bluuebunny/crossref_metadata_2025 huggingface dataset:
Environmental fatigue crack growth in titanium aluminides and hydrogen evolution behaviour
Authors: Minoshima K. , Obara K. , Minamino N. , Komai K.
Published: March 2003

The influence of environment on fatigue crack growth behaviour was investigated both in nearly lamellar and in duplex titanium aluminides, and the hydrogen evolution kinetics was analysed by thermal desorption spectroscopy. The tensile strength of the duplex material decreases in the order of the extent of the water molecule content in the environment: the strength in vacuum is the highest, and decreases in the order of laboratory air and finally in water. In the case of the lamellar material, the fatigue crack growth rate in dry air is higher in the R–C crack plane orientation than that in the L–C crack plane orientation. The crack growth rate becomes higher when the crack grows as the lamellae tear. However, in the case of the duplex material, the crack growth rate in the R–C crack plane orientation is smaller in the low ΔK (ΔKeff ) region. When cathodic charging is applied, the fatigue crack growth rate becomes higher than in dry air, particularly in the higher stress intensity factor range. The hydrogen evolution rate is increased by cathodic charging, with lower temperature peaks and higher ones. The peaks at lower temperatures are correlated with the decomposition of hydrides and de‐training of hydrogen from microstructural imperfections such as microvoids. As‐received materials also show an evolution peak at a higher temperature, and the evolution rate is almost independent of cathodic charging. In addition, the evolution rate at a high temperature (above 800 °C) is increased by cathodic charging. The hydrogen is considered to have an important role on fatigue crack growth acceleration.
Atomic force microscopy and the mechanism of fatigue crack growth
Authors: Jono M. , Sugeta A. , Uematsu Y.
Published: March 2003

Fatigue crack test was performed using a grain‐orientated 3% silicon iron under constant amplitude loading. Growth behaviour of the fatigue crack and slip deformation behaviour near the crack tip were observed in detail by using an atomic force microscope. In the lower K region, only one preferential slip system of this material operated and the fatigue crack grew along that slip plane. It was found that constraint of slip deformation due to cyclic strain hardening resulted in crack arrest and crack branching. The fatigue crack grew in a zigzag manner as a result of such successive crack branching and deflection. In the high K region, two preferential slip systems operated simultaneously to an almost identical extent and the fatigue crack grew in a direction perpendicular to the loading axis. The slipping distance in one loading cycle was measured quantitatively by using the image processing technique and the crack growth mechanism is discussed.
An algorithm for evaluating crack closure from local compliance measurements
Authors: SKORUPA M. , BERETTA S. , CARBONI M. , MACHNIEWICZ T.
Published: March 2003

ABSTRACT A numerical procedure for evaluating fatigue crack closure from load‐local deformation data is presented. The local compliance is assumed to vary during the entire loading cycle, including the portion when the crack is fully open. The closure mechanism‐related characteristic load levels are identified based on comparisons between the open crack compliance variations upon loading and unloading. This type of compliance analysis is conditioned by an appropriate smoothness of the measured data. Hence, the choice and optimization of the procedure for smoothing of the measured data. Hence, the choice and optimization of the procedure for smoothing the recorded signals is thoroughly addressed in this paper. The performance of the algorithm is evaluated from comparisons between the computed closure parameters and their reference values which have been previously shown to correlate the observed crack growth rate behaviour.
Crack initiation: the choice of tests available to calibrate Dang Van's criterion
Authors: Peridas G. , Hills D. A.
Published: March 2003

ABSTRACT Dang Van has proposed a method for predicting fatigue life under high cycle fatigue conditions, where the time for a crack to propagate to the critical length is negligible compared to the time it takes for it to initiate. His argument is that if shakedown is achieved at the grain level, a crack will never initiate. Since the exact stress state at the grain level is very difficult to quantify, the determination of the fatigue limit is achieved using calibration experiments. These experiments need not be restricted to bending and torsion as is common practice. Indeed, any form of specimen geometry and loading path may be used to estimate fatigue life. The comparison of many such tests can not only lead to a more accurate definition of the fatigue limit, but also indicates the suitability of the criterion. Moreover, by ensuring that calibration and specimen data lie close together on the fatigue locus, a better prediction of initiation conditions is achieved.
The nature of initiation and propagationS‐Ncurves at and below the fatigue limit
Authors: Singh A.
Published: March 2003

The traditional stress‐life method of life prediction relies on anS–Ncurve of stress versus total life. However, the total life of a sample can be divided into two phases—an initiation phase and a propagation phase that leads to ultimate failure. Although this break‐up of total life into two phases has been recognized in theory, there has been no experimental method to generate initiation and propagationS–Ncurves. In this paper a methodology to generate initiation and propagationS–Ncurves is presented. Acoustic emission technology is used to detect the transition from the initiation phase to the propagation phase. The phenomenon of fatigue limits is also explored and it is shown that the fatigue limit of the traditionalS–Ncurve corresponds to the fatigue limit of the initiation phase and that initiated cracks continue to propagate at stress levels below the initiation endurance limit. It is also shown that no damage is accrued at stress levels below the fatigue limit. A method to extend the propagation life curve below the initiation endurance limit is also presented. The proposed two‐phaseS–Ncurve will greatly extend the life‐predicting capability of the stress‐life method and can explain some of the contradictions observed in experiments.
Fatigue life recovery in aluminium alloy aircraft structure
Authors: Sharp P. K. , Liu Q. , Barter S. A. , Baburamani P. , Clark G.
Published: March 2003

Abstract This paper presents the results of several DSTO research programmes which investigated the effectiveness of the fatigue life enhancement method used on RAAF F/A‐18 aircraft – glass bead peening. The research identified ways in which process improvement could enhance this effectiveness, and developed a procedure for mid‐life reworking of critical airframe parts to effectively restore the original fatigue life. The procedure included removing a very thin layer of material (and with it, any undesirable manufacturing features and accumulated fatigue cracking) from the surface. Further life recovery can be achieved, if required, by applying an optimized peening procedure. This process has been developed to allow restoration of fatigue life to critical airframe components which are thought to be accumulating fatigue crack damage faster than desired.
History influence exponent in cumulative fatigue damage determined using two‐step loading experiments
Authors: Košút J.
Published: March 2003

ABSTRACTThe only material parameter, which appears in quadratic phenomenological hypothesis of cumulative fatigue damage, is the history influence exponent ϑ reflecting the material sensitivity to loading sequence and loading level interactions. Methods of its determination on the basis of two‐step loading experiments are presented. One method is based on the least‐squares method using linearizing transformation, other two ones are based on concept of individual fatigue life curves and statistical concept of inexact structural model. Parameter ϑ has been evaluated in 24 cases representing various materials and different test conditions. Possibility of existence of a common ϑ value holding for all materials is discussed. Such a value does not follow from analysed experimental data. However, ϑ = 0.5 is recommended for improving the reliability of engineering fatigue life estimates in case when required experimental material data are not available.
Influence of effective stress intensity factor range on mixed mode fatigue crack propagation
Authors: Bian L. C. , Soh A. K.
Published: March 2003

ABSTRACT The behaviour of fatigue crack propagation of rectangular spheroidal graphite cast iron plates, each consisting of an inclined semi‐elliptical crack, subjected to axial loading was investigated both experimentally and theoretically. The inclined angle of the crack with respect to the axis of loading varied between 0° and 90°. In the present investigation, the growth of the fatigue crack was monitored using the AC potential drop technique, and a series of modification factors, which allow accurate sizing of such defects, is recommended. The rate of fatigue crack propagation db/dN is postulated to be a function of the effective strain energy density factor range, ΔSeff. Subsequently, this concept is applied to predict crack growth due to fatigue loads. The mixed mode crack growth criterion is discussed by comparing the experimental results with those obtained using the maximum stress and minimum strain energy density criteria. The threshold condition for nongrowth of the initial crack is established based on the experimental data.
Interfacial damage in fibre‐reinforced composites subjected to tension fatigue loading
Authors: SHI Z. F. , ZHOU L. M.
Published: March 2003

ABSTRACT The interfacial behaviour of fibre‐reinforced composites subjected to tension fatigue loading is studied based on the shear‐lag model. The governing equations of this problem are obtained and solved. In order to describe the interfacial debonding, the Paris fatigue crack growth formula as well as a modified degradation model for the coefficient of friction is adopted. Finally some important values related to interfacial debonding are obtained. In the present investigation, Poisson's contraction is considered.
Determination of weight functions for elastic T‐stress from reference T‐stress solutions
Authors: WANG X.
Published: March 2003

ABSTRACT This paper presents the application of the weight function method for the calculation of elastic T‐stress. First, the background of the weight function method for the calculation of T‐stress is summarized. Then an analysis of known weight functions for T‐stress revealed that it is possible to approximate them with one universal mathematical form with three unknown parameters with high accuracy. The existence of this weight function form significantly simplified the determination of weight functions for T‐stress. For any particular crack geometry, the unknown parameters can be determined from reference T‐stress solutions. The general weight function expression, with suitable reference T‐stress solutions, was used to derive the weight functions for single edge cracked plate, double edge cracked plate and center cracked plate specimens. These weight functions were then further used to calculate the T‐stress solutions for cracked specimens under several nonlinear stress fields and were compared to available numerical data.
Evaluation of giga‐cycle fatigue properties of some maraging steels by intermittent ultrasonic fatigue testing
Authors: ISHII H. , YAGASAKI T. , AKAGI H.
Published: March 2003

ABSTRACT In evaluating the giga‐cycle fatigue strength of some high strength steels, information on the size distribution of nonmetallic inclusions contained in the material is indispensable. To save time and effort of obtaining such data concerning the inclusions, a convenient dissolution method to evaluate the maximum inclusion size is proposed, in place of a conventional method of measuring the inclusion sizes on many cross‐sectional areas. Meanwhile, to save time‐consuming work of obtaining giga‐cycle fatigue properties of some metallic materials, an intermittent ultrasonic fatigue testing method has also been developed. In the present paper, these two newly developed methods were successfully combined to assess the long life fatigue properties of maraging steels as a function of inclusion size.
A low‐cycle fatigue life prediction model of ultrafine‐grained metals
Authors: DING H.‐Z. , MUGHRABI H. , HÖPPEL H. W.
Published: March 2003

A (high strain) low‐cycle fatigue (LCF) life prediction model of ultrafine‐grained (UFG) metals has been proposed. The microstructure of a UFG metal is treated as a two‐phase ‘composite’ consisting of the ‘soft’ matrix (all the grain interiors) and the ‘hard’ reinforcement (all the grain boundaries). The dislocation strengthening of the grain interiors is considered as the major strengthening mechanism in the case of UFG metals. The proposed model is based upon the assumption that there is a fatigue‐damaged zone ahead of the crack tip within which the actual degradation of the UFG metal takes place. In high‐strain LCF conditions, the fatigue‐damaged zone is described as the region in which the local cyclic stress level approaches the ultimate tensile strength of the UFG metal, with the plastic strain localization caused by a dislocation sliding‐off process within it. The fatigue crack growth rate is directly correlated to the range of the crack‐tip opening displacement. The empirical Coffin–Manson and Basquin relationships are derived theoretically and compared with experimental fatigue data obtained on UFG copper (99.99%) at room temperature under both strain and stress control. Good agreement is found between the model and the experimental data. It is remarkable that, although the model is essentially formulated for high strains (LCF), it is also found to be applicable at low strains in the high‐cycle fatigue (HCF) regime.
A quantitative model for predicting the morphology of surface initiated rolling contact fatigue cracks in back‐up roll steels
Authors: FROLISH M. F. , FLETCHER D. I. , BEYNON J. H.
Published: March 2003

ABSTRACT A previously presented qualitative model for the rolling contact fatigue and spalling failure of back‐up rolls has been quantified in terms of crack lengths and growth directions. The morphologies of surface initiated fatigue cracks have been predicted using published data on the mode I and mode II thresholds in low carbon and roll steels, respectively, and the theoretical determination of the mode I and mode II stress intensity factors at the tips of the inclined surface cracks. The predictions have been validated by using the results of the metallographic examination of rolling contact fatigue cracks produced in test discs used in experimental simulations and the examination of spalled material from a back‐up roll.
On the fitting and extrapolation ofJ‐resistance data derived through the linear normalization technique
Authors: TARPANI J. R. , BOSE FILHO W. W. , SPINELLI D.
Published: March 2003

In this paper, an assessment is made regarding the effects ofJ–Rcurve fitting and extrapolation methods in twoJ‐integral criteria – namely crack initiation,Ji, and tearing instability,J50– which were obtained through the linear normalization technique. Power‐law, logarithmic and linear fits were concurrently applied toJ–Δadata derived from sub‐sized compact tensile specimens machined from a nuclear grade steel and tested at 300 °C. Research results show that the logarithmicJ–Rfit is the most conservative approach within a broad range of elastic–plastic fracture resistance, compared to the conventional power‐law fit. On the other hand, the linear fitting method provided the most non‐conservativeJ‐predictions. The values ofJiandJ50have been successfully correlated with the net energy absorbed during Charpy impact testing of the materials.
Invasion‐resistance in experimental grassland communities: species richness or species identity?
Authors: Crawley  , Brown  , Heard  , Edwards
Published: March 2003

The question as to why some communities are more invasible than others has pro‐found implications for conservation biology and land management. The theoretical issues involved go right to the heart of our understanding of species coexistence and community assembly. The experiment reported here indicates that for productive, small‐scale grassland plots, species identity matters more than species richness in determining both the number of invading species and the total biomass of invasives.
Resonance effects and outbreaks in ecological time series
Authors: Blarer  , Doebeli
Published: March 2003

Organismal response to environmental variability is an important aspect of ecological processes. We propose new mechanisms whereby environmental variability can cause cyclic population outbreaks due to the nonlinearity of the organismal response. We consider stage‐structured populations that respond to variable environments with variable diapause or dormancy, and in which cyclic changes of the environment induce a resonance‐like boost in the population size. If there is also a stochastic component of variation in the environment, the population outbreaks are magnified by the phenomenon of “stochastic resonance”. The results show that large population fluctuations may not be due to extrinsic or intrinsic factors alone, but to a nonlinear interaction between the external environment and internal population processes. Indeed, in the presence of such nonlinearities even very small environmental fluctuations can cause massive fluctuations in population size. Our theoretical results may help to explain periodic population cycles and outbreak dynamics found in many infectious diseases and pest species. We also discuss the evolution of the response parameters that regulate diapause or dormancy and promote the outbreak dynamics in variable environments.
The interpretation of stem diameter–height allometry in trees: biomechanical constraints, neighbour effects, or biased regressions?
Authors: Henry H.A.L. , Aarssen L.W.
Published: March 2003

Static diameter–height allometry data have been used by many ecologists to demonstrate that diameter should increase at a faster rate than height during tree growth, as predicted by biomechanical models. We review the available evidence and examine the potential problems that arise in the interpretation of this relationship. In particular, we reveal how few studies investigating patterns of diameter–height allometry in trees have adequately controlled for neighbour effects. We also demonstrate how the interpretation of diameter–height allometry has suffered from a lack of uniformity in the selection of regression models, and how the use of least squares regression to estimate allometric scaling exponents can be biased. We conclude that most of the published data on static diameter–height relationships in trees tell us virtually nothing about either age (developmental) effects or neighbour effects; they are completely confounded. Further studies are required to analyse the long‐term dynamic growth trajectories of individual trees in relation to local neighbour effects, and greater effort must be made to establish the consistent use of unbiased statistical methods between studies.
Dispersal and spatial scale affect synchrony in spatial population dynamics
Authors: Paradis E. , Baillie S.R. , Sutherland W.J. , Gregory R.D.
Published: March 2003

A large body of theoretical studies has shown that synchrony among populations is critical for the long‐term persistence of species in fragmented habitats. Although the effects of dispersal and environmental factors on synchrony have been investigated theoretically, empirical studies of these relationships have been lacking. We explored the interplay between environmental and demographic factors (fecundity, survival, dispersal) on population synchrony for 53 species of birds. We show that the interspecific differences in mean synchrony were determined by global environmental factors whose action was probably mediated by the abundance of each species. After removing the effect of these global factors on synchrony, the residual synchrony was strongly correlated with dispersal distance. The relationship between dispersal and synchrony was stronger for the species nesting in wet habitats than for those nesting in dry habitats. Our results indicate that different factors synchronize bird populations at different spatial scales, thus highlighting the role of scale in understanding spatial population dynamics and extinction risks.
Habitat fragmentation and extinction thresholds on fractal landscapes
Authors: Hill M.F. , Caswell H.
Published: March 2003

Habitat fragmentation is a potentially critical factor in determining population persistence. In this paper, we explore the effect of fragmentation when the fragmentation follows a fractal pattern. The habitat is divided into patches, each of which is suitable or unsuitable. Suitable patches are either occupied or unoccupied, and change state depending on rates of colonization and local extinction. We compare the behaviour of two models: a spatially implicit patch‐occupancy (PO) model and a spatially explicit cellular automaton (CA) model. The PO model has two fixed points: extinction, and a stable equilibrium with a fixed proportion of occupied patches. Global extinction results when habitat destruction reduces the proportion of suitable patches below a critical threshold. The PO model successfully recreates the extinction patterns found in other models. We translated the PO model into a stochastic cellular automaton. Fractal arrangements of suitable and unsuitable patches were used to simulate habitat fragmentation. We found that: (i) a population on a fractal landscape can tolerate more habitat destruction than predicted by the patch‐occupancy model, and (ii) the extinction threshold decreases as the fractal dimension of the landscape decreases. These effects cannot be seen in spatially implicit models. Landscape struc‐ture plays a vital role in mediating the effects of habitat fragmentation on persistence.
Field evidence for apparent competition mediated via the shared parasites of two gamebird species
Authors: Tompkins D.M. , Draycott R.A.H. , Hudson P.J.
Published: March 2003

Although apparent competition mediated via shared parasites is potentially an important force influencing community structure, there is limited evidence to demonstrate its occurrence in the field. Here we show that the intensity of infection by the caecal nematode Heterakis gallinarum picked up by naive grey partridges, both maintained in pens and released on six gamebird estates in the UK, is significantly correlated with the intensity of infection recorded in the previous year from pheasants on those estates. Furthermore, the worm burdens picked up appeared to be sufficient to negatively influence host condition. These results provide evidence that infection from pheasants determines the worm burdens of partridges in the field, supporting the hypothesis that parasite‐mediated apparent competition with the pheasant may be a factor influencing the decline and subsequent recovery of wild grey partridges.
An integrated approach to management in epidemiology and pest control
Authors: Shea Katriona , Thrall Peter H. , Burdon Jeremy J.
Published: March 2003

Management of biological systems involves the application of ecological and evolutionary principles within a decision theory framework. In the present review, we focus on epidemiology and pest (insect and plant) control. By studying their similarities and differences, it is possible to outline a broad conceptual background for the antagonistic interactions involved, and hence to begin to develop a classification system of predictive value in management situations. In particular, we address issues of scale in space and time, and relate these to contrasting practical problems associated with deploying biocontrol agents and disease control. Additionally, we discuss three areas where an integrated approach to natural enemies, using ecological and evolutionary insights, and decision theory have much to offer: (i) management and resistance, (ii) the problems of emerging diseases, and (iii) the links between disease and behaviour.
The evolutionary dynamics of local infection and global reproduction in host–parasite interactions
Authors: Boots  , Sasaki
Published: March 2003

A fundamental question in both evolutionary biology and parasitology is why do different levels of virulence evolve in different parasites. Here we use explicitly spatial lattice models to show how the spatial relationships of infection and host reproduction determine the degree of virulence that will occur. When the reproduction of the host acts over larger spatial scales than the infection process higher virulence is predicted. In contrast to both the mean‐field and the case where infection acts over larger spatial scales than reproduction, the transmission and virulence predicted are always finite as “self‐shading” of infected individuals always occurs. This process may help to explain the evolution of the high virulence of larval diseases of insects where reproduction clearly acts over greater distances than infection.
The young sperm gambit
Authors: Siva‐Jothy
Published: March 2003

Thermodynamic and oxidative damage to the condensed nuclear DNA of spermatozoa may result in age‐related deterioration that results in zygote death. By definition, any such spermatozoan‐age effects are direct (i.e. nonheritable) and, if they occur, females may be selected to avoid such costs. This paper examines the potential of spermatozoan‐age effects to provide alternative explanations for several reproductive phenomena.
On the role of small mammals in mediating climatically driven vegetation change
Authors: Curtin C.G. , Kelt D.A. , Frey T.C. , Brown J.H.
Published: March 2003

Biotic and abiotic processes jointly influence natural systems, yet opportunities to integrate studies of both processes are uncommon. For two decades we have excluded different subsets of the small mammal community from a series of plots near a grassland‐desert ecotone in the northern Chihuahuan Desert. These studies spanned a period of historically high winter rainfall, allowing us to distinguish the effects of climate and small mammals on the composition and patch structure of vegetation. Removal of only kangaroo rats (Dipodomys) or of all small mammals led to increased cover of large herbaceous vegetation. The size of vegetative patches increased in all plots but this increase was three times greater where all rodents were removed. Thus, the activity of small mammals that forage under and near shrub canopies appear to significantly inhibit the expansion of existing vegetative patches, and may have a stronger influence on habitat structure than previously recognized.
Stochastic matrix models for conservation and management: a comparative review of methods
Authors: Fieberg John , Ellner Stephen P.
Published: March 2003

Stochastic matrix models are frequently used by conservation biologists to measure the viability of species and to explore various management actions. Models are typically parameterized using two or more sets of estimated transition rates between age/size/stage classes. While standard methods exist for analyzing a single set of transition rates, a variety of methods have been employed to analyze multiple sets of transition rates. We review applications of stochastic matrix models to problems in conservation and use simulation studies to compare the performance of different analytic methods currently in use. We find that model conclusions are likely to be robust to the choice of parametric distribution used to model vital rate fluctuations over time. However, conclusions can be highly sensitive to the within‐year correlation structure among vital rates, and therefore we suggest using analytical methods that provide a means of conducting a sensitivity analysis with respect to correlation parameters. Our simulation results also suggest that the precision of population viability estimates can be improved by using matrix models that incorporate environmental covariates in conjunction with experiments to estimate transition rates under a range of environmental conditions.
Depletion of seed patches by Merriam’s kangaroo rats: are GUD assumptions met?
Authors: Price Mary V. , Correll Rachel A.
Published: March 2003

Depletion of experimental seed patches by granivorous animals often is used as a qualitative assay of foraging activity. An optimal foraging model suggests that seed amounts remaining when foragers leave patches (“giving‐up‐density”, GUD) also provide quantitative measures of foraging economics, diet strategies and foraging abilities. Such quantitative uses of GUDs rest on several largely untested assumptions. We tested two of these with Merriam’s kangaroo rats: that gain curves are smoothly decelerating, and that foragers leave patches at a constant harvest rate. Harvest rates indeed declined with patch residence time, but in the piecewise linear fashion expected of systematic search. Animals also revisited areas within patches less frequently than expected with random search. In the field, they depleted patches in multiple visits and did not use a constant‐rate leaving rule. These deviations from model assumptions cast doubt on inferences about foraging ecology that have been based on quantitative GUD theory.
Experimental support for a resource‐based mechanistic model of invasibility
Authors: Davis Mark A. , Pelsor Melissa
Published: March 2003

Recent theory has suggested a mechanistic relationship between resource availability, competition and invasibility. In a field experiment, in which we manipulated resources and competition, we confirmed that changes in resource availability affected competition intensity, which in turn affected invasibility. We found that fluctuations in resource availability of as short as a few weeks had a large impact on plant invasion success (survival and percentage cover), including up to 1 year following the fluctuations. If resource availability is a primary mechanism controlling invasibility, it may serve as a unifying concept that can integrate earlier ideas regarding invasibility. The results emphasize the important role of history in the invasion process, particularly the occurrence of stochastic, short‐lived events that temporarily reduce or suspend competition and increase invasibility. Therefore, it may be very difficult, or even impossible, to reconstruct the ecology of particular invasions after the fact.
Interactions between density‐dependent processes, population dynamics and control of an invasive plant species, Tripleurospermum perforatum (scentless chamomile)
Authors: Buckley Yvonne M. , Hinz Hariet L. , Matthies Diethart , Rees Mark
Published: March 2003

Tripleurospermum perforatum is an invasive weedy species which exhibits strong over‐compensating density dependence. Interactions between density‐dependent survival, probability of flowering and fecundity were modelled and their impact on the population dynamics were examined. When only fecundity was density‐dependent, the dynamics were similar to those observed in the model containing all three density‐dependent terms. Density‐dependent survival was a stabilizing process when acting in combination with density‐dependent fecundity and probability of flowering; removing density‐dependent survival from the model produced two‐point cycles. The addition of a seed bank was also stabilizing. Simulations of control strategies at different life‐history stages indicated that full control would be difficult due to the strong over‐compensating density dependence, with severe reductions in fecundity and late season survival necessary in order to reduce equilibrium seed density and biomass.
Individual and population level determinants of immigration success on local habitat patches: an experimental approach
Authors: Gundersen Gry , Andreassen Harry P. , Ims Rolf A.
Published: March 2003

The effect of population density on immigration success of young root voles was studied in a factorial experiment where immigrants of three different types were translocated from donor populations to recipient habitat patches with experimentally manipulated population sizes. The different types of immigrants were: (1) residents, animals that had remained in their natal patch; (2) colonists, animals that already had successfully dispersed and settled in a new patch; and (3) transients, animals roaming around in a non‐habitable matrix area of the donor population. Generally, we found evidence for a negative density‐dependent immigration success in terms of survival, rate of sexual maturation and body growth rate. With respect to the survival rate, the degree of density dependence differed among the three immigrant types. Our findings are discussed with respect to the degree of rescue effect immigrants may have on a recipient population depending on population density, and also its relevance concerning translocation of individuals for conserving endangered populations.
Feedbacks between nutrient cycling and vegetation predict plant species coexistence and invasion
Authors: Miki Takeshi , Kondoh Michio
Published: March 2003

To investigate the role of species‐specific litter decomposability in determining plant community structure, we constructed a theoretical model of the codevelopmental dynamics of soil and vegetation. This model incorporates feedback between vegetation and soil. Vegetation changes the nutrient conditions of soil by affecting mineralization processes; soil, in turn, has an impact on plant community structure. The model shows that species‐level traits (decomposability, reproductive and competitive abilities) determine whether litter feedback effects are positive or negative. The feedback determines community‐level properties, such as species composition and community stability against invasion. The model predicts that positive feedback may generate multiple alternative steady states of the plant community, which differ in species richness or community composition. In such cases, the realized state is determined by initial abundance of co‐occurring species. Further, the model shows that the importance of species‐level traits depends on environmental conditions such as system fertility.
Co‐occurrence of multiple, supposedly incompatible modes of sex determination in a lizard population
Authors: Shine R. , Elphick M. J. , Donnellan S.
Published: March 2003

Sex is determined genetically in some species (genotypic sex determination, or GSD) and by the environment (environmental sex determination, or ESD) in others. The two systems are generally viewed as incompatible alternatives, but we have found that sex determination in a species of montane lizard (Bassiana duperreyi, Scincidae) in south‐eastern Australia is simultaneously affected by sex chromosomes and incubation temperatures, as well as being related to egg size. This species has strongly heteromorphic sex chromosomes, and yet incubation at thermal regimes characteristic of cool natural nests generates primarily male offspring. We infer that incubation temperatures can over‐ride genetically determined sex in this species, providing a unique opportunity to explore these alternative sex‐determining systems within a single population.
Network structure and biodiversity loss in food webs: robustness increases with connectance
Authors: Dunne Jennifer A. , Williams Richard J. , Martinez Neo D.
Published: March 2003

Food‐web structure mediates dramatic effects of biodiversity loss including secondary and `cascading' extinctions. We studied these effects by simulating primary species loss in 16 food webs from terrestrial and aquatic ecosystems and measuring robustness in terms of the secondary extinctions that followed. As observed in other networks, food webs are more robust to random removal of species than to selective removal of species with the most trophic links to other species. More surprisingly, robustness increases with food‐web connectance but appears independent of species richness and omnivory. In particular, food webs experience `rivet‐like' thresholds past which they display extreme sensitivity to removal of highly connected species. Higher connectance delays the onset of this threshold. Removing species with few trophic connections generally has little effect though there are several striking exceptions. These findings emphasize how the number of species removed affects ecosystems differently depending on the trophic functions of species removed.
From parasitism to mutualism: partner control in asymmetric interactions
Authors: Johnstone Rufus A. , Bshary Redouan
Published: March 2003

Intraspecific cooperation and interspecific mutualism often feature a marked asymmetry in the scope for exploitation. Cooperation may nevertheless persist despite one‐sided opportunities for cheating, provided that the partner vulnerable to exploitation has sufficient control over the duration of interaction. Here we develop a simple, game theoretical model of this form of partner control. We show that as a victim's ability to terminate an encounter increases, selection can favour reduced exploitation, resulting in a switch from parasitism to mutualism. For a given level of control, exploitation is likely to be less intense and the interaction to last longer when there are greater mutualistic benefits to be gained, and when the benefits of cheating are lower relative to the costs inflicted on the victim. Observations of interactions between cleaner‐fish and non‐predatory species of client are shown to match these predictions.
Island theory, matrix effects and species richness patterns in habitat fragments
Authors: Cook William M. , Lane Kurt T , Foster Bryan L. , Holt Robert D.
Published: March 2003

Island biogeography theory, created initially to study diversity patterns on islands, is often applied to habitat fragments. A key but largely untested assumption of this application of theory is that landscape matrix species composition is non‐overlapping with that of the islands. We tested this assumption in successional old field patches in a closely mowed matrix, and because our patches are appropriately viewed as sets of contiguous habitat units we studied patterns of species richness per unit area. Previous studies at our site did not find that diversity patterns on patch ‘islands’ conformed to predictions of island biogeography theory. Our results indicate that when matrix species are removed from the patch samples, diversity patterns conform better to theory. We suggest that classical island theory remains an appropriate tool to study diversity patterns in fragmented habitats, but that allowances should be made for spill‐over colonization of ‘islands’ from the ‘sea’.
Reductions in grassland species evenness increase dicot seedling invasion and spittle bug infestation
Authors: Wilsey Brian J. , Polley H. Wayne
Published: March 2003

Previous experiments that tested whether diverse plant communities have lower invasibility have all varied species richness. We experimentally varied evenness of four grassland species (three grasses and one forb) by planting a field experiment in Texas, and monitored the number of unplanted dicot and monocot species that invaded plots for two growing seasons. By varying evenness, we eliminated any sampling effect in our diversity treatment, because all plots contained the same plant species. Experimentally reducing evenness led to a greater number of dicot invaders, which emerged in plots throughout the growing season, but had less of an effect on monocot invaders, which emerged in flushes when experimental plants were semi‐dormant. Frequency of Solidago canadensis (altissima) stems with spittle bugs significantly increased with reductions in evenness during the first year, apparently because the greater number of Solidago stems in high evenness plots diluted the spittle‐bug effect. These results support the view that higher diversity plant communities are more resistant to dicot invaders and insect herbivores.
Trading off the ability to exploit rich versus poor food quality
Authors: Tessier Alan J. , Woodruff Pamela
Published: March 2003

Abstract Lakes differ in the quality of food for planktonic grazers, but whether grazers adapt to this resource heterogeneity is poorly studied. We test for evidence of specialization to resource environment within a guild of suspension feeding daphniids inhabiting lakes that differ in food web structure. Using bioassays, we demonstrate that food quality for grazers increases from deep to shallow to temporary lakes, which also represents a gradient of increasing predation risk. We compare growth rates and reproductive performance of daphniid taxa specific to each of the three lake types and find they differ greatly in minimum resource requirements, and in sensitivity to the resource gradient. These differences express a trade‐off in ability to exploit rich vs. poor resources. Taxa from deep lakes, poor in resources, have low minimal needs, but they do relatively poorly in rich resource environments. We conclude that grazer distribution is consistent with an adaptive match of exploitation ability to resource environments.
Threatened and endemic species: are they good indicators of patterns of biodiversity on a national scale?
Authors: Bonn Aletta , Rodrigues Ana S. L. , Gaston Kevin J.
Published: March 2003

Endemic and/or threatened species are often targeted to set conservation priorities. It is tempting to assume that a reserve network focusing on these species will be an effective umbrella for overall species richness of a country. For South Africa and Lesotho we tested whether complementary networks selected for threatened and/or endemic bird species satisfactorily represent all bird species, both in terms of capturing areas where other species are present or areas where they are more abundant (and, presumably, more viable). We found that areas selected for threatened and endemic species perform considerably better than areas selected at random. However, they do not guarantee the representation of overall bird species diversity, particularly not in peak abundance locations. Although nationally threatened and endemic species are important conservation targets, our results indicate that reserve networks focusing solely on these species may not be sufficient to preserve overall species diversity in a country.
Insect perception of ambient ultraviolet‐B radiation
Authors: Mazza Carlos A. , Izaguirre Miriam M. , Zavala Jorge , Scopel Ana L. , Ballaré Carlos L.
Published: March 2003

Solar ultraviolet‐B radiation (UV‐B, 290–315 nm) has a strong influence on the interactions between plants and animal consumers. Field studies in various ecosystems have shown that the intensity of insect herbivory increases when the UV‐B spectral band of solar radiation is experimentally attenuated using filters. This effect of UV‐B on insect herbivory has been attributed to UV‐B‐induced changes in the characteristics of plant tissues, and to direct damaging effects of UV‐B photons on the animals. We tested for effects of UV‐B radiation on insect behaviour using field experiments with the thrips Caliothrips phaseoli. When placed in a ‘choice’ tunnel under natural daylight, these insects showed a clear preference for low‐UV‐B environments, and this preference could not be accounted for by differences between environments in total irradiance. These results provide the first evidence of ambient UV‐B photoperception in an insect, challenging the idea that animals are unable to detect variations in the narrow UV‐B component of solar radiation.
Are three‐dimensional spider webs defensive adaptations?
Authors: Blackledge Todd A. , Coddington Jonathan A. , Gillespie Rosemary G.
Published: March 2003

Spider webs result from complex behaviours that have evolved under many selective pressures. Webs have been primarily considered to be foraging adaptations, neglecting the potential role of predation risk in the evolution of web architecture. The ecological success of spiders has been attributed to key innovations in how spiders use silk to capture prey, especially the invention of chemically adhesive aerial two‐dimensional orb webs. However, araneoid sheet web weavers transformed the orb architecture into three‐dimensional webs and are the dominant group of aerial web‐building spiders world‐wide, both in numbers and described species diversity. We argue that mud‐dauber wasps are major predators of orbicularian spiders, and exert a directional selective pressure to construct three‐dimensional webs such that three‐dimensional webs are partly defensive innovations. Furthermore, patterns of diversification suggest that escape from wasp predators may have facilitated diversification of three‐dimensional web‐building spiders.
The area‐independent effects of habitat complexity on biodiversity vary between regions
Authors: Johnson Mark P. , Frost Natalie J. , Mosley Matthew W. J. , Roberts Malcolm F. , Hawkins Stephen J.
Published: March 2003

Potential explanatory variables often co‐vary in studies of species richness. Where topography varies within a survey it is difficult to separate area and habitat‐diversity effects. Topographically complex surfaces may contain more species due to increased habitat diversity or as a result of increased area per se. Fractal geometry can be used to adjust species richness estimates to control for increases in area on complex surfaces. Application of fractal techniques to a survey of rocky shores demonstrated an unambiguous area‐independent effect of topography on species richness in the Isle of Man. In contrast, variation in species richness in south‐west England reflected surface availability alone. Multivariate tests and variation in limpet abundances also demonstrated regional variation in the area‐independent effects of topography. Community composition did not vary with increasing surface complexity in south‐west England. These results suggest large‐scale gradients in the effects of heterogeneity on community processes or demography.
Invasive California poppies (Eschscholzia californicaCham.) grow larger than native individuals under reduced competition
Authors: Leger Elizabeth A. , Rice Kevin J.
Published: March 2003

Invasive plants can be larger and more fecund in their invasive range than in their native range, although it is unknown how often this is a result of a genetically controlled shift in traits, a plastic response to a favourable environment, or a combination thereof. Here we present data from common garden experiments that compare the size and fecundity of native and invasive California poppies,Eschscholzia californicaCham. Individuals from 20 populations, half from California (native) and half from Chile (invasive), were grown both with and without competition from other plants in a container experiment and at two field locations. There were no differences in survival between native and invasive plants at any location. We found significant increases in size and fecundity in invasive populations at two of three locations when poppies were grown without competition from other plants. Our results indicate that genetic shifts in traits have occurred in invasive populations, and that the invasive plants are better at maximizing growth and reproduction in open environments.
Could Bt transgenic crops have nutritionally favourable effects on resistant insects?
Authors: Sayyed Ali H. , Cerda Hugo , Wright Denis J.
Published: March 2003

We present an idea that larvae of some Bacillus thuringiensis (Bt ) resistant populations of the diamondback moth, Plutella xylostella (L.), may be able to use Cry1Ac toxin derived from Bt as a supplementary food protein. Bt transgenic crops could therefore have unanticipated nutritionally favourable effects, increasing the fitness of resistant populations. This idea is discussed in the context of the evolution of resistance to Bt transgenic crops.