bluuebunny/crossref_metadata_2025 huggingface dataset:
Clinical and genetic heterogeneity in hereditary haemochromatosis: association between lymphocyte counts and expression of iron overload
Authors: Porto Graça , Cardoso Carla S. , Gordeuk Victor , Cruz Eugénia , Fraga José , Areias Jorge , Oliveira José Carlos , Bravo Fernanda , Gangaidzo Innocent T. , MacPhail A.P. , Gomo Zvenyika A.R. , Moyo Victor M. , Melo Graça , Silva Cidália , Justiça Benvindo , De Sousa Maria
Published: November 2004

Abstract: To identify a new marker of expression of disease, independent of HFE genotype in patients with hereditary haemochromatosis (HHC), the total peripheral blood lymphocyte counts were analysed according to iron status in two groups of subjects with HFE mutations. The groups consisted of 38 homozygotes for C282Y, and 107 heterozygotes for the C282Y or compound heterozygotes for C282Y and H63D. For control purposes, total lymphocyte counts and iron status were also examined in 20 index patients with African dietary iron overload, a condition not associated with HFE mutations, and in 144 members of their families and communities. Mean lymphocyte numbers were lower in C282Y homozygous HHC index subjects with cirrhosis and higher iron stores than in those without cirrhosis and with lower iron burdens [(1.65 ± 0.43) × 106/mL vs. (2.27 ± 0.49) × 106/mL; p = 0.008]. Similarly, mean lymphocyte counts were significantly lower in C282Y heterozygotes and C282Y/H63D compound heterozygotes with iron overload and increased serum ferritin concentrations compared to those with normal serum ferritin concentrations (p < 0.05). Statistically significant negative correlations were found, in males, between lymphocyte counts and the total body iron stores, either in C282Y homozygous HHC patients (p = 0.031 in a multiple regression model dependent on age) and in C282Y heterozygotes or C282Y/H63D compound heterozygotes with iron overload (p = 0.029 in a simple linear model). In contrast, lymphocyte counts increased with increasing serum ferritin concentrations among the index subjects with African iron overload (r = 0.324, not statistically significant) and among the members of their families and communities (r = 0.170, p = 0.042). These results suggest that a lower peripheral blood lymphocyte count is associated with a greater degree of iron loading in HFE haemochromatosis but not in African iron overload, and they support the notion that the lymphocyte count may serve as a marker of a non‐HFE gene that influences the clinical expression of HFE haemochromatosis.
Stability of ecosystem properties in response to above‐ground functional group richness and composition
Authors: Wardle David A. , Bonner Karen I. , Barker Gary M.
Published: November 2004

While there has been a rapidly increasing research effort focused on understanding whether and how composition and richness of species and functional groups may determine ecosystem properties, much remains unknown about how these community attributes affect the dynamic properties of ecosystems. We conducted an experiment in 540 mini‐ecosystems in glasshouse conditions, using an experimental design previously shown to be appropriate for testing for functional group richness and composition effects in ecosystems. Artificial communities representing 12 different above‐ground community structures were assembled. These included treatments consisting of monoculture and two‐ and four‐species mixtures from a pool of four plant species; each plant species represented a different functional group. Additional treatments included two herbivore species, either singly or in mixture, and with or without top predators. These experimental units were then either subjected to an experimentally imposed disturbance (drought) for 40 d or left undisturbed. Community composition and drought both had important effects on plant productivity and biomass, and on several below‐ground chemical and biological properties, including those linked to the functioning of the decomposer subsystem. Many of these compositional effects were due to effects both of plant and of herbivore species. Plant functional group richness also exerted positive effects on plant biomass and productivity, but not on any of the below‐ground properties. Above‐ground composition also had important effects on the response of below‐ground properties to drought and thus influenced ecosystem stability (resistance); effects of composition on drought resistance of above‐ground plant response variables and soil chemical properties were weaker and less consistent. Despite the positive effects of plant functional group richness on some ecosystem properties, there was no effect of richness on the resistance of any of the ecosystem properties we considered. Although herbivores had detectable effects on the resistance of some ecosystem properties, there were no effects of the mixed herbivore species treatment on resistance relative to the single species herbivore treatments. Increasing above‐ground food chain length from zero to three trophic levels did not have any consistent effect on the stability of ecosystem properties. There was no evidence of either above‐ground composition or functional group richness affecting the recovery rate of ecosystem properties from drought and hence ecosystem resilience. Our data collectively point to the role of composition (identity of functional group), but not functional group richness, in determining the stability (resistance to disturbance) of ecosystem properties, and indicates that the nature of the above‐ground community can be an important determinant of the consistency of delivery of ecosystem services.
Extension of ideal free resource use to breeding populations and metapopulations
Authors: Doncaster C. Patrick
Published: November 2004

The concept of an ideal and free use of limiting resources is commonly invoked in behavioural ecology as a null model for predicting the distribution of foraging consumers across heterogeneous habitat. In its original conception, however, its predictions were applied to the longer timescales of habitat selection by breeding birds. Here I present a general model of ideal free resource use, which encompasses classical deterministic models for the dynamics in continuous time of feeding aggregations, breeding populations and metapopulations. I illustrate its key predictions using the consumer functional response given by Holling's disc equation. The predictions are all consistent with classical population dynamics, but at least two of them are not usually recognised as pertaining across all scales. At the fine scale of feeding aggregations, the steady state of an equal intake for all ideal free consumers may be intrinsically unstable, if patches are efficiently exploited by individuals with a non‐negligible handling time of resources. At coarser scales, classical models of population and metapopulation dynamics assume exploitation of a homogeneous environment, yet they can yield testable predictions for heterogeneous environments too under the assumption of ideal free resource use.
Developmental instability and within‐population variation in sex expression by andromonoecious Euphorbia xanti
Authors: Kiflawi Moshe
Published: November 2004

Within an area spanning approximately 1 km2, individuals
belonging to a population of andromonoecious Euphorbia xanti range in their sex expression from all‐staminate to all‐cosexual. In this study I attempt to evaluate this variation using measures of non‐directional asymmetry as potential indicators of developmental instability. Assuming that developmental homeostasis and sex allocation are sensitive to the same, or correlated, stress factors, I expected an adaptive explanation to be manifested in a monotonic relationship between asymmetry and ‘femaleness’ (i.e. the per‐plant proportion of cosexual cyathia). Unexpectedly, per‐plant mean asymmetry peaked at intermediate sexuality (femaleness of approx. 0.5). At the same time, intermediate sexuality was qualitatively associated with relatively low probabilities of cosexual cyathia developing fruit. I discuss two hypothetical explanations for these observations.
How to determine a regional species pool: a study in two Swedish regions
Authors: Dupré Cecilia
Published: November 2004

The species pool hypothesis has been proposed as one of the possible explanations for the local species richness of plant communities. For testing and validating this theory, it is of crucial importance to determine the dimension of the regional pool, which is the reservoir of species that are potentially able to exist in a community.  The main purpose of this study was to develop and test different methods for the determination of the regional species pool. Two regions in Sweden served as study areas, Öland and Uppland. In both regions, three different vegetation types were treated: dry calcareous grasslands, coastal meadows and deciduous forests. For the determination of the regional pool two main groups of methods are proposed: 1) six ecological approaches, based on Ellenberg species indicator values, and 2) two phytosociological approaches, based on the occurrence of species in different syntaxa in the framework of the Braun‐Blanquet system. The different screening methods were tested using Sørensen's index expressing the similarity between the community species pool and the regional species pool. Two types of error were recognized which may result in low index values.  For the six ecological methods Sørensen's index values were below 50%. The methods differed considerably from each other in accuracy, due to large differences in errors of both types. The phytosociological methods resulted in higher similarity values of up to almost 70%. The two approaches differed in error type but gave similar results.
Patterns of spread in insect‐pathogen systems: the importance of pathogen dispersal
Authors: White Andrew , Watt Allan D. , Hails Rosie S. , Hartley Sue E.
Published: November 2004

A general model of insect‐pathogen dynamics is presented which includes explicit host and pathogen dispersal. Four distinctive, wave‐like, patterns of dispersal are produced which can be categorised by two universal parameters – the speed of advance, and the position of the leading edge, of the wave of dispersal of the host relative to that of the disease. These patterns are (1) the pathogen becomes extinct, allowing the host to disperse at the carrying capacity across the land surface, (2) the host disperses more rapidly than the pathogen, producing host densities at the carrying capacity in a region behind the leading edge of the wave, with these densities reduced due to interaction with the pathogen in the wave interior, (3) the host and pathogen disperse at the same speed but the leading edge of the host extends beyond that of the pathogen, allowing the host to reach ‘high’ density at the leading edge only, and (4) the host and pathogen disperse at the same speed but the leading edge of the pathogen extends beyond that of the host, producing ‘low’ density host dispersal across the land surface. A biological description explaining the causes of these patterns has important consequences regarding the use of pathogens for biological control of insect pests.  The model is modified to represent a specific insect‐pathogen system, the winter moth, Operophtera brumata, and its nuclear polyhedrosis virus. The same patterns, categorised by the same universal parameters, are observed. Thus, it is suggested that the strength of infection and the relative dispersal rates of the host and pathogen are influential in determining the patterns of host outbreaks observed in this insect.
Body size–density relationship for Mytilus edulis in an experimental food‐regulated situation
Authors: Alunno‐Bruscia Marianne , Petraitis Peter S. , Bourget Edwin , Fréchette Marcel
Published: November 2004

We grew mussels (Mytilus edulis) under two different food
regimes and eight population density levels to estimate the joint effects
of density and biomass on their growth and survival and to determine the
shape of the biomass–density (B–N) relationship.
Mussels were reared for 22 months, between December 1994 and October
1996, in 1‐L experimental chambers supplied with natural seston. Growth
in shell length, individual wet mass and ash free dry mass (m)
decreased with decreasing food availability and increasing population
density. Survival was negatively correlated with density but did not
differ significantly between food regimes during the first year.
Variations in concentration of available food did not alter the effects
of crowding on mussels, as judged from the slopes of the body
size–density curves. The general patterns exhibited by
B–N curves were not consistent with expectations since we
found 1) no classical competition–density (C–D) effect as
reported in plants at intermediate competition levels, and 2) a slope of
−0.648 for m–N curves in both food regimes, which was
higher than expected from self‐thinning (ST) theory. This value does not
support present food‐driven ST theory. This study introduces an unusual
m–N region which combines properties of both ST and C–D effect.
Movement by mating females of a host alternating aphid: a response to leaf fall
Authors: Glinwood Robert , Pettersson Jan
Published: November 2004

Autumn remigrants of the host alternating bird cherry‐oat aphid, Rhopalosiphum padi, colonise individual plants of the winter host,
Prunus padus. However, they do not select leaves at different
stages of senescence, which is surprising as leaf fall is a definitive
deadline for production and development of mating females. For successful
development mating females should be mobile and have the capacity to
evaluate leaf senescence. Although remigrants do not selectively colonise
leaves at different stages of senescence, mating females were found in
very low numbers on leaves that were about to be shed. In choice test and
olfactometer bioassays, abscising leaves were avoided by mating females.
In the field mature leaves were available for colonisation during the
period of leaf fall of P. padus. We conclude that mating females abandon abscising leaves in favour of leaves at an earlier stage of senescence, increasing the time available for mating. Our results support the role of remigrants as pioneers which identify good winter host plants, and of mating females as mobile searchers for optimal feeding and mating sites.
Variation of butterfly diet breadth in relation to host‐plant predictability: results from two faunas
Authors: Beccaloni George W. , Symons Fernley B.
Published: November 2004

Host‐plant data for North American and Australian butterflies were used to test the hypothesis that larval diet breadth increases with decreasing resource predictability (where the latter was estimated by host‐plant growth‐form/duration). For each region in turn we compared the diet breadths of butterflies which utilise herbaceous host‐plants with those of species having woody hosts. For North America alone we also compared the diet breadths of species having annual hosts with those utilising perennial hosts, and the diets of species having herbaceous‐annual hosts with those using woody‐perennial hosts. Studies of diet breadth may be biased by the host taxonomic level which contributes most to the diet index used. For example, the results of analyses which employ indices based on numbers of families of hosts utilised may differ from those using indices based on counts of host species or genera. To investigate this potential problem we performed cross‐species analyses where diet breadth was defined in turn as the number of host species, genera, or families eaten. We found that using different taxonomic levels did give inconsistent results. To avoid this we employed phylogenetic diet breadth indices in comparative analyses of Independent Contrasts. The former incorporate information from the whole of the host‐plant phylogeny, whilst the comparative method eliminates any confounding effects of butterfly phylogeny. The results indicated that there is a phylogenetic component to butterfly diet breadth. They also largely differed from those of the cross‐species investigations, although there were similarities (i.e. results differed between regions and varied according to whether the whole fauna or just endemics were investigated). Our results suggested that in both regions, non‐endemics which feed on herbaceous plants have wider diet breadths than non‐endemics which utilise woody hosts. However, we found no consistent evidence that the diet breadths of endemics increase with decreasing resource predictability (as estimated here).
Is the assembly of ant communities mediated by parasitoids?
Authors: Feener, Jr. Donald H.
Published: November 2004

Studies of species interactions in ant communities have been a major source of evidence for the importance of interspecific competition in natural communities. One consequence of the overwhelming evidence for competition in ant communities is that the role of such “top‐down” processes as predation and parasitism has been ignored. Recent evidence, however, suggests that the composition and dynamics of ant communities are influenced by highly specialized parasitoids that mediate the outcome of competition among ant species. Here I review this evidence and develop a general framework for integrating the roles of competition and parasitism in the assembly of ant communities. I then use invasions by the red imported fire ant (Solenopsis wagneri) and the Argentine ant (Linepithema
humile) to show how this new framework can be used to develop testable hypotheses regarding the ecological success (or failure) of invasive ant species.
Forest fragmentation truncates a food chain based on an old‐growth forest bracket fungus
Authors: Komonen Atte , Penttilä Reijo , Lindgren Mariko , Hanski Ilkka
Published: November 2004

We studied the effect of forest fragmentation on the insect community inhabiting an old‐growth forest specialist bracket fungus, Fomitopsis
rosea, in eastern Finland. Samples of the fungus from large
non‐isolated control areas were compared with samples from forest
fragments in two isolation time classes; 2–7 yr and 12–32 yr
since isolation. Fomitopsis rosea hosted a species‐rich community
with relatively many specialized old‐growth forest insects. The
numerically dominant food chain consisted of F. rosea, the tineid
moth Agnathosia mendicella and the tachinid fly Elfia
cingulata, a specialist parasitoid of A. mendicella. The
frequency of F. rosea on suitable fallen spruce logs and the
frequency of A. mendicella in fruiting bodies were significantly lower in the forest fragments than in the control areas. The median number of trophic levels decreased from three in the control areas to one in the fragments that had been isolated for the longest period of time. The parasitoid was completely missing from the fragments isolated for 12–32 yr. Our results show that in boreal forests habitat loss and fragmentation truncate food chains of specialized species in the course of time since isolation.
Importance of habitat structure to the arthropod food‐web in Douglas‐fir canopies
Authors: Halaj Juraj , Ross Darrell W. , Moldenke Andrew R.
Published: November 2004

This study tested the hypothesis that habitat structure dictates the distribution and community composition of arboreal arthropods. A diverse arthropod assemblage of Douglas‐fir canopies, which included Araneae, Psocoptera, Collembola and Homoptera, was chosen as a model system. Habitat structural diversity, defined as needle density and branching complexity of Douglas‐fir branches, was manipulated in a four‐month experiment by needle removal, thinning and tying of branches. Abundance of canopy spiders declined significantly following needle density reduction and branch thinning, branch tying significantly increased spider abundance. Distinct habitat utilization patterns were found among individual spider guilds. Orb weavers (Araneidae) dominated spider assemblages in structurally simple habitats, whereas tied branches were colonized primarily by sheet‐web weavers (Linyphiidae) and nocturnal hunting spiders (Anyphaenidae and Clubionidae). Spider species richness and average body size of several spider species increased in structurally more complex habitats. Arboreal spiders appeared to be limited by strong bottom‐up effects in the form of habitat quality and, to a lesser degree, prey availability. Habitat manipulations did not affect densities or biomass of flying arthropod colonists in the branch vicinity. Needle removal and branch thinning led to a significant decline in the abundance of Psocoptera and Collembola. Tying of branches resulted in an eight‐fold increase in Collembola numbers, organisms most sensitive to habitat alterations. Canopy habitat structure modified vertical dispersal of Collembola from forest litter, which may have significant implications for arboreal consumers. Our results lend strong support to the importance of habitat structural diversity in explaining general patterns of arthropod abundance and diversity on plants.
Predator‐prey theory: hidden twins in ecology and microbiology
Authors: Jost Christian
Published: November 2004

Consumption of a resource by an organism is a key process in both microbiology and population ecology. Recently, there has been a debate in population ecology about the importance of organism density in functions describing this process. Actually, microbiologists have had this debate over the last 40 years. In reviewing their main results I show that even for the most simple systems there is no unique correct function to describe consumption. Organism density influences consumption to various degrees. I conclude that, for predictions based on model simulation, one should use a pluralistic approach, working with different models to identify robust predictions (that is, common to all studied models) and guide further research to understand model‐specific predictions.
The roles of community biomass and species pools in the regulation of plant diversity
Authors: Grace James B.
Published: November 2004

Considerable debate has developed over the importance of community biomass and species pools in the regulation of community diversity. Attempts to explain patterns of plant diversity as a function of community biomass or productivity have been only partially successful and, in general, have explained only a fraction of the observed variation in diversity. At the same time, studies that have focused on the importance of species pools have led some to conclude that diversity is primarily regulated in the short term by the size of the species pool rather than by biotic interactions. In this paper, I explore how community biomass and species pools may work in combination to regulate diversity in herbaceous plant communities. To address this problem, I employ a simple model in which the dynamics of species richness are a function of aboveground community biomass and environmentally controlled gradients in species pools. Model results lead to two main predictions about the role of biomass regulation: (1) Seasonal dynamics of richness will tend to follow a regular oscillation, with richness rising to peak values during the early to middle portion of the growing season and then declining during the latter part of the season. (2) Seasonal dieback of aboveground tissues facilitates the long‐term maintenance of high levels of richness in the community. The persistence of aboveground tissues and accumulation of litter are especially important in limiting the number of species through the suppression of recruitment. Model results also lead to two main predictions about the role of species pools: (1) The height and position of peak richness relative to community biomass will be influenced by the rate at which the species pool increases as available soil resources increase. (2) Variations in nonresource environmental factors (e.g. soil pH or soil salinity) have the potential to regulate species pools in a way that is uncorrelated with aboveground biomass. Under extreme conditions, such nonresource effects can create a unimodal envelope of biomass–richness values. Available evidence from the literature provides partial support for these predictions, though additional data are needed to provide more convincing tests.
Press perturbation experiments and the indeterminacy of ecological interactions: effects of taxonomic resolution and experimental duration
Authors: Attayde José Luiz , Hansson Lars‐Anders
Published: November 2004

The outcomes of press perturbation experiments on community dynamics are difficult to predict because there is a high degree of indeterminacy in the strength and direction of ecological interactions. Ecologists need to quantify uncertainties in estimates of interaction strength, by determining all the possible values a given interaction strength could take and the relative likelihood of each value. In this study, we assess the degree to which fish effects on zooplankton and phytoplankton are indeterminate in direction using a combination of experimental data and Monte Carlo simulations. Based on probability distributions of interaction strength (i.e. effect magnitude), we estimated the probability of each fish interaction being negative, positive or undetermined in direction. We then investigated how interaction strength and its predictability might vary with experimental duration and the taxonomic resolution of food web data. Results show that most effects of fish on phyto‐ and zooplankton were indeed indeterminate, and that the effects of fish were more predictable in direction as the taxonomic resolution of food web data decreased and the experimental duration increased. Results also show that most distributions of interaction strength were not normal, suggesting that normal based statistical procedures for testing hypothesis about interaction strength may be misleading, as well as predictions of food web models assuming normal distributions of interaction strength. By considering the probability distributions and confidence intervals of interaction parameters, ecologists would better understand the outcomes of species interactions and make more realistic predictions about our perturbations in natural food webs.
Predation and dispersal of large and small seeds of a tropical palm
Authors: Brewer Steven W.
Published: November 2004

Seed size may vary greatly among individuals within plant species. What effects the extremes of this variation have for seeds taken by small mammals are poorly understood. Not all seeds removed by small mammals are necessarily eaten. Small rodents are common seed predators, but they may disperse a significant proportion of seeds by scatter hoarding them via burial. Size‐dependent predation and dispersal of seeds has not been directly tested within a plant species for tropical rodents. This study tested whether or not large and small nuts of Astrocaryum mexicanum (Palmae) differed in their fates due to handling by the spiny pocket mouse Heteromys desmarestianus (Heteromyidae). Exclosures were used to give small rodents exclusive access to A. mexicanum nuts. H. desmarestianus preferentially consumed large over small A. mexicanum nuts, but cached (in burrows and by scatter hoarding) similar proportions of these nuts by size. Small nuts tended to be buried farther away from exclosures than large nuts. Although sample sizes of buried nuts were small, the rodents retrieved all buried large nuts, but 30% of the small nuts remained buried long enough to germinate. I also examined predispersal predation by insects and found that insects appear to have no size preference for A. mexicanum nuts, but insect predation appears to hinder nut development. Thus, nuts attacked by insects develop to be significantly smaller, with a low proportion of undamaged endosperm, than uninfested nuts. It is hypothesized that the preferential predation of large A. mexicanum nuts by H. desmarestianus is a response by these rodents to insect predation.
Synchrony in short‐term fluctuations of moose calf body mass and bank vole population density supports the mast depression hypothesis
Authors: Selås Vidar , Sonerud Geir A. , Histøl Trond , Hjeljord Olav
Published: November 2004

Inter‐annual variations in body mass of moose, Alces alces, in Norway and Sweden have been considered as most likely due to direct or indirect effects of weather, but so far predictions of autumn body mass of moose calves on the basis of weather data have given a poor fit to data. A striking, but hitherto unnoticed, feature of several time series on body mass of moose calves from south‐eastern Norway is an apparently regular 3–4‐year fluctuation pattern. This short‐term fluctuation could be due to regular variations in forage quality, e.g. caused by a cyclic seed production of some important food plants, as envisaged by the “mast depression” hypothesis. One plant species important as food for moose calves in autumn is bilberry, Vaccinium myrtillus, which usually produces high seed crops (masts) at intervals of 3–4 years. Populations of the bank vole, Clethrionomysglareolus, which feeds on bilberry shoots in winter, are known to peak in bilberry post‐mast years. In two study areas in Norway, there was a positive correlation between the autumn body mass of moose calves and the autumn population index of bank vole in the succeeding year. In the northern area there was an additional positive effect of summer precipitation, whereas in the southern area there was an additional negative effect of summer temperature. In both areas, however, the effect of weather was less pronounced than that of the bank vole index. These results support the mast depression hypothesis.
Phenological complementarity, species diversity, and ecosystem function
Authors: Stevens M. Henry H. , Carson Walter P.
Published: November 2004

Increasing species diversity frequently enhances ecosystem function. Phenological complementarity, the asynchrony of species resource use and growth, may explain how species diversity influences ecosystem function but remains largely untested. We used an early successional plant community containing species with a variety of phenologies to test whether increasing species diversity enhances ecosystem function by increasing phenological complementarity. Over a two‐year period, we increased environmental heterogeneity within an abandoned field with variation in disturbance, soil nutrients, water, light availability, and disturbance in 160 permanent plots, and measured percent cover of each plant species three times in each growing season. We did not manipulate species composition directly, and thus diversity and complementarity in each plot were the result of pre‐existing conditions and responses of individuals to experimental treatments. Species diversity was measured in two ways, as the total number of species per plot and as the evenness of species abundances. Phenological complementarity was measured as the negative logarithm of the variance ratio. We tested whether the number of plant species per plot, species evenness, and their phenological complementarity in the first year predicted total annual cover in the second year. Total annual cover increased only moderately with number of species and evenness, consistent with studies that randomize species composition among replicate plots. Any effect that species number or evenness had on total annual cover, however, was not due to phenological complementarity. Rather, diversity was unrelated to phenological complementarity. These results indicate that naturally occurring variation in species diversity had little effect on whether phenological complementarity can enhance ecosystem function.
Talus fragmentation mitigates the effects of pikas, Ochotona princeps, on high alpine meadows
Authors: Roach W. John , Huntly Nancy , Inouye Richard
Published: November 2004

North American pikas (Ochotona princeps) are central‐place foragers that are restricted to talus slopes in the mountains of western North America. They graze meadows surrounding talus producing a gradient of grazing pressure extending out from the talus edge. We asked: 1) how does the physical structure of the talus slope affect pika foraging behavior? and 2) how do differences in foraging behavior affect the relative abundance of different plant functional groups?  Pikas at three talus‐meadow interfaces differed in the distances they traveled for forage, depending upon the structure of the talus slope. Pikas fed preferentially on vegetation within the talus, ventured into the surrounding meadow when intra‐talus vegetation was rare, and ranged farther when haying than when grazing. They ventured into the surrounding meadow less when talus fields were more fragmented and intra‐talus vegetation was more abundant.  We tested the effects of pikas on meadow vegetation by excluding pikas from small plots located at various distances from talus edges. When habitat structure forced pikas to venture out from the talus to forage, they decreased total vegetative biomass by as much as 80% and increased the relative importance of cushion plants and sedges. These effects were undetectable at distances beyond one meter. These patterns appear to result from a change in the relative performance of different plant functional groups caused by herbivory.
Competitive ability of neotropical Drosophila predicted from larval development times
Authors: Krijger Coenraad L. , Peters Yvonne C. , Sevenster Jan G.
Published: November 2004

Interspecific competition is a major aspect of the ecology of insect communities exploiting ephemeral and fragmented resources. To generate testable predictions on the role of competition in such communities, information is required about the competitive abilities of the species involved. In this study we test whether larval development time, an important life‐history trait, can be used to predict interspecific competitive ability in neotropical Drosophila species. Based on earlier work, we predicted a negative relationship between larval development time and competitive ability. Pairwise competition experiments were conducted with seven Drosophila species (in all possible combinations) that are known to coexist in central Panama and represent a wide range of larval development times. We used a novel approach to determine experimental densities, standardising total larval food requirements, and thereby increasing the resolution for detecting general traits involved in exploitation competition. The effects of competition were measured by comparing values for three fitness‐related parameters (thorax length, larval survival, and total mass of emerged adults) against their values at two levels of intraspecific competition. The outcomes of the pairwise competition were significantly explained by the differences in larval development times: within species pairs, species with a longer development suffered more from interspecific competition than those with shorter development. It is argued that larval development time is a major determinant of competitive rank order among drosophilid species, allowing testable community‐wide predictions. In addition, this relationship provides a basis for studying the role of life‐history trade‐offs in community‐level processes. Given the generality of the Drosophila biology, development time is expected to play a similar role in other insect communities exploiting ephemeral and fragmented resources.
Female‐biased dispersal, low female recruitment, unpaired males, and the extinction of small and isolated bird populations
Authors: Dale Svein
Published: November 2004

Small and isolated populations are usually assumed to be at a high risk of extinction due to environmental or demographic stochasticity, genetic problems, or too little immigration. In birds, natal dispersal is usually female‐biased, but the consequences of such a pattern on vulnerability to extinction of isolated populations has not received much attention before. In this paper I derive predictions as to how female‐biased natal dispersal may differentially affect the extinction risk of populations and species with contrasting distributions, migratory behaviours, life histories and mating systems. Female‐biased dispersal will lead to male‐biased sex ratios in small, isolated or fragmented populations, in particular because recent research has shown that females often have a limited ability to search for mates and may therefore effectively be lost from the breeding population if they disperse into areas empty of conspecifics. I reviewed published studies on birds and found that a high proportion of unpaired males is common in isolated populations or populations in small habitat fragments. Dispersal of females may therefore increase the vulnerability to extinction of small or isolated populations, or populations at the periphery of a species’ distribution range. I also predict that vulnerability to extinction should be greater for migratory than for resident species and greater for short‐lived than for long‐lived species because of differences in the time available for females to locate unpaired males. Further, extinction risk may also be greater for birds than for mammals due to differences in which sex disperses and patterns of parental care. Finally, mating system will also affect vulnerability to extinction when natal dispersal leads to biased sex ratios. I review available evidence for these predictions (e.g. songbird declines in North America) and discuss implications for conservation.
Population dynamics of a vertically transmitted, parasitic sex ratio distorter and its amphipod host
Authors: Kelly A. , Dunn A. M. , Hatcher M. J.
Published: November 2004

We investigate the population dynamics of a vertically transmitted, parasitic sex ratio distorter (Nosema granulosis) and its amphipod host (Gammarus duebeni), using field measurements to quantify and test alternative theoretical models of the interaction. We measure parasite, host population and transmission parameters at four locations on the Isle of Cumbrae, Scotland at monthly intervals for two years. We develop a simple infinite population model and test its predictions for parasite prevalence using field measurements of the parameters. Parasite prevalence is maintained at relatively low levels at all four sites (20–42%), consistently below that predicted. The parasite imposes a slight fitness cost on its host by reducing female fecundity, but this cost cannot account for the relatively low prevalences observed. We also investigate the importance of population structure, comparing parasite prevalence across sites and sampling intervals to look for evidence of spatial and temporal asynchrony as predicted by metapopulation models. We find significant temporal and spatial heterogeneity in parasite prevalence although there was also evidence that parasite dynamics were synchronous across sites. These data suggest that the parasite is unlikely to drive local population dynamics through cycles of extinction and recolonization at the scale measured. As the host (adult) population sex ratio was male‐biased, local population crashes are unlikely to be induced by the parasite, contrary to theoretical predictions.
Correlation of foliage and litter chemistry of sugar maple, Acer saccharum, as affected by elevated CO2 and varying N availability, and effects on decomposition
Authors: King J. S. , Pregitzer K. S. , Zak D. R. , Kubiske M. E. , Holmes W. E.
Published: November 2004

Rising atmospheric carbon dioxide has the potential to alter leaf litter chemistry, potentially affecting decomposition and rates of carbon and nitrogen cycling in forest ecosystems. This study was conducted to determine whether growth under elevated atmospheric CO2 altered the quality and microbial decomposition of leaf litter of a widely distributed northern hardwood species at sites of low and high soil nitrogen availability. In addition, we assessed whether the carbon–nutrient balance (CNB) and growth differentiation balance (GDB) hypotheses could be extended to predict changes in litter quality in response to resource availability. Sugar maple (Acer saccharum) was grown in the field in open‐top chambers at 36 and 55 Pa partial pressure CO2, and initial soil mineralization rates of 45 and 348 μg N g−1 d−1. Naturally senesced leaf litter was assessed for chemical composition and incubated in the laboratory for 111 d. Microbial respiration and the production of dissolved organic carbon (DOC) were quantified as estimates of decomposition. Elevated CO2 and low soil nitrogen resulted in higher litter concentrations of nonstructural carbohydrates and condensed tannins, higher C/N ratios and lower N concentrations. Soil N availability appears to have had a greater effect on litter quality than did atmospheric CO2, although the treatments were additive, with highest concentrations of nonstructural carbohydrates and condensed tannins occurring under elevated CO2–low soil N. Rates of microbial respiration and the production of DOC were insensitive to differences in litter quality. In general, concentrations of litter constituents, except for starch, were highly correlated to those in live foliage, and the CNB/GDB hypotheses proved useful in predicting changes in litter quality. We conclude the chemical composition of sugar maple litter will change in the future in response to rising atmospheric CO2, and that soil N availability will exert a major control. It appears that microbial metabolism will not be directly affected by changes in litter quality, although conclusions regarding decomposition as a whole must consider the entire soil food web.
Fatal interactions between Scots pine and Sphagnum mosses in bog ecosystems
Authors: Ohlson Mikael , Økland Rune Halvorsen , Nordbakken Jørn‐Frode , Dahlberg Barbro
Published: November 2004

In this study, we explore how Sphagnum mosses and Scots pine, Pinus sylvestris, interact on different spatial and temporal scales in a boreal bog ecosystem. We were particularly interested in relationships between the occurrence of Sphagnum‐dominated habitats and the occurrence of Scots pines of different age and size.

 Juvenile and adult pines occurred in different habitats. While juveniles mainly occurred in Sphagnum‐dominated habitats, predominantly with Sphagnum rubellum, adult pines were found in habitats dominated by lichens, or with a sparse vegetation cover. Examination of surface peat cores sampled close to adult pines revealed that almost all pines (97%) had established in a Sphagnum‐dominated environment and that the habitat had changed since pine establishment. Scots pine is thus capable of changing and exterminating the Sphagnum‐dominated environment preferred for germination and establishment. Pines impede Sphagnum growth and peat accumulation significantly once they have reached a stem diameter of approximately 20 mm. It takes from 30 to 90 yr for a pine to reach that size.

 Our results show the importance of interactions between Scots pine and Sphagnum mosses in bog ecosystems. We conclude that interactions between trees and Sphagnum mosses are important driving forces behind the vegetation change that has characterised boreal bogs during the Holocene.
Influence of host ecology and morphology on the diversity of Neotropical bird lice
Authors: Clayton D. H. , Walther B. A.
Published: November 2004

Host‐parasite systems can be powerful arenas in which to explore factors influencing community structure. We used a comparative approach to examine the influence of host ecology and morphology on the diversity of chewing lice (Insecta: Phthiraptera) among 52 species of Peruvian birds. For each host species we calculated two components of parasite diversity: 1) cumulative species richness, and 2) mean abundance. We tested for correlations between these parasite indices and 13 host ecological and morphological variables. Host ecological variables included geographic range size, local population density, and microhabitat use. Host morphological variables included body mass, plumage depth, and standard dimensions of bill, foot and toenail morphology, all of which could influence the efficiency of anti‐parasite grooming. Data were analysed using statistical and comparative methods that control for sampling effort and host phylogeny. None of the independent host variables correlated with louse species richness when treated as a dependent variable. When richness was treated as an independent variable, however, it was positively correlated with mean louse abundance. Host body mass was also positively correlated with mean louse abundance. When louse richness and host body mass were held constant, mean louse abundance correlated negatively with the degree to which the upper mandible of the host's bill overhangs the lower mandible. This correlation suggests that birds with longer overhangs are better at controlling lice during preening. We propose a specific functional hypothesis in which preening damages lice by exerting a shearing force between the overhang and the tip of the lower mandible. This study is the first to suggest a parasite‐control function of such a detailed component of bill morphology across species. Avian biologists have traditionally focused almost exclusively on bills as tools for feeding. We suggest that the adaptive radiation of bill morphology should be reinterpreted with both preening and feeding in mind.
Effects of avian predation threat, water flow and cover on growth and habitat use by chub, Leuciscus cephalus, in an experimental stream
Authors: Allouche Stéphanie , Gaudin Philippe
Published: November 2004

Trophic interactions, including “top‐down” predator‐prey interactions, are particularly important in influencing the structure of fish communities. While the varied impacts of piscivorous fish have been well investigated, the effects of fish‐eating birds on riverine fish behaviour and population dynamics still remain controversial, mainly because they are undervalued. Summer experiments were conducted in an experimental outdoor stream to evaluate the effects of avian predation threat, stream flow, and overhead cover on growth and behavioural tactics of wild juvenile chub (Leuciscus cephalus). Groups of fifteen chub maintained in riffle‐pool sequences were submitted to combinations of different conditions, namely absence or presence of a simulated fish‐eating bird, low or high flow, and absence or presence of medium or high cover. In the absence of predation threat, chub foraged in the riffles and maximized feeding opportunities. Under predation threat, they sheltered, foraged less and grew slowly and as expected, they increased their use of the riffles at high flow as water turbulence is an efficient shelter from birds but only in the absence of cover. In the presence of cover, fish sheltered exclusively under pool covers and were more prone to take risks at low flow because of higher costs in terms of lost feeding opportunities associated with these conditions. This result indicates that flow velocity altered cover use tactics through its impact on food supply, suggesting that it may affect the outcome of predator‐prey relationships. So, chub use cover in a flexible manner according to both the benefits in terms of predator avoidance and the costs in terms of lost feeding opportunities. A striking finding of the experiments is the drastic reduction in the range of growth variances amongst fish when they are maintained under predation threat, suggesting a homogenization of fitness between individuals. From all our results, we argue that in lowland streams, under summer field situations, fish‐eating birds may affect local prey population dynamics more through sub‐lethal effects on growth rates than directly through death rates.