionary biologists to denote insect traits. The existing term “electivity” could be incorporated into the lexicon of plant–insect interactions to supplant the ecological use of “preference”. The term “preference” would then denote a behavioural trait of the insect. The mirror‐image trait of the plant would be “acceptability”. This could be a step towards a common terminology that would be usable by both ecologists and evolutionists.
The senescence ofDaphniafrom risky and safe habitats
Authors: Dudycha
Published: March 2003

Evaluating life history in an ecological context is critical for understanding the diversity of life histories found in nature. Lifespan and senescence differ greatly among taxa, but their ecological context is not well known. Life history theory proposes that senescence is ultimately caused by a reduction of the effectiveness of natural selection as organisms age. A key prediction is that different levels of extrinsic mortality risk lead to the evolution of different senescence patterns. I quantified both mortality risk and investment in late‐life fitness ofDaphnia pulex‐pulicaria, a common freshwater zooplankter. I found thatDaphniafrom high‐risk pond habitats invest relatively little in late‐life fitness, whereas those from low‐risk lake habitats invest relatively more in late‐life fitness. This suggests that ecological approaches can be useful for understanding senescence variation.
Experimental confirmation of ecosystem model predictions comparing transient and equilibrium plant responses to elevated atmospheric CO2
Authors: Newton P.C.D. , Clark H. , Edwards G.R. , Ross D.J.
Published: March 2003

Ecosystem models predict that short‐term responses to elevated atmospheric CO2 may differ substantially from the “real” long‐term responses expected at equilibrium. Experimental validation of these model predictions is difficult as the data available are from short‐term studies that do not include biogeochemical feedbacks typical of long‐term exposure. Using a reciprocal transplant design at a natural CO2 spring, we generated combinations of atmospheric and soil conditions that represented both short‐ and long‐term elevated CO2 conditions. Plant responses were significantly different between these treatments, confirming model predictions that there is not a simple relationship between transient and equilibrium responses to elevated CO2.
Amino acid uptake: a widespread ability among boreal forest plants
Authors: Persson Jörgen , Näsholm Torgny
Published: March 2003

Amino acids constitute a potentially important source of nitrogen for plants in boreal forest ecosystems. Accordingly, it may be suggested that distinct plant species differing abilities to take up amino acids constitutes an important factor in determining plant ecosystem composition. Using GC‐MS and isotopically labelled amino acids, we measured the simultaneous uptake of 15 different amino acids by 31 common boreal forest plant species. The results from this study show that all plant species tested, representing a wide variety of plant types, have the ability to take up amino acids from an incubation solution. Furthermore, uptake rates were unrelated to mycorrhizal associations as well as habitat soil amino acid concentrations and plant nitrogen availability dependence as measured by Ellenberg nitrogen indicator values. These results suggest that mycorrhiza is of minor importance for discrete plant amino acid uptake rates and further points out the potential importance of amino acids to plant nitrogen nutrition in boreal forest ecosystems.
Phenology is a major determinant of tree species range
Authors: Chuine Isabelle , Beaubien Elisabeth G.
Published: March 2003

Global warming is expected to have a major impact on plant distributions, an issue of key importance in biological conservation. However, very few models are able to predict species distribution accurately, although we know species respond individually to climate change. Here we show, using a process‐based model (PHENOFIT), that tree species distributions can be predicted precisely if the biological processes of survival and reproductive success only are incorporated as a function of phenology. These predictions showed great predictive power when tested against present distributions of two North American species – quaking aspen and sugar maple – indicating that on a broad scale, the fundamental niche of trees coincides with their realized niche. Phenology is shown here to be a major determinant of plant species range and should therefore be used to assess the consequences of global warming on plant distributions, and the spread of alien plant species.
Constraints on colonization and species richness along a grassland productivity gradient: the role of propagule availability
Authors: Foster Bryan L.
Published: March 2003

An important goal in ecology is to discern under what habitat conditions community structure is primarily regulated by local ecological interactions and under what conditions community structure is more regulated by the pool of available colonists. I conducted a seed addition experiment in successional grassland to evaluate the relative significance of neighbourhood biotic interactions and propagule availability in regulating plant colonization and species richness along a natural gradient of grassland productivity. In undisturbed field plots, seed additions of 34 species led to an increase in species richness in locations of low productivity, an effect that declined in magnitude as productivity increased. In disturbed plots, seed additions led to a relatively constant increase in species richness at all levels of productivity. The results support the hypothesis that the role of propagule availability in regulating colonization dynamics and species richness declines in significance relative to local‐scale competitive interactions as habitat productivity increases.
Modelling the dynamic spatio‐temporal response of predators to transient prey patches in the field
Authors: Winder Linton , Alexander Colin J. , Holland John M. , Woolley Chris , Perry Joe N.
Published: March 2003

The spatio‐temporal dynamics of two aphid species (Metopolophium dirhodum and Sitobion avenae) and a generalist predator (Pterostichus melanarius) were observed in a field‐scale study using a grid of 256 sampling locations with a 12‐m spacing. Using Spatial Analysis by Distance Indices we demonstrate that populations show ephemeral spatial pattern at the field scale. We observed a positive, lagged beetle response to this aphid pattern; conversely, the aphids displayed a negative, lagged response to beetle spatial pattern. Examination of the local structure of the spatio‐temporal dynamics revealed a strong response by the beetle population to aphid patches. The temporal structure of spatial associations between the species shows a strong correspondence with those from a conceptual model of predator–prey spatial interaction. The spatially coupled dynamics were sufficiently strong for the predator to have a negative effect on the intrinsic rate of increase of their prey.
Competitive interactions and persistence of two nematode species that parasitize Drosophila recens
Authors: Perlman S.J. , Jaenike J.
Published: March 2003

Drosophila recens is parasitized in the wild by two nematodes, Howardula aoronymphium, a host generalist, and Parasitylenchus nearcticus, a host specialist known only from D. recens. In order to understand how these two parasite species coexist, we compared their ability to infect and grow in D. recens, their effects on host fecundity and survival, and whether one parasite species was competitively superior in double infections. The specialist nematode P. nearcticus had greater rates of infection and reproduction than the generalist H. aoronymphium, and completely sterilized females in single and mixed infections. The specialist was competitively superior in mixed infections, as generalist motherworms were significantly smaller than in single infections. These results suggest that P. nearcticus might competitively exclude H. aoronymphium if D. recens were the only host available. It is likely that H. aoronymphium persists in D. recens by transmission from other, more suitable host species.
Non‐hybrid offspring from matings between hemiclonal hybrid waterfrogs suggest occasional recombination between clonal genomes
Authors: Vorburger Christoph
Published: March 2003

The hemiclonal waterfrog Rana esculenta, a hybrid between R. ridibunda and R. lessonae, eliminates the lessonae genome from the germline and clonally transmits the ridibunda genome (hybridogenesis). Such genomes are prone to accumulate deleterious mutations, which may explain why offspring from matings between hybrids are typically inviable. Here I present field data from a population for which experimental crossings showed that some R. esculenta pairs produce viable R. ridibunda offspring. I demonstrate: (1) that R. ridibunda metamorphs are also produced and survive under natural conditions; (2) that their genotypes are consistent with combinations of clonal ridibunda genomes found in hybrids; and (3) that all R. ridibunda are female. These females possibly recombine the clonal genomes they inherited and, upon mating with syntopic R. lessonae, produce new hemiclones with novel combinations of alleles. Hence, occasional recombination between otherwise clonal ridibunda genomes seems plausible and may provide an escape from the evolutionary dead end they were proposed to be trapped in.
Cohort effects and population dynamics
Authors: Lindström Jan , Kokko Hanna
Published: March 2003

Cohort effects originate from environmental conditions, and can have long‐term consequences for the cohort's performance. It has been proposed that cohort effects tend to increase population fluctuations. However, differences among individuals, which cohort effects introduce into a population, usually have stabilizing effects. There are thus two different predictions regarding the impact of cohort effects on population fluctuations. We argue that it is important to distinguish between environmental variability and its long‐term effects on individual quality, and approach the question with a population model that can include or exclude such effects. We show that the influence of cohort effects depends on the inherent dynamics: cohort effects can have stabilizing effects if dynamics are inherently unstable. However, the most common outcome is destabilization whenever cohort effects act on top of inherently stable dynamics. Intriguingly, both alternatives are due to individual differences affecting the structure of density dependence in a similar way.
Consumer movement through differentially subsidized habitats creates a spatial food web with unexpected results
Authors: Callaway Duncan S. , Hastings Alan
Published: March 2003

Energy and nutrient flow between habitats, or allochthonous input, can have a significant impact on food web dynamics. Previous theory demonstrated that resource abundance decreases in habitats where consumers are subsidized. Here we examine the effect of subsidies that are available in localized parts of a habitat (such as near the shore in a marine‐subsidized terrestrial ecosystem) with a two‐patch model in which consumers move between patches, resources are stationary, and consumers receive the subsidy in only one of the two patches. In contrast to previous theory, our results show that subsidized consumers can increase resource abundance, though only in the subsidized patch. Furthermore, the total resource population responds positively to increasing consumer movement. These results demonstrate the importance of spatial heterogeneity in food web dynamics and the need for further examination of the role of space in multispecies trophic webs.
Interspecific infanticide deters predators
Authors: Janssen Arne , Faraji Farid , Van Der Hammen Tessa , Magalhães Sara , Sabelis Maurice W.
Published: March 2003

It is well known that young, small predator stages are vulnerable to predation by conspecifics, intra‐guild competitors or hyperpredators. It is less known that prey can also kill vulnerable predator stages that present no danger to the prey. Since adult predators are expected to avoid places where their offspring would run a high predation risk, this opens the way for potential prey to deter dangerous predator stages by killing vulnerable predator stages. We present an example of such a complex predator–prey interaction. We show that (1) the vulnerable stage of an omnivorous arthropod prey discriminates between eggs of a harmless predator species and eggs of a dangerous species, killing more eggs of the latter; (2) prey suffer a minor predation risk from newly hatched predators; (3) adult predators avoid ovipositing near killed predator eggs, and (4) vulnerable prey near killed predator eggs experience an almost fourfold reduction of predation. Hence, by attacking the vulnerable stage of their predator, prey deter adult predators and thus reduce their own predation risk. This provides a novel explanation for the killing of vulnerable stages of predators by prey and adds a new dimension to anti‐predator behaviour.
Biodiversity and ecosystem functioning at local and regional spatial scales
Authors: Bond Emily M. , Chase Jonathan M.
Published: March 2003

Local niche complementarity among species (the partitioning of species based upon niche differentiation) is predicted to affect local ecosystem functioning positively. However, recent theory predicts that greater local diversity may hinder local ecosystem functioning when diversity is enhanced through source–sink dynamics. We suggest community assembly as a way to incorporate both the local and regional processes that determine biodiversity and its consequent effects on ecosystem functioning. From this, we propose a hump‐shaped relationship between diversity and ecosystem functioning at local scales, but a linear increase of functioning with diversity at regional scales due to regional complementarity.
Herbivore and pathogen damage on grassland and woodland plants: a test of the herbivore uncertainty principle
Authors: Schnitzer Stefan A. , Reich Peter B. , Bergner Belle , Carson Walter P.
Published: March 2003

Researchers can alter the behaviour and ecology of their study organisms by conducting such seemingly benign activities as non‐destructive measurements and observations. In plant communities, researcher visitation and measurement of plants may increase herbivore damage in some plant species while decreasing it in others. Simply measuring plants could change their competitive ability by altering the amount of herbivore damage that they suffer. Currently, however, there is only limited empirical evidence to support this `herbivore uncertainty principle' (HUP). We tested the HUP by quantifying the amount of herbivore and pathogen damage in 13 plant species (> 1400 individuals) at four different visitation intensities at Cedar Creek Natural History Area, Minnesota, USA. Altogether, we found very little evidence to support the HUP at any intensity of visitation. Researcher visitation did not alter overall plant herbivore damage or survival and we did not detect a significant visitation effect in any of the 13 species. Pathogen damage also did not significantly vary among visitation treatments, although there was some evidence that high visitation caused slightly higher pathogen damage. Based on our results, we question whether this phenomenon should be considered a `principle' of plant ecology.
Community structure and the evolution of aposematic coloration
Authors: Merilaita Sami , Kaitala Veijo
Published: March 2003

Studies on the evolution of aposematic coloration (prey coloration advertising for unpalatability) have mainly focused on predator psychology in simplified single‐prey species systems. We chose, instead, to model population dynamics on the community level. We studied the invasion by an aposematic phenotype in the presence and absence of another prey species. The single‐prey and two‐prey models differed in two major ways. First, with two prey species the invasion was possible only with a weak aposematic signal, whereas with a single prey species there was no such an upper limit for signal strength. Second, with a single prey species, increase of the aposematic phenotype always resulted in rapid extinction of the predator. Resource value and growth rate of the alternative prey species affected the invasion. These results suggest that community structure is an important determinant of the conditions for invasion of aposematism, and may have contributed to its initial evolution.
Minimizing energy expenditure facilitates vertebrate persistence on oceanic islands
Authors: McNab Brian K.
Published: March 2003

The characteristics of terrestrial vertebrates on oceanic islands are examined. They often include a reduced body size, a tolerance of conspecifics, flightlessness, a reduced basal rate of metabolism, and a propensity to enter torpor. On oceanic islands ectotherms frequently replace endotherms. These changes reduce the energy expenditure and resource requirements of vertebrates. Such reductions are permitted by the absence of mammalian predators and facilitate the survival of island endemics in the face of a restricted resource base and a variable environment through an increase in population size. Some insular species increase body size, but this occurs only when the resource base is large, due either to a fortuitously abundant resource, or to the absence of other species that exploit normally abundant resources. Some questions are posed to guide future work. They examine of the characteristics that permit species to disperse over water barriers, the conditions that require a reduction in resource use, the rapidity of response by immigrants to island conditions, whether supertramps show physiological differentiation with respect to island distance or size, and whether island size is absolute or relative to the size of the immigrants.
Opposing clines for high and low temperature resistance in Drosophila melanogaster
Authors: Hoffmann Ary A. , Anderson Alisha , Hallas Rebecca
Published: March 2003

In insects, species comparisons suggest a weak association between upper thermal limits and latitude in contrast to a stronger association for lower limits. To compare this to latitudinal patterns of thermal responses within species, we considered latitudinal variation in heat and cold resistance in Drosophila melanogaster. We found opposing clines in resistance to these temperature extremes in comparisons of 17–24 populations from coastal eastern Australia. Knockdown time following heat shock increased towards the tropics, whereas recovery time following cold shock decreased towards temperate latitudes. Mortality following cold shock also showed a clinal pattern. Clinal associations with latitude were linear and related to minimum temperatures in the coldest month (for cold resistance) and maximum temperatures in the warmest month (for heat resistance). This suggests that within species both high and low temperature responses can vary with latitude as a consequence of direct or indirect effects of selection.
Reproductive timing and individual fitness
Authors: Brommer Jon E. , Merilä Juha , Kokko Hanna
Published: March 2003

Estimation of individual fitness – i.e. description of the extent to which an individual's genes are represented in future generations – is a feature central to most evolutionary studies. Lifetime reproductive success (LRS) is a commonly used estimate of individual fitness, but because it is rate‐insensitive (i.e. timing of reproductive events is not incorporated), it may give a biased estimate of fitness when reproductive timing is an important component of fitness. A review of all empirical studies which have used a recently derived, rate‐sensitive estimate of individual fitness, λind revealed that λind ranks the fitness of phenotypes differently from LRS, and that this difference may lead to different conclusions about strength of selection acting on phenotypic traits. However, although λind may be a better estimate of individual fitness than LRS in certain situations (e.g. in growing populations), its application is not always unproblematic. For instance, in contrast to rate‐insensitive estimates of individual fitness, the λind is sensitive to the age at which offspring are censused and there is little consensus among published studies on when offspring should be counted. Further, rate‐sensitivity does not necessarily improve a fitness estimate in spatio‐temporal variable environments. We suggest that the ultimate test on the applicability of λind vs. LRS as practical measures of individual fitness in quantifying selection should come from studies which correlate these estimates with actual number of descendants left more than one generation further in future.
Unusual synchronization of Red Sea fish energy expenditures
Authors: Stone Lewi , He DaiHai , Becker K. , Fishelson Lev
Published: March 2003

The highly gregarious goat fish Parupeneus forskalii found in the Red Sea at Eilat, Israel exhibit highly synchronous swimming and feeding activity. Five fish were studied under controlled conditions and highly resolved time‐series of their energy expenditures were measured. All fish demonstrated strong phase synchronization in that their activity levels, although erratic in time and intensity, were collectively coordinated and peaked simultaneously together. The synchronization of these wildly varying, and possibly chaotic signals of energy expenditures, were quantified using phase analysis. We suggest that, ecologically, this collective synchronization is a strategy that increases food‐catch.
Interplay between nitrogen deposition and grazing causes habitat degradation
Authors: Van Der Wal René , Pearce Imogen , Brooker Rob , Scott Dave , Welch David , Woodin Sarah
Published: March 2003

Abstract Increased atmospheric nitrogen (N) deposition has been held responsible for the large‐scale invasion of graminoids (grasses, sedges and rushes) in a wide range of habitats from forests to upland heaths, causing dramatic changes in plant species composition. Concurrently with an increase in N deposition over the last century, livestock grazing has intensified in many parts of the world following policy reform, leading to large‐scale degradation of natural and seminatural ecosystems. On the basis of a series of experiments conducted in a Scottish montane ecosystem, we discovered that grazing and N deposition do not operate independently, and the interplay between them is leading to the replacement of valuable moss‐dominated habitat by grasses and sedges. Our study indicates that in setting ‘critical loads’ of N, widely used to minimize habitat degradation, it is necessary to account for substantial amplification of N‐deposition effects by grazing.
Parasite evolution and extinctions
Authors: Boots Michael , Sasaki Akira
Published: March 2003

We examine the evolution of diseases that show the frequency‐dependent transmission process that is commonly applied to sexually and vector‐transmitted infections. As is commonly found, the basic reproductive ratio (R0) of the parasite is maximized by evolution. This has important implications, as it implies that for a wide range of circumstances diseases that show frequency‐dependent transmission may be selected to evolve towards driving their hosts to extinction. This contrasts with the results obtained in spatially explicit models where although parasite‐driven host extinction may occur, it is unlikely to evolve. We further show that an evolutionary constraint between transmission and virulence is required for evolution to lead to an endemic coexistence of both the host and the disease. Furthermore, this constraint needs to be saturating, such that transmission is ‘bought’ at an increasing cost in terms of virulence, to avoid evolution to extinction.
Positive effects of plant species diversity on productivity in the absence of legumes
Authors: Van Ruijven Jasper , Berendse Frank
Published: March 2003

We investigated the effect of species richness on productivity in randomly assembled grassland communities without legumes. Aboveground biomass increased with increasing species richness and different measures of complementarity showed strong increases with plant species richness. Increasing productivity could not be attributed to a relative increase of highly productive species. Instead, the increase appeared to be caused by the increased performance of several low‐productive species. Our results provide evidence that niche complementarity can strongly increase productivity in grasslands, although the communities contained only grasses and forbs.