bluuebunny/crossref_metadata_2025 huggingface dataset:
Fatigue strength of die‐cast magnesium components
Authors: RENNER F. , ZENNER H.
Published: March 2003

There is a commercial interest to extend the use of die‐cast magnesium from low stress applications, such as interior components of motor vehicles, to components carrying significant loads. In high stress applications it is the strength and fatigue properties of die‐cast magnesium alloys that limit their use. Manufacturing defects, such as microscopic shrink holes, pores and oxide inclusions, impair the strength of components under fatigue loads, but are unavoidable with present‐day magnesium casting technology. In the present study, the effects of different rib thicknesses and notch radii on the fatigue strength were investigated on realistic cast specimens with unmachined surfaces. The tests were performed on ribbed specimens of magnesium alloys AZ91 and AM60 under pulsating bending stress with a constant amplitude at a stress ratio R = 0. As indicated by the results of the investigation, the real material must be considered together with its defects in designing die‐cast magnesium components. For this purpose, the influence of defects must therefore be given a higher priority than the local stresses at the surface.
The effect of a corner radius on an asymptotic solution to the fretting of complete contacts including the plastic process zone
Authors: NAVARRO C. , HILLS D. A. , DOMINGUEZ J.
Published: March 2003

An asymptotic elastic solution provides a good way of encapsulating the behaviour of the process zone at the edge of a complete contact. In this paper we examine the effect of a finite edge radius on the validity of such an approach, and provide clear recommendations on the tolerable edge radius if the asymptotic approach is to remain viable.
Stress concentration formula useful for any dimensions of shoulder fillet in a round bar under tension and bending
Authors: NODA N. A. , TAKASE Y.
Published: March 2003

In this work stress concentration factors (SCFs), Kt for a round bar with a fillet are considered on the basis of exact solutions, now available for special cases, and accurate numerical results. Then, a convenient Kt formula useful for any dimensions of the fillet is proposed. The conclusions can be summarised as follows: (i) For the limiting cases of deep (d) and shallow (s) fillet, the body force method is used to calculate the Kt values. Then, the formula are obtained as Ktd and Kts. (ii) On the one hand, upon comparison of Kt and Ktd, it is found that Kt is nearly equal to Ktd if the fillet is deep or blunt. (iii) On the other hand, if the fillet is sharp or shallow, Kt is mainly controlled by Kts and the fillet depth. (iv) The fillet shape is classified into several groups according to the fillet radius and fillet depth. Then the least squares method is applied for calculation of Kt/Ktd and Kt/Kts. (v) Finally, a convenient formula is proposed that is useful for any dimensions of fillet in a round bar. The formula give SCFs with less than 1% error in most cases for any dimensions of fillet under tension and bending.
The anatomy of the interspecific abundance–range size relationship for the British avifauna: II. Temporal dynamics
Authors: Blackburn Tim M. , Gaston Kevin J. , Greenwood Jeremy J.D. , Gregory Richard D.
Published: March 2003

In a companion paper, we started an examination of the anatomy of the interspecific relationship between local abundance and geographical range size in the British avifauna by analysing its spatial dynamics. Here, we use the same data to extend this study to a consideration of the temporal dynamics of the relationship. Most species of British breeding bird show a positive intraspecific abundance–range size relationship through time: i.e. in years when a species is locally more abundant it also occupies a higher proportion of census sites. However, the majority of such relationships are not statistically significant, and other relationships that are statistically significant are negative. Therefore, intraspecific abundance–range size relationships do not simply mirror the relationship across species. Where they do arise, positive relationships are more likely to be associated with positive intraspecific relationships between range size and maximum rather than minimum abundance. The interspecific abundance–range size relationship is remarkably consistent across years, and is always significantly positive. The relationships for woodland and farmland census sites show correlated variation, so that in years when the linear regression slope and coefficient of determination are high across species on farmland plots, they also tend to be high across species on woodland plots. Common species tend to be common on both farmland and woodland plots, and tend to be common in all years. Likewise, rare species tend to be rare in all habitats and years. This concordance means that the positive interspecific abundance–range size relationship can be viewed as occurring largely independently of intraspecific relationships. It follows from the above that developing an understanding of intraspecific abundance–range size relationships may be of only limited value in ascertaining the determinants of positive interspecific abundance–range size relationships. We conclude that for interspecific relationships, it will be important to know why some species are consistently common and others rare, whereas for intraspecific relationships it will be important to understand the dynamic links between local abundances across sites.
Discrete consumers, small scale resource heterogeneity, and population stability
Authors: Nisbet Roger M. , De Roos Andre M. , Wilson William G. , Snyder Robin E.
Published: March 2003

We present a consumer‐resource model in which individual consumers subsist on a continuum of resource distributed over a very large number of small “bite‐sized” patches, each patch being sufficiently small that all its resource is eaten whenever a consumer visits. This form of consumer–resource interaction forces a heterogeneous distribution of resource among the patches, and may dampen out the large amplitude, consumer‐resource cycles that are predicted by traditional models of well‐mixed, spatially homogeneous systems. The resource equilibrium does not increase with enrichment, a prediction that distinguishes this model from models that invoke direct or indirect consumer density dependence as a stabilizing mechanism.
Species richness and the proportion of predatory animal species in temporary freshwater pools: relationships with habitat size and permanence
Authors: Spencer  , Blaustein  , Schwartz  , Cohen
Published: March 2003

The proportion of predatory animal species is often believed not to vary systematically across communities. However, we predict that larger temporary freshwater pools, and pools that are more permanent, will contain a higher proportion of predatory animal species. In 24 temporary rockpools in Northern Israel (supporting communities dominated by ostracods, copepods, cladocerans, flatworms, dipterans and amphibians), the mean proportion of macroscopic predatory species (averaged over a series of samples) increased with increasing pool area. For the highest possible proportion of predatory species (including microscopic species with uncertain diets), the relationship with pool area was not statistically significant. We did not find significant relationships between permanence and the proportion of either macroscopic or all possible predatory species. Larger pools and pools that were more permanent had more species. Species richness and the proportion of macroscopic predators were positively correlated. These patterns imply that species‐poor ecosystems are likely to be functionally different from species‐rich systems.
Effects of plant species richness on invasion dynamics, disease outbreaks, insect abundances and diversity
Authors: Knops J.M.H. , Tilman D. , Haddad N.M. , Naeem S. , Mitchell C.E. , Haarstad J. , Ritchie M.E. , Howe K.M. , Reich P.B. , Siemann E. , Groth J.
Published: March 2003

Declining biodiversity represents one of the most dramatic and irreversible aspects of anthropogenic global change, yet the ecological implications of this change are poorly understood. Recent studies have shown that biodiversity loss of basal species, such as autotrophs or plants, affects fundamental ecosystem processes such as nutrient dynamics and autotrophic production. Ecological theory predicts that changes induced by the loss of biodiversity at the base of an ecosystem should impact the entire system. Here we show that experimental reductions in grassland plant richness increase ecosystem vulnerability to invasions by plant species, enhance the spread of plant fungal diseases, and alter the richness and structure of insect communities. These results suggest that the loss of basal species may have profound effects on the integrity and functioning of ecosystems.
Quantifying the effects of multiple processes on local abundance: a cohort approach for open populations
Authors: Schmitt R.J. , Holbrook S.J. , Osenberg C.W.
Published: March 2003

A challenge for species with demographically open populations is to evaluate the relative importance of various processes that together set local abundance. We developed a cohort‐based framework for quantifying the influence of an external supply of colonists and subsequent density‐independent and density‐dependent mortality on local abundance. Two complementary approaches – based on limitation and elasticity – revealed the nature of interactions and nonlinearities among these processes. Data for an Indo‐Pacific reef fish were used to document the settler–survivor relationship and to quantify natural variation in settlement. Limitation by density‐dependence was two‐fold and 20‐fold greater than by supply or density‐independent mortality, respectively. Elasticity analyses showed that adult abundance was 40% more sensitive to small proportionate changes in supply than in density‐dependence. These techniques provide a way to compare across systems, which could enhance our ability to draw general conclusions regarding the processes that shape local abundance of species with open populations.
Biodiversity and reduced extinction risks in spatially isolated rodent populations
Authors: Ritchie M.E.
Published: March 2003

Ecologists have rarely explored the potential influence of local (alpha) biodiversity on the stability and local extinction of spatially isolated populations. Twenty years of annual counts of a small, grazing rodent (Utah prairie dogs, Cynomys parvidens) from 20 different isolated local populations (colonies) in southern Utah, U.S.A. were analysed. These prairie dogs exhibited large fluctuations and repeated extinctions at individual colonies during the census period. Frequency of extinction at a colony declined dramatically as the number of locally occurring plant species increased. This pattern was not explained by differences among colonies in plant productivity, plant species composition, colony size, or variability in annual counts. Thus, lower extinction risk of consumer populations may be associated with greater resource diversity, and maintaining high local plant diversity may help sustain spatially isolated herbivore populations in fragmented habitats.
Density dependence in resource exploitation: empirical test of Levins’ metapopulation model
Authors: Doncaster C.P. , Gustafsson L.
Published: March 2003

Levins’ model of metapopulation dynamics is modified to incorporate variable degrees of density dependence in the per capita exploitation of resource patches. We demonstrate a simple means of testing for this density dependence in a sample of metapopulations, each at its equilibrium balance of local colonization to extinction. The fraction of habitable unoccupied patches equilibrates to a constant number under the null model of density independent colonization, and to a constant proportion under strong density dependence. We compare the null model to two density dependent alternatives, using data on exploitation of nest boxes by collared flycatchers Ficedula albicollis. The analysis shows how predicted trends in the equilibrium unoccupied fraction are similar for both spatial interference and net immigration. This needs to be recognized, since the null hypothesis of a constant unused resource applies also to the dynamics of consumable resources, where it is expressed in a constant stock of uneaten prey at the dynamic equilibrium of predators to prey.
Patterns of endemism in riverine fish of the Northern Hemisphere
Authors: Oberdorff T. , Lek S. , Gu???gan J.‐F.
Published: March 2003

Loss of endemic species represents a symptom of general degrading ecosystem conditions that is the indirect result of biodiversity alteration. Here, we developed a predictive model relating species richness of endemic riverine fishes to measured biological, climatic, and historical variables using data from 118 rivers distributed all over the Northern Hemisphere. In a minimally adequate multiple general least square model, total riverine fish species richness, historical biogeography (Pleistocene glaciations), and comtemporary climate accounted for 63% of the variability in endemic species richness; the strongest correlate being riverine fish species richness. Our findings suggest that (i) endemism and richness patterns are generally similar (fish diversity “hot‐spots” areas sustain higher endemic species richness); (ii) glaciation in the Pleistocene have had a significant negative influence on endemic species richness in the more septentrional areas; and (iii) certain basins situated in desertic areas (subtropical dry‐zone of deserts) have unusually high numbers of endemics. These last areas should not be overshadowed when setting conservation priorities.
Do leaf‐cutting ant nests make “bottom–up” gaps in neotropical rain forests?: a critical review of the evidence
Authors: Farji‐Brener  , Illes
Published: March 2003

We discuss the role of leaf cutting ant nests (Atta sp.) on the formation of understory “bottom–up” gaps in tropical rain forests. We (i) describe the characteristics of Atta nests that qualify them as promoters of understory gaps; (ii) review the effects of Atta nests on the dynamics of systems other than tropical forests; and (iii) discuss the hypotheses that in neotropical rain forests Atta nests affect plant productivity, and also influence distribution and community composition of the flora. Finally, we discuss the importance of Atta nests in the dynamics of forests and compare their effects with the better‐known effects of treefall gaps. Atta nests modify soil structure, soil nutrient content, light intensity at ground level, seed bank composition and understory structure. The presence of the nest and associated refuse also accelerates the soil nutrient cycles. The size, high frequency, turnover rate and duration of these nests could make them a good substrate for particular species of plants. In spite of their potential importance, little information exists on the effects of Atta nests on plant productivity and/or plant composition in neotropical rain forests.
On the relationship between niche and distribution
Authors: Pulliam H.R.
Published: March 2003

Applications of Hutchinson’s n‐dimensional niche concept are often focused on the role of interspecific competition in shaping species distribution patterns. In this paper, I discuss a variety of factors, in addition to competition, that influence the observed relationship between species distribution and the availability of suitable habitat. In particular, I show that Hutchinson’s niche concept can be modified to incorporate the influences of niche width, habitat availability and dispersal, as well as interspecific competition per se. I introduce a simulation model called NICHE that embodies many of Hutchinson’s original niche concepts and use this model to predict patterns of species distribution. The model may help to clarify how dispersal, niche size and competition interact, and under what conditions species might be common in unsuitable habitat or absent from suitable habitat. A brief review of the pertinent literature suggests that species are often absent from suitable habitat and present in unsuitable habitat, in ways predicted by theory. However, most tests of niche theory are hampered by inadequate consideration of what does and does not constitute suitable habitat. More conclusive evidence for these predictions will require rigorous determination of habitat suitability under field conditions. I suggest that to do this, ecologists must measure habitat specific demography and quantify how demographic parameters vary in response to temporal and spatial variation in measurable niche dimensions.
Vascular plant species richness along latitudinal and altitudinal gradients: a contribution from New Zealand temperate rainforests
Authors: Ohlemüller R. , Wilson J.B.
Published: March 2003

Vascular plant species richness is known to often decrease with both increasing latitude and increasing altitude. However, a number of studies have shown the reverse trend and the primary cause of these gradients remains unknown. In the present work, generalized linear models were used to assess the relative importance of latitude and altitude as well as of a number of other factors (mean annual precipita‐tion, slope, substrate and forest type) on species richness in temperate rainforests of New Zealand. The effect of Southern beech (Nothofagus spp.) as dominant canopy species on total species richness was shown to be much smaller than postulated in most previous studies. Within the region studied, altitude had by far the strongest effect on species richness. This effect was independent of latitude and was significant for woody but not for herbaceous vegetation.
A rough guide to population change in exploited fish stocks
Authors: Cook R.
Published: March 2003

Interpreting how populations will change in response to exploitation is essential to the sound management of fish stocks. While deterministic models can be of use in evaluating sustainable fishing rates, the inherent variability of fish populations limits their value. In this paper a probabilistic approach is investigated which avoids having to make strong assumptions about the functional relationship between spawning stock size and the annual number of young fish (recruits) produced. Empirical probability distributions for recruits are derived, conditioned on stock size, and used to indicate likely stock changes under different fishing mortality rates. The method is applied to cod (Gadus morhua) in the North Sea to illustrate how population change can be inferred and used by fishery managers to choose fishing mortality rates which are likely to achieve sustainable exploitation.
Community relaxation in fragmented landscapes: the relation between species richness, area and age
Authors: Gonzalez A.
Published: March 2003

Estimates of species loss due to habitat destruction are normally based on calculations employing the species–area relation, S = cAz. The validity of this approach is based on the assumption that the value of the exponent (z) defining the slope of the species–area relation in nonfragmented communities is at a steady state and that z is thus a constant. However, departure from such an assumption renders this approach unreliable. Here I report the results from a natural field experiment using “model” bryophyte‐based microlandscapes designed to follow the species richness dynamics of microarthropod communities postfragmentation. Community isolation due to fragmentation initiated a delayed community relaxation process and resulted in substantial local extinction. Over the period of the experiment z declined in the control communities and yet remained fairly stable in the fragmented communities. I conclude that predictions of species loss due to habitat fragmentation that do not take into account the fact that often z may not be a constant may lead to error‐prone predictions of future species loss.
Functional redundancy in heterogeneous environments: implications for conservation
Authors: Wellnitz Todd , Poff N. LeRoy
Published: March 2003

It has been argued that one of the best ways to conserve biological diversity is to maintain the integrity of functional processes within communities, and this can be accomplished by assessing how much ecological redundancy exists in communities. Evidence suggests, however, that the functional roles species play are subject to the influences of local environmental conditions. Species mayappearto perform the same function (i.e. be redundant) under a restricted set of conditions, yet their functional roles may vary in naturally heterogeneous environments. Incorporating the environmental context into ecological experiments would provide a critical perspective for examining functional redundancy among species.
Plants protect their roots by alerting the enemies of grubs
Authors: Van Tol Rob W.H.M. , Van Der Sommen Anton T.C. , Boff Mari I.C. , Van Bezooijen Jan , Sabelis Maurice W. , Smits Peter H.
Published: March 2003

Plant roots in the soil are under attack from many soil organisms. Although many ecologists are aware of the presence and importance of natural enemies in the soil that protect the plants from herbivores, the existence and nature of tritrophic interactions are poorly understood. So far, attention has focused on how plants protect their above‐ground parts against herbivorous arthropods, either directly or indirectly (i.e. by getting help from the herbivore’s enemies). This article is the first in showing that indirect plant defences also operate underground. We show that the roots of a coniferous plant (Thuja occidentalis) release chemicals upon attack by weevil larvae (Otiorhynchus sulcatus) and that these chemicals thereby attract parasitic nematodes (Heterorhabditis megidis).
Developmental stability across the breeding distribution of the scissor‐tailed flycatcher (Tyrannus forficatus)
Authors: González‐Guzmán Laura I. , Mehlman David W.
Published: March 2003

A central question in ecology is how individual fitness interacts with the spatial variation in population density and habitat characteristics across species’ ranges. We used fluctuating asymmetry (FA) as a measure of developmental stability (DS) in individuals ofTyrannus forficatusto estimate the suitability of sites of varying abundance and position within the species’ range. FA in the inner‐tail feathers of males and females is not spatially correlated across the species’ range. FA in males increases towards the centre of the range and is not correlated with abundance. FA in females is not correlated with position in the range or abundance. Our results suggest that optimal sites are found throughout the range of the species, whereas suboptimal sites are mainly found towards the centre of the range. Additionally, our results suggest that abundance may not reflect the suitability of sites across species’ ranges.
High seed dispersal rates in faunally intact tropical rain forest: theoretical and conservation implications
Authors: Webb Campbell O. , Peart David R.
Published: March 2003

Dispersal limitation may promote high tree‐species diversity in rain forest by slowing local competitive exclusion, but evidence is scarce. By comparing the species lists of tree plots with those of nested seedling plots in rain forest at Gunung Palung, we found that the proportion of local seedlings arising from active dispersal events (via wind or animals, but not simply dropped from parent trees) was high: 68% of species and 46% of the individual seedlings. Local seedling species richness was not limited by the local richness of adults. Using these data, a spatially explicit simulation model indicated that dispersal limitation may not contribute substantially to the maintenance of tree diversity in this forest. We predict that the loss of animal seed dispersers would reduce local species richness of seedlings by 60%. While this reduction could possibly increase the influence of dispersal limitation, it would interfere with other mechanisms maintaining diversity.
Area and the latitudinal diversity gradient for terrestrial birds
Authors: Hawkins Bradford A. , Porter Eric E.
Published: March 2003

We tested the hypothesis that area represents the primary explanation for the latitudinal diversity gradient using breeding terrestrial birds of North America, the northern Palearctic, Australia and the Afrotropics as our focal group. We tested two propositions inherent to the area hypothesis: (1) tropical biomes are larger than extra‐tropical biomes, and (2) there is a significant species‐area relationship for birds at the biome scale of resolution. Using a more realistic definition of biomes which incorporates the effects of both energy and water, we find no support for either proposition, leading us to conclude that the area per se does not explain terrestrial latitudinal diversity gradients.
Plant species richness and community productivity: why the mechanism that promotes coexistence matters
Authors: Mouquet Nicolas , Moore Joslin L. , Loreau Michel
Published: March 2003

This paper stresses that the mechanism of coexistence is the key to understanding the relationship between species richness and community productivity. Using model plant communities, we explored two general kinds of mechanisms based on resource heterogeneity and recruitment limitation, with and without any trade‐off between reproductive and competitive abilities. We generated different levels of species richness by changing model parameters, in particular the number of species in the regional pool, the degree of recruitment limitation, and the level of heterogeneity. Different diversity–productivity patterns are obtained with different coexistence mechanisms, indicating there is no reason to expect any general relationship between species richness and productivity. We discuss these results in the context of the within‐site and across‐site aspects of the relationship between species richness and productivity. Furthermore, we extend these results to hypothesize the relationship between species richness and productivity for other coexistence mechanisms not explicitly considered here.
Local adaptation and the geometry of host–parasite coevolution
Authors: Gandon Sylvain
Published: March 2003

Metapopulation dynamics can strongly affect the ecological and evolutionary processes involved in host–parasite interactions. Here, I analyse a deterministic host–parasite coevolutionary model and derive analytic approximations for the level of local adaptation as a function of (1) host migration rate, (2) parasite migration rate, (3) parasite specificity and (4) parasite virulence. This analysis confirms the results of previous simulation studies: the difference between host and parasite migration rates may explain the level of local adaptation of both species. I also show that both higher specificity and higher virulence generally lead to higher levels of local adaptation of the species which is already ahead in the coevolutionary arms race. The present analysis also provides a simple geometric interpretation for local adaptation which captures the complexity of the temporal dynamics of host–parasite coevolution.
Allee effect makes possible patchy invasion in a predator–prey system
Authors: Petrovskii Sergei V. , Morozov Andrew Y. , Venturino Ezio
Published: March 2003

The dynamics of interacting ecological populations results from the interplay between various deterministic and stochastic factors and this is particularly the case for the phenomenon of biological invasion. Whereas the spread of invasive species via propagation of a population front was shown to appear as a result of deterministic processes, the spread via formation, interaction and movement of separate patches has been recently attributed to the influence of environmental stochasticity. An appropriate understanding of the comparative importance of deterministic and stochastic mechanisms is still lacking, however. In this paper, we show that the patchy invasion appears to be possible also in a fully deterministic predator–prey model as a result of the Allee effect.
Natural enemy diversity and pest control: patterns of pest emergence with agricultural intensification
Authors: Wilby Andrew , Thomas Matthew B.
Published: March 2003

Concern over declining biodiversity and the implications for continued provision of ecosystem services has led, recently, to intense research effort to describe relationships between biodiversity and ecosystem functioning. Here we extend this effort to the relationship between natural enemy species diversity and natural pest control. From simple modelled food‐webs and simulations of natural enemy species loss we derive specific predictions concerning the effect of herbivore life‐history traits, such as life‐cycle type and concealment, on the shape (reflecting diversity effects) and variance (reflecting species composition effects) of the relationship between natural enemy diversity and pest‐control. We show that these predictions are consistent with the emergence of different pest types following intensification of rice production in Asia. We suggest that basic biological insights can help define the structure of ecological processes and allow more accurate predictions of the effect of species loss on the delivery of ecosystem services.
Penguins as oceanographers unravel hidden mechanisms of marine productivity
Authors: Charrassin Jean‐Benoît , Park Young‐Hyang , Maho Yvon Le , Bost Charles‐André
Published: March 2003

A recent concept for investigating marine ecosystems is to employ diving predators as cost‐effective, autonomous samplers of environmental parameters (such as sea‐temperature). Using king penguins during their foraging trips at sea, we obtained an unprecedented high resolution temperature map at depth off the Kerguelen Islands, Southern Ocean, a poorly sampled but productive area. We found clear evidence of a previously unknown subsurface tongue of cold water, flowing along the eastern shelf break. These new results provide a better understanding of regional water circulation and help explain the high primary productivity above the Kerguelen Plateau.
Related concentrations of specific immunoglobulins against the Lyme disease agentBorrelia burgdorferi sensu latoin eggs, young and adults of the kittiwake (Rissa tridactyla)
Authors: Gasparini Julien , McCoy Karen D. , Tveraa Torkild , Boulinier Thierry
Published: March 2003

The capacity for mothers to transmit induced resistance against a specific parasite to their young may be an essential maternal effect that determines the fitness of offspring. In a previous study, antibodies against the Lyme disease agentBorrelia burgdorferi sensu latowere detected in kittiwake (Rissa tridactyla) eggs in relation to the exposure of birds to the tick vectorIxodes uriae. However, as yet, there has been no demonstration of a direct relationship between antibody concentrations in parents and young in a natural population. Here, we show, using the kittiwake–Borreliasystem, the existence of a positive relationship between antibody concentrations in maternal serum and that in eggs and chick serum. No such relationship was found between paternal serum and eggs or young. These results suggest the existence of an adaptive maternal effect, an effect that should have important implications for the ecology and evolution of host–parasite interactions.
Evolution of resistance to Bt crops: directional selection in structured environments
Authors: Ives Anthony R. , Andow D. A.
Published: March 2003

Abstract Widespread use of transgenic crops that express an insecticidal endotoxin from Bacillus thuringiensis increases the risk of evolution of resistance by the European corn borer and other insect pests. To delay resistance evolution, the high‐dose/refuge strategy is being implemented for Bt maize and Bt cotton. We develop a general modelling framework to understand the invasion and spread of alleles conferring resistance. We show that at least three processes are involved in explaining the effectiveness of the high‐dose/refuge strategy: the intensity of selection, assortative (non‐random) mating due to spatial subdivision, and variation in male mating success also due to spatial subdivision. Understanding these processes leads to a greater range of possible resistance management tactics. For example, efforts to encourage adults to leave their natal fields may have the unwanted effect of speeding rather than slowing resistance evolution. Furthermore, when Bt maize causes high mortality to susceptible target pests, spraying insecticides in refuges to reduce pest populations may not greatly disrupt resistance management.
Resistance to a fungal pathogen and host plant specialization in the pea aphid
Authors: Ferrari Julia , Godfray H. Charles J.
Published: March 2003

Herbivores that show host race formation on different plant species have proven to be valuable model systems for studying the evolution of specialization and speciation. Here, we use the pea aphid, Acyrthosiphon pisum, to investigate a possible link between specialization on two host plant species, Lotus uliginosus and Trifolium pratense, and resistance to a natural enemy, the fungal pathogen Erynia neoaphidis. Pea aphids collected on either plant species in the field showed in most cases poor survival on the alternate host plant. Furthermore, pea aphids specialized on T. pratense were very resistant to E. neoaphidis, whereas aphids specialized on L. uliginosus were susceptible. This susceptibility was not influenced by the actual food plant on which the assays were conducted. We discuss how selection from natural enemies may influence the process of specialization and race formation, and how specialization can affect the evolution of resistance.
Trophic control of grassland production and biomass by pathogens
Authors: Mitchell Charles E.
Published: March 2003

Current theories of trophic regulation of ecosystem net primary production and plant biomass incorporate herbivores, but not plant pathogens. Obstacles to the incorporation of pathogens include a lack of data on pathogen effects on primary production, especially outside agricultural and forest ecosystems, and an apparent inability to quantify pathogen biomass. Here, I report the results of an experiment factorially excluding foliar fungal pathogens and insect herbivores from an intact grassland ecosystem. At peak in control plots, 8.9% of community leaf area was infected by pathogens. Disease reduction treatment dramatically increased root production and biomass by increasing leaf longevity and photosynthetic capacity. In contrast, herbivory reduction had no detectable effects at the ecosystem or leaf scale. Additionally, biomass of foliar fungal pathogens in the ecosystem was comparable with that of insect herbivores. These results identify pathogens as potential regulators of ecosystem processes and promote the incorporation of pathogens into trophic theory.
A novel system for efficient gene expression and monitoring of bacteria in aquatic environments
Authors: Espinosa‐Urgel Manuel , Kolter Roberto
Published: March 2003

In a previous study, we reported the identification of Escherichia coli genes with increased expression in an aquatic environment. Here, we describe the use of one of these genes, gapC, as an expression system in freshwater habitats. We have identified the transcriptional start site of gapC and analysed the synthesis of the GapC protein during incubation in aquatic medium. The promoter of gapC was used to construct fusions to the reporter genes lacZ and gfp. Analysis of these fusions indicates the potential of gapC as a valuable tool for the detection of E. coli in freshwater habitats, as well as for expressing other genes in aquatic environments.
Phylogenetic relationship and antifouling activity of bacterial epiphytes from the marine alga Ulva lactuca
Authors: Egan Suhelen , Thomas Torsten , Holmström Carola , Kjelleberg Staffan
Published: March 2003

It is widely accepted that bacterial epiphytes can inhibit the colonization of surfaces by common fouling organisms. However, little information is available regarding the diversity and properties of these antifouling bacteria. This study assessed the antifouling traits of five epiphytes of the common green alga, Ulva lactuca. All isolates were capable of preventing the settlement of invertebrate larvae and germination of algal spores. Three of the isolates also inhibited the growth of a variety of bacteria and fungi. Their phylogenetic positions were determined by 16S ribosomal subunit DNA sequencing. All isolates showed a close affiliation with the genus Pseudoalteromonas and, in particular, with the species P. tunicata. Strains of this bacterial species also display a variety of antifouling activities, suggesting that antifouling ability may be an important trait for members of this genus to be highly successful colonizers of animate surfaces and for such species to protect their host against fouling.
Burkholderia cepacia complex: distribution of genomovars among isolates from the maize rhizosphere in Italy
Authors: Fiore Alessia , Laevens Severine , Bevivino Annamaria , Dalmastri Claudia , Tabacchioni Silvia , Vandamme Peter , Chiarini Luigi
Published: March 2003

Burkholderia cepacia is a ‘complex’ in which seven genomic species or genomovars have so far been identified. It appears that all seven B. cepacia genomovars are capable of causing infections in vulnerable persons; in particular, the importance of Burkholderia multivorans (genomovar II) and B. cepacia genomovar III among cystic fibrosis isolates, especially epidemic ones, has been emphasized. In order to acquire a better comprehension of the genomovar composition of environmental populations of B. cepacia, 120 strains were isolated from the rhizosphere of maize plants cultivated in fields located in northern, central and southern Italy. The identification of the different genomovars was accomplished by a combination of molecular polymerase chain reaction (PCR)‐based techniques, such as restriction fragment length polymorphism (RFLP) analysis of 16S rDNA (ARDRA), genomovar‐specific PCR tests and RFLP analyses based on polymorphisms in the recA gene whole‐cell protein electrophoresis. ARDRA analysis allowed us to distinguish between all B. cepacia genomovars except B. cepacia genomovar I, B. cepacia genomovar III and Burkholderia ambifaria (genomovar VII). The latter genomovars were differentiated by means of recA PCR tests and RFLP analyses. Among the rhizospheric isolates of B. cepacia, we found only B. cepacia genomovar I, B. cepacia genomovar III, Burkholderia vietnamiensis (genomovar V) and B. ambifaria. B. cepacia genomovars I and III and B. ambifaria were recovered from all three fields, whereas B. vietnamiensis was detected only in the population isolated from the field located in central Italy. Among strains isolated from northern and southern Italy, the most abundant genomovars were B. ambifaria and B. cepacia genomovar III respectively; in contrast, the population isolated in central Italy showed an even distribution of strains among genomovars. These results indicate that it is not possible to differentiate clinical and environmental strains, or pathogenic and non‐pathogenic strains, of the B. cepacia complex simply on the basis of genomovar status, and that the environment may serve as a reservoir for B. cepacia genomovar III infections in vulnerable humans.
Natural roles of biosurfactants
Authors: Ron Eliora Z. , Rosenberg Eugene
Published: March 2003

Microorganisms produce a variety of surface‐active agents (or surfactants). These can be divided into low‐molecular‐weight molecules that lower surface and interfacial tensions efficiently and high‐molecular‐weight polymers that bind tightly to surfaces. These surfactants, produced by a wide variety of microorganisms, have very different chemical structures and surface properties. It is therefore reasonable to assume that different groups of biosurfactants have different natural roles in the growth of the producing microorganisms. Moreover, as their chemical structures and surface properties are so different, each emulsifier probably provides advantages in a particular ecological niche. Several bioemulsifiers have antibacterial or antifungal activities. Other bioemulsifiers enhance the growth of bacteria on hydrophobic water‐insoluble substrates by increasing their bioavailability, presumably by increasing their surface area, desorbing them from surfaces and increasing their apparent solubility. Bioemulsifiers also play an important role in regulating the attachment–detachment of microorganisms to and from surfaces. In addition, emulsifiers are involved in bacterial pathogenesis, quorum sensing and biofilm formation. Recent experiments indicate that a high‐molecular‐weight bioemulsifier that coats the bacterial surface can be transferred horizontally to other bacteria, thereby changing their surface properties and interactions with the environment.
Lipopolysaccharide O‐chain microheterogeneity ofSalmonellaserotypes Enteritidis and Typhimurium
Authors: Parker Craig T. , Liebana Ernesto , Henzler David J. , Guard‐Petter Jean
Published: March 2003

Variability in the lipopolysaccharide (LPS) of the two most prevalentSalmonellaserotypes causing food‐borne salmonellosis was assessed using gas chromatography analysis of neutral sugars from 43Salmonella entericaserovar Enteritidis (S. Enteritidis)and20Salmonella entericaserovar Typhimurium (S. Typhimurium) isolates.Four substantially different types of O‐chain chemotypes were detected using cluster analysis of sugar compositions; these were low‐molecular‐mass (LMM) LPS, glucosylated LMM LPS, high‐molecular‐mass (HMM) LPS and glucosylated HMM LPS. Nineteen out of 20S. Typhimurium isolates yielded glucosylated LMM.In contrast,S. Enteritidis produced a more diverse structure, which varied according to the source and history of the isolate: 45.5% of egg isolates yielded glucosylated HMM LPS; 100% of stored strains lacked glucosylation but retained chain length in some cases; and 83.3% of fresh isolates from the naturally infected house mouseMus musculusproduced glucosylated LMM LPS. A chain length determinant (wzz) mutant ofS. Enteritidis produced a structure similar to that ofS. Typhimurium and was used to define what constituted significant differences in structure using cluster analysis. Fine mapping of theS. Enteritidis chromosome by means of a two‐restriction enzyme‐ribotyping technique suggested that mouse isolates producing glucosylated LMM LPS were closely related to orally invasive strains obtained from eggs, and that stored strains were accumulating genetic changes that correlated with suppression of LPS O‐chain glucosylation. These results suggest that the determination of LPS chemotype is a useful tool for epidemiological monitoring ofS. Enteritidis,which displays an unusual degree of diversity in its LPS O‐chain.
25S rDNA‐based molecular monitoring of glomalean fungi in sewage sludge‐treated field plots
Authors: Jacquot‐Plumey E. , Van Tuinen D. , Chatagnier O. , Gianinazzi S. , Gianinazzi‐Pearson V.
Published: March 2003

Recycling of sewage wastes in agriculture is likely to affect the biological activity of soils through contamination of ecosystems by pathogens and metallic or organic micropollutants. The impact of sewage sludge spreading under field conditions on arbuscular mycorrhiza (AM) formation by a community of glomalean fungi was evaluated using a nested polymerase chain reaction (PCR) and discriminating primers based on 25S rDNA polymorphisms to detect different fungal species within root systems. Medicago truncatula was grown in soil of field plots amended or not with a composted sewage sludge, spiked or not with organic or metallic micropollutants. Overall AM development in roots decreased with sewage sludge application, and the relative abundance of five AM fungal morphotypes in root fragments was modified by the input of composted sludges. Sewage sludge spiked or not with organic pollutants had a generally positive effect on the relative diversity of AM fungal populations in planta, whereas after spreading of the sludge spiked with metallic pollutants, no variation was observed in the abundance of different species.
The probiotic potential against vibriosis of the indigenous microflora of rainbow trout
Authors: Spanggaard Bettina , Huber Ingrid , Nielsen Janne , Sick Esben B. , Pipper Christian B. , Martinussen Torben , Slierendrecht Wilhelmina J. , Gram Lone
Published: March 2003

The antibacterial properties of the indigenous microflora of rainbow trout (Oncorhynchus mykissWalbaum) and the potential use of inhibitory bacteria as fish probiotics were investigated. A total of 1018 bacteria and yeasts were isolated on tryptone soy agar (TSA) from skin, gills and intestine. Forty‐five of these inhibited growth of the fish pathogenic bacteriumVibrio anguillarumin a well diffusion assay. The antagonism was most prominent amongPseudomonasspp., as 28 (66%) of the antagonistic bacteria belonged to this genus, despite constituting only 15% of the total tested flora. As pseudomonads are typically siderophore producers, chrome azurol S (CAS) agar was used as a semi‐selective medium for isolation of antagonistic bacteria. On this medium, 75% of the iron‐chelating strains were inhibitory toV. anguillarum. Eight strains out of a subset of 11 antagonists caused a 3–6 log unit reduction in the density ofV. anguillarum[measured by polymerase chain reaction (PCR) detection in a most probable number (MPN) regimen] in a broth co‐culture assay. Survival of rainbow trout infected with vibriosis was improved 13–43% by six out of nine antagonistic strains testedin vivo.All disease‐protecting strains were pseudomonads, isolated from CAS plates, whereas twoCarnobacteriumspp. that were antagonistic inin vitrowell diffusion assays did not alter the accumulated mortality of rainbow trout. The addition of live bacterial cultures to fish‐rearing water may thus improve survival of the fish; however,in vitroantagonism could not completely predict anin vivoeffect. Further studies on the underlying mechanism of activity are required to design appropriate selection criteria for fish probiotic bacteria.
Carbon isotopic fractionations associated with thermophilic bacteria Thermotoga maritima and Persephonella marina
Authors: Zhang Chuanlun L. , Ye Qi , Reysenbach Anna‐Louise , Götz Dorothee , Peacock Aaron , White David C. , Horita Juske , Cole David R. , Fong Jon , Pratt Lisa , Fang Jiasong , Huang Yongsong
Published: March 2003

Stable carbon isotopes can provide insight into carbon cycling pathways in natural environments. We examined carbon isotope fractionations associated with a hyperthermophilic fermentative bacterium, Thermotoga maritima, and a thermophilic chemolithoautotrophic bacterium Persephonella marina. In T. maritima, phospholipid fatty acids (PLFA) are slightly enriched in 13C relative to biomass (ɛ = 0.1–0.8‰). However, PLFA and biomass are depleted in 13C relative to the substrate glucose by ~8‰. In P. marina, PLFA are 1.8–14.5‰ enriched in 13C relative to biomass, which suggests that the reversed tricarboxylic acid (TCA) cycle or the 3‐hydroxypropionate pathway may be used for CO2 fixation. This is supported by small fractionation between biomass and CO2 (ɛ = −3.8‰ to −5.0‰), which is similar to fractionations reported for other organisms using similar CO2 fixation pathways. Identification of the exact pathway will require biochemical assay for specific enzymes associated with the reversed TCA cycle or the 3‐hydroxypropionate pathway.