bluuebunny/crossref_metadata_2025_split huggingface dataset:
Individual variation in the competitive ability of interference‐prone foragers: the relative importance of foraging efficiency and susceptibility to interference
Authors: Caldow R. W. G. , Goss‐Custard J. D. , Stillman R. A. , Durell S. E. A. Le V. Dit , Swinfen R. , Bregnballe T. 
Published: March 2003

Summary1.Individual variation in the competitive ability of foraging animals arises from variation in their intrinsic foraging efficiency and in their susceptibility to interference from competitors. Empirical and theoretical studies have concentrated on quantifying the latter and examining its role in determining the distribution and dynamics of animal populations, but have seldom considered the role of variation in foraging efficiency. Using the frequency of occurrence of oystercatchers in supplementary feeding habitats as an index of their competitive ability, we assessed the relative importance of foraging efficiency and susceptibility to interference in determining the overall competitive ability of an individual.2.Individual mussel‐feeding oystercatchers varied in their tendency to supplement their low‐tide intake by feeding on other prey on upshore tidal flats and in fields. Individuals that opened mussels by stabbing occurred on upshore flats more often than birds which hammered mussels, whereas young birds were more likely to visit the fields than older birds. This difference between habitats emphasized the need to understand the individual differences underlying these class effects.3.Foraging efficiency increased with age and differed between feeding methods. Dominance also increased with age, but did not differ between feeding methods. An individual's foraging efficiency was not related to its dominance.4.Individual variation in the usage of either upshore flats or fields, and of each habitat separately, was related to individual variation in foraging efficiency, but not to variation in dominance. Individuals of poor intrinsic foraging ability made greater use of supplementary feeding habitats than did more efficient foragers.5.Our results show that, even in an interference‐prone system and across a wide range of circumstances, individual variation in foraging efficiency is the major determinant of overall competitive ability. We believe, therefore, that this source of individual variation is of greater importance in determining variation in mortality risk within a population than the effort invested in its study hitherto would suggest. We suggest that a modelling approach is necessary to establish the competitive conditions under which susceptibility to interference might become the more important determinant of competitive ability and, hence, whether such conditions are ever likely to occur in natural populations. We argue that greater emphasis needs to be placed on identifying the determinants of foraging efficiency and its variation between individuals.
Immunocompetence and condition‐dependent sexual advertisement in male house sparrows (Passer domesticus)
Authors: Gonzalez Guillermo , Sorci Gabriele , Møller Anders Pape , Ninni Paola , Haussy Claudy , De Lope Florentino 
Published: March 2003

Summary1. We tested the condition‐dependent hypothesis of sexual advertisement in house sparrows (Passer domesticus). Male house sparrows have a bib of black feathers which serves as both a badge of social status and as a cue for female choice. We manipulated environmental conditions during the premoult period of juvenile house sparrows kept in outdoor aviaries. Birds were assigned to two treatments differing in the amount of dietary proteins, which are known to affect the expression of immune response in birds. We tested whether birds in the protein‐rich group had better immune responses and developed larger bibs than birds reared on a protein‐poor diet. We also checked whether immune response was a predictor of survival and parasite resistance.2. Individuals with higher cellular immune response at capture had greater probability to survive during the 3 months of the experiment, and they had a higher probability to recover from infection withHaemoproteussp. (a blood parasite). Conversely, birds with high immunoglobulin concentrations at capture had a higher probability of mortality.3. Birds on the protein‐rich diet had a higher cellular immune response compared to birds in the protein‐poor treatment. Humoral immune response showed the opposite pattern, being higher for birds in the protein‐poor treatment. We did not find any effect of food quality on the development of the badge, assessed as the size of the trait and its colour properties.4. In conclusion, our results support the view that immune defences are important for survival and parasite resistance in natural populations, and that they might be costly to produce. On the other hand, we did not find support for the condition‐dependent hypothesis of sexual advertisement, suggesting that the badge may not be a costly trait to produce. However, badge size could reflect other aspects of condition. The kind of pigments involved in colour signals may be the key factor determining the production costs of such traits.
Long‐term observations on the dynamics of three species of polychaetes living on tidal flats of the Wadden Sea: the role of weather and predator–prey interactions
Authors: Beukema J. J. , Essink K. , Dekker R. 
Published: March 2003

Summary 1. On the tidal flats of the Wadden Sea, the predatory polychaete Nephtys hombergii (Sav.) is sensitive to low winter temperatures, whereas its main prey, the smaller polychaetes Scoloplos armiger (O.F.M.) and Heteromastus filiformis (Clap.) are not. 2. Monitoring macrofauna dynamics during the last three decades (1970–97) in two distant Wadden Sea areas revealed that Nephtys abundance was severely reduced in all eight coldest winters (generally to an average of < 0·1 g ash‐free dry weight m–2), whereas its biomass reached mean values around 1 g AFDW m–2 during periods with mild winters (≤ 5 days with an air temperature remaining below 0°C all day). Over‐winter changes in Nephtys abundance were strongly related to temperature and 70% of the variability in late winter abundance of this species could be explained by temperature characteristics of the foregoing winter. 3. Biomass values of Nephtys observed in late winter predicted to a high degree (90%) the generally somewhat higher values in the subsequent summer. This seasonal change of predator abundance was generally not related to prey density. Only locally (in an exposed low‐biomass area) and rarely (one‐tenth of the years of observation) did low prey supply (if it was lower than simultaneous predator biomass) coincide with a decline, rather than the usual seasonal increase in Nephtys biomass. 4. A negative influence of predator abundance on prey biomass was observed in both prey species over the entire range of observed predator biomass values. The usual seasonal (winter to summer) increase of prey biomass was significantly smaller at high than at low Nephtys biomass and was even reduced to nil at the highest predator density observed. Scoloplos biomass was about four times higher after years with low (< 0·1 g m–2) than after years with high (> 1·0 g m–2) Nephtys biomass. Similar differences were observed in biomass of Heteromastus. 5. It is concluded that winter temperatures act as an important structuring factor in the Wadden Sea zoobenthic community by directly governing the densities of an important infaunal predator and indirectly affecting the abundance of at least two important prey species. Cold winters intervene by starting a new cycle of predator–prey interaction on average once per 3 or 4 years. 6. Predator abundance was more frequently determined by weather conditions than by food supply. Prey abundance was primarily governed by predator abundance and food supply and was only indirectly (via predator abundance) affected by the incidence of cold winters.
Predicting the relationship between local and regional species richness from a patch occupancy dynamics model
Authors: Hugueny B. , Cornell H.V. 
Published: March 2003

Summary 1. A linear relationship between the number of species in ecological communities (local richness) and the species pools from which the communities are drawn (regional richness) suggests that species interactions are not sufficient to limit local richness and that communities are not saturated with species. Instead, this relationship implies that communities are open to regional influences and are interlinked by dispersal. 2. Here we show how the linear relationship between local and regional richness in real, noninteractive, assemblages of cynipid gall wasps on California oaks, can be predicted from a simple patch‐occupancy model. 3. One cynipid assemblage has been surveyed for 3 years, allowing for crude estimates of colonization and extinction rates per patch. Using the mainland/island model of patch occupancy dynamics, these rates are combined with the observed number of cynipid species associated with each oak species (regional richness) to predict the expected local species richness in each patch. Assuming that species are independently distributed among localities, the expected variance in species richness among localities is also computed. 4. The model is then tested on an independent data set. When differences in sampling effort (number of surveyed trees per locality) were accounted for, the regression equation relating observed (n = 41) to predicted local species richness does not differ statistically from the line of perfect agreement. The residuals are also distributed according to the predicted variance. 5. Although not statistically significant, the variance in local richness appears to be slightly underestimated by the model. One explanation may be that cynipid species display some positive covariance in their distribution among localities, that is, groups of species occur together in given localities more frequently than would be expected by chance. Variance ratio tests identified statistically positive covariance within cynipid assemblages for three oaks species. 6. The close fit of the model to the data supports the theoretical scenario for noninteractive communities, that the slope of the local–regional richness relationship and patch‐occupancy processes are different expressions of the same phenomenon.
The interactive effects of food and predators on reproduction and overwinter survival of arctic ground squirrels
Authors: Karels Tim J. , Byrom Andrea E. , Boonstra Rudy , Krebs Charles J. 
Published: March 2003

Summary 1. We examined the effects of food and predators on population limitation in the arctic ground squirrel (Spermophilus parryii plesius Richardson) in the boreal forest of the south‐western Yukon. We focused on ground squirrel reproduction and overwinter survival. 2. Squirrel populations were monitored by live‐trapping and radio‐telemetry from 1993 to the spring of 1996 on four control and four experimental areas (one predator exclosure treatment, two food addition treatments, and one predator exclosure plus food addition treatment). 3. Predator exclusion increased body condition, percentage lactating, percentage weaning litters, litter size, and doubled population density relative to controls, but had no effect on juvenile growth rate, overwinter survival, or juvenile emergence date. 4. Food addition advanced juvenile emergence date and increased adult body condition, percentage lactating, percentage weaning litters, litter size, population density relative to controls (4–7 fold), but had no effect on juvenile growth rate or overwinter survival. 5. Predator exclusion combined with food addition increased adult body condition, percentage lactating, percentage weaning litters, and population density relative to controls (19‐fold). 6. We conclude that arctic ground squirrels in the boreal forest are limited by an interaction between food and predation, acting primarily through changes in reproduction, and that their impact on density was multiplicative.
Increased pre‐natal maternal corticosterone promotes philopatry of offspring in common lizards Lacerta vivipara
Authors: De Fraipont Michelle , Clobert Jean , John Henry , Alder ‐ , Meylan Sandrine 
Published: March 2003

Summary 1. There is growing evidence that dispersal is highly phenotypically plastic, i.e. that dispersal is condition‐dependent. In the common lizard, dispersal has even been shown to be influenced by the maternal environment during pregnancy. Juveniles in good condition or issued from mothers in good condition disperse earlier or in higher numbers. 2. We hypothesized that plasma corticosterone was the proximate mechanism by which condition and dispersal are linked, and tested this by manipulating the level of circulating corticosterone in pregnant females of the common lizard. 3. After parturition, we measured juvenile attractiveness towards the mother and juvenile dispersal of corticosterone (B) and placebo (P) implanted females. 4. Offspring of B females did disperse in lower number than those of P females. B offspring were also more attracted by the mother's odour than P offspring. 5. In quite a few cases, the behavioural response of juveniles was dependent on the interaction between the hormonal treatment and the mother snout–vent length or condition (body weight corrected for snout–vent length). 6. Corticosterone constitutes therefore one of the proximate mechanisms involved in the prenatal control of juvenile dispersal in this species. Along with other results, it is proposed that prenatal control of dispersal has evolved in order to avoid competition between mothers and their offspring.
Effects of food availability on the distribution of migratory warblers among habitats in Jamaica
Authors: Johnson Matthew D. , Sherry Thomas W. 
Published: March 2003

SummaryTheoretical arguments suggest that distributions of migratory birds in winter should match patterns of food availability, but in reality the match between migrants and their food may be imperfect because, for various reasons, birds may be unable to ‘track’ food resources. We tested the hypothesis that food availability influences the distribution of migratory canopy‐foraging insectivorous warblers wintering in Jamaica.Over a wide spatial scale (24 sites on the island), warbler abundance varied significantly among sites and habitats and was significantly dependent on measures of arthropod biomass. Alternative factors (vegetation characteristics, resident bird competitor abundance, predator abundance) were not correlated with warbler abundance.Over a short temporal scale (about 2 weeks) at a single site, warbler abundance increased as predicted quantitatively after a natural, rapid increase in arthropod biomass.Over a longer temporal scale (the duration of a winter), changes in density and persistence of individually marked American redstarts (Setophaga ruticilla, L.) on six sites corresponded with concurrent fluctuations in arthropod biomass.These results document a strong association between arthropod biomass and warbler abundance in time and space, suggesting that warblers wintering in Jamaica distribute themselves in response to food resources.We hypothesize that dominance hierarchies and variable movement strategies operate in concert with birds’ responses to food to influence the distribution of wintering warblers at different spatial scales. Whether food availability determines habitat quality remains to be investigated.
Phenotypic quality and senescence affect different components of reproductive output in roe deer
Authors: Hewison A. J. M. , Gaillard J. M. 
Published: March 2003

SummaryFemales should adjust their reproductive effort prior to substantial investment. For roe deer, due to delayed implantation, this adjustment may occur at ovulation/fertilization during the summer rut, or at implantation in mid‐winter. We investigated the effects of climate and maternal phenotype (mass, condition, age) on potential litter size (number of fertilized ovulations) and implantation failure (number of fertilized ovulations less number of fetuses) of 818 individual roe does from nine populations across Britain.Potential litter size varied from 1 to 4, with 0–100% of does per population polyovulating. Among prime‐aged does, 16.7–54.5% subsequently failed to implant at least one blastocyst.Individual fecundity was determined by the combined effects of maternal phenotypic quality and age, but acting at different stages of the reproductive process: potential litter size increased with increasing maternal body mass; implantation failure was independent of phenotypic quality, but varied in relation to maternal age, being lowest for prime‐aged does, somewhat higher for yearlings, but higher still for senescent females.Implantation failure increased with increasing initial potential litter size, perhaps related to physiological malfunction. Implantation was often an all‐or‐nothing process, with females either implanting all or failing to implant any of their fertilized blastocysts.Climatic factors were not consistently correlated with individual female fecundity within populations, but between populations implantation failure increased with climatic severity.For species such as roe deer, where females rely on food intake rather than fat reserves for reproduction, we suggest that a two‐step process shapes patterns of reproductive output: body mass first sets an upper limit to potential litter size at conception, then reproductive output is limited mainly by senescence and climatic severity through implantation failure.
Evidence for individualistic species assembly creating convergent predator:prey ratios among pond invertebrate communities
Authors: Jeffries Michael J. 
Published: March 2003

summary

Predator : prey ratios are cited as examples of a community level pattern, which suggests underlying assembly rules. Consistent ratios may result from either holistic community interactions or individualistic species assembly. This study tested for evidence of holistic or individualistic explanations for the predator : prey ratios among invertebrate communities of temporary ponds.
Macroinvertebrate species were recorded from 30 adjacent experimental ponds, in January and early summer over 4 years. After the first 2 years either additional predatory or prey taxa were added to treatment ponds to skew the natural predator : prey ratios. Species richness and ratios were monitored for the following 2 years comparing treatment ponds subject to augmented predator or prey richness against unmanipulated control ponds.
The majority of species added to treatments established in their respective ponds initially creating unusually high or low predator : prey ratios. In the 2 years following manipulation the ratios in treatment and control ponds converged. The convergence resulted from the spread of the additional species across all the ponds rather than acquisition or extinction of species within treatment ponds compensating for the skewed ratios.
Convergent predator : prey ratios resulted from the spread of the augmented local species pool across the site rather than holistic community level adjustment within separate ponds. The results support individualistic models of community assembly as the explanation for convergent predator : prey ratios in pond habitats.
Dynamics of common wasps in New Zealand beech forests: a model with density dependence and weather
Authors: Barlow N. D. , Beggs J. R. , Barron M. C. 
Published: March 2003

Summary

Nest densities of the common wasp ( Vespula vulgaris
(L.)) were monitored at six sites for 13 years in the honeydew‐rich southern beech (
Nothofagus spp.) forests of New Zealand's South Island.
Densities of wasp nests were among the highest in the world, up to 30 ha
−1 , and at any one site varied approximately twofold over the study period.
Strong but not overcompensating density dependence was identified, together with a negative effect of spring rainfall.
The density dependence appeared to act from the number of autumn queens produced during one year to the number of mature (autumn) nests the next year, rather than through variations in queen production per nest. The same stage was also responsible for most of the variability in autumn nest numbers from year to year, with queen production per nest contributing less.
Yearly changes in nest density could be summarized by a simple Ricker model including both density dependence and weather, thereby incorporating both previous hypotheses for the determination of wasp abundance.
Body‐size determinants of niche overlap and intraguild predation within a complex food web
Authors: Woodward Guy , Hildrew Alan G. 
Published: March 2003

SummaryBody‐size may be an important feature of the structure of food webs. Detailed food web data are however scarce, particularly those including ontogenetic dietary shifts within species. We examined the predator guild in a well characterized food web, that of Broadstone Stream (UK), to assess the importance of body‐size within and among species in relation to intraguild predation and niche overlap.In agreement with recent food web theory, mutual predation and cannibalism were frequent and occurred in many pairwise permutations. This intraguild predation was strongly asymmetric, being determined by relative body‐size within and among species, and seasonal ‘ontogenetic reversals’ in trophic status arose when generations overlapped.Predator size determined dietary overlap, with ontogenetic shifts often outweighing taxonomic differences. Small predators had the narrowest diets, regardless of species, and were limited to feeding on a restricted subset of the total prey size‐spectrum. Niche overlap decreased as pairwise differences in body‐size increased among and within species. Overlap in diet also tracked seasonal changes in resource availability, being highest in summer, when prey were abundant and small, and declining progressively over time, as prey became scarcer and/or larger. The small predators also became more detritivorous as prey abundance declined and the larger prey species attained size‐refugia.The body‐size constraints driving feeding relationships within the predator guild, in terms of both resource partitioning and intraguild predation, lend support to recent niche models of food web structure (Warren 1996;Williams & Martinez 2000). The highly interconnected food web of Broadstone Stream appeared to be structured by relatively simple rules, with seasonal and ontogenetic shifts in the size‐spectrum accounting for most of the changes in predator diet and trophic position. Encounter rate in time (prey and predator mobility) and space (microhabitat use) and foraging mode also influenced prey vulnerability and niche overlap, but were secondary to the effect of body‐size.