bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mechanisms determining the spatial distribution of microtine predators on the Arctic tundra
Authors: Wiklund Christer G. , Kjellén Nils , Isakson Erik 
Published: March 2003

1.  We studied the spatial distribution of avian microtine predators using data from 19 study areas on the tundra of northern Siberia. 2.  Numbers of snowy owls, and long‐tailed skuas and pomarine skuas depended strongly on lemming density. However, a significant relationship between lemming density and number of rough‐legged buzzards appeared first after removal of the effect of snowy owl abundance on the distribution of rough‐legged buzzards. 3.  We applied a recently developed method (Manly 1995) to examine co‐occurrences of species and found that rough‐legged buzzards and snowy owls did not co‐occur while snowy owls, long‐tailed skuas and pomarine skuas did. 4.  There are large differences in nest construction and chick behaviour between rough‐legged buzzards and the three other species. Moreover, the snow owl is a polyphagous predator preying also on large birds including raptor chicks. Therefore, we propose that reduced risk of nest predation favours rough‐legged buzzards nesting away from snowy owls. 5.  Variations in abundance of the two lemming species did not seem to influence the distributions of snowy owls and rough‐legged buzzards. Neither was it likely that latitudinally related factors such as breeding season length affected the distribution of rough‐legged buzzards.
Measuring the degree of sexual segregation in group‐living animals
Authors: Conradt Larissa 
Published: March 2003

1.  So far, no measure exists to quantify sexual segregation in animal populations. Studies on segregation have relied on ecological measures of overlap and association to estimate the degree of segregation. 2.  However, existing ecological measures of overlap and association are stochastically related to sex ratio, population density or group size. These stochastic relations can lead to confounding results, making the existing measures unsuitable for quantitative studies on segregation. 3.  In the present paper a new measure of segregation, the ‘segregation coefficient’, is suggested, which is free of stochastic relations. 4.  The segregation coefficient is suitable for quantitative studies on segregation. It also makes possible, for the first time, comparisons between the degree of social segregation and the degree of habitat or spatial segregation in a population of animals.
A dynamic refuge model and population regulation by insect parasitoids
Authors: Lynch Liam D. , Bowers Roger G. , Begon Michael , Thompson David J. 
Published: March 2003

1.  The population dynamic effects of refuges, which hosts enter and leave by diffusive movement, in host–parasitoid interactions are explored using simple models in continuous time. 2.  This type of refuge has a stabilizing effect on a host–parasitoid interaction, which is contrary to the implications of some previous models. 3.  Stability can be explained by considering how depletion processes lead to a refuge proportion (proportion of hosts protected at a given instant) that increases as parasitoid density increases. This effect is synonymous with pseudointerference in the context of the model. 4.  Very high rates of movement of host larvae largely destroy this stability process. Stability is greatest at intermediate levels of movement. 5.  Density‐dependent host movement can alter the effect of these refuges such that they are either more stabilizing, or tend to destabilize, the dynamics of host–parasitoid systems, depending on the type of density dependence assumed. The conclusion that intermediate movement rates are likely to generate stability with this general type of refuge is not altered in the presence of any type of density dependence, unless the density dependence is at levels which we consider unrealistically high and unlikely to be encountered in nature. 6.  It is the assumption that larvae do not move into the refuge prior to becoming vulnerable to parasitism that ensures top‐down population control in the model. Thus, parasitoids attacking very early instars make good candidates for biological control when faced with a structural refuge.
Hatching asynchrony in Eurasian kestrels in relation to the abundance and predictability of cyclic prey
Authors: WIEBE KAREN L. , KORPIMÄKI ERKKI , WIEHN JÜRGEN 
Published: March 2003

Summary

Many hypotheses for hatching asynchrony within avian clutches centre around food supply during the laying or nestling periods, but food is measured in few studies.
Patterns of clutch size and hatching asynchrony were analysed in the Eurasian kestrel [Falco tinnunculus (Linnaeus)] breeding in Finland, where small mammal populations show dramatic 3‐year cycles.
Data collected over 12 years showed that clutches were larger when food was more abundant in spring.
While controlling for clutch size, hatching was more synchronous in the low phase of the vole cycle compared to the increase phase, contrary to conventional interpretation of the brood reduction hypothesis. The age of the female, but not the age of the male was associated with the degree of asynchrony.
Paradoxically, numbers of voles were most stable in the ‘low’ phase, which may lead to a unique situation, where food is more predictable when scarce. Kestrels have small, synchronous clutches in the ‘low’ phase and large asynchronous clutches in the ‘increase’ phase.
Based on results of this study, it is suggested that it is important to consider the predictability of food, and not only the mean abundance of food when evaluating hypotheses for hatching asynchrony. Patterns of clutch size and hatching asynchrony at the population level can be used to evaluate current energy‐related hypotheses for asynchrony.
Pre‐breeding survival of barnacle geese Branta leucopsis in relation to fledgling characteristics
Authors: JEUGD HENK P. VAN DER , LARSSON KJELL 
Published: March 2003

Summary

Pre‐breeding survival, i.e. survival from fledging up to the third winter, of barnacle geese (Branta leucopsis) was analysed by using more than 45000 resightings of 1302 individually marked birds. Since observations from the wintering areas only were used, the survival estimates obtained were not confounded by natal dispersal.
Post‐fledging survival, i.e. survival from fledging to the start of the first winter, differed significantly among the 10 cohorts analysed. These differences were related to the mean body weight and mean tarsus length of cohorts just before fledging. In further analyses, when data from all cohorts were combined, body weight and tarsus length of individuals just before fledging were found to be positively related to Post‐fledging survival.
Post‐fledging survival rates were, on average, lower than survival rates after the first winter. This age effect was largest in cohorts with low mean body weight at capture and was absent in cohorts with high mean body weight at capture.
The age effect on survival could only partly be explained by individuals with low body weight having lower survival and successively disappearing from the cohort. It was therefore concluded that other factors must have contributed to the age effect as well.
It is hypothesized that lightweight birds are more vulnerable to, for example, diseases, parasites and predation during the first months of their life. Once they have survived this critical period, the effects of characters related to low body weight at capture seem to disappear.
Body size of juveniles just before fledging has previously been found to be greatly affected by weather factors and the availability of high quality food during the growth period. Hence, a considerable part of the observed variation in Pre‐breeding survival appears to be outside the individual's control.
Co‐existence between Iberian lynx and Egyptian mongooses: estimating interaction strength by structural equation modelling and testing by an observational study
Authors: PALOMARES F. , FERRERAS P. , TRAVAINI A. , DELIBES M. 
Published: March 2003

Summary

We used path analysis to investigate the causal relationships between Iberian lynx and Egyptian mongoose track numbers, and to estimate the direct effect of the former on the latter in the Doñana area (2750 km2), south‐western Spain. Relative abundance of rabbits, shrub cover and protection level were also considered in the path analysis.
An observational study consisting of a repeated track survey with a split‐plot design was also carried out between 1990 and 1996 to independently test the results obtained by path analysis.
Maximum likelihood ratio tests of the goodness‐of‐fit of the model to the data and other indices used all indicated that the proposed theoretical model depicting the relationships among variables was adequate. Egyptian mongooses suffered a significant direct negative effect from lynx, and rabbits and protection level positively, and significantly affected lynx track abundance; rabbits were also significantly affected by shrub cover (all these standardized partial correlation coefficients ranged from 0.34 to 0.41). Shrub cover did not directly affect either lynx or mongoose tracks.
Relatively important indirect effects were found between shrubs and lynx (0.12), and between rabbits and mongooses (−0.14), and protection and mongooses (−0.17). The two latter non‐intuitive indirect effects were through the lynx path, as a consequence of the negative direct influence of lynx on mongooses.
The repeated track censuses confirmed that lynx presence significantly affects number of mongoose tracks. In areas without lynx, mean number of mongoose tracks were 3.6 times higher than in areas with lynx.
It remains an interesting question which of several proposed hypotheses explains co‐existence of both species in south‐western Spain.
Allometry of territory size and metabolic rate as predictors of self‐thinning in young‐of‐the‐year Atlantic salmon
Authors: Steingrímsson Stefán Ó. , Grant James W. A. 
Published: March 2003

1. Self‐thinning is a progressive decline in population density caused by competitively induced losses in a cohort of growing individuals and can be depicted as: log10 (density) = c − β log10 (body mass). 2. In mobile animals, two mechanisms for self‐thinning have been proposed: (i) the space hypothesis predicts that maximum population density for a given body size is the inverse of territory size, and hence, the self‐thinning slope is the negative of the slope of the allometric territory‐size relationship; (ii) the energetic equivalence hypothesis predicts that the self‐thinning slope is the negative of the slope of the allometric metabolic rate relationship, assuming a constant supply of energy for the cohort. 3. Both hypotheses were tested by monitoring body size, population density, food availability and habitat for young‐of‐the‐year Atlantic salmon (Salmo salar) in Catamaran Brook, New Brunswick. The results were consistent with the predictions of the space hypothesis. Observed densities did not exceed the maximum densities predicted and the observed self‐thinning slope of −1·16 was not significantly different from the slope of −1·12, predicted by the allometry of territory size for the population under study. 4. The observed self‐thinning slope was significantly steeper than −0·87, predicted by the allometry of metabolic rate, perhaps because of a gradual decline in food abundance over the study period. The decline in density was more rapid in very shallow sites and may have been partly caused by a seasonal change in water depth and an ontogenetic habitat shift rather than solely by competition for food or space. 5. The allometry of territory size may be a useful predictor of self‐thinning in populations of mobile animals competing for food and space.
Egg viability as a constraint on hatching synchrony at high ambient temperatures
Authors: Stoleson Scott H. , Beissinger Steven R. 
Published: March 2003

Summary 1. We tested experimentally the effects of exposure to high ambient temperatures, for periods of 1–9 days, on the viability of eggs of the green‐rumped parrotlet. 2. The hatchability of 534 newly laid parrotlet eggs declined after exposure of 3 or more days. 3. The probability of hatching was not significantly affected by the duration or proportion of time exposed to temperatures below physiological zero or above normal incubation range. 4. Incubation periods were negatively correlated with exposure time, suggesting some embryo development occurred prior to incubation. 5. Results presented here corroborate similar studies of temperate zone birds, and suggest that the decline in egg viability may be particularly extreme in hot climates, such as tropical lowlands. 6. We suggest that the relationship between ambient temperature and egg viability could contribute to both seasonal and latitudinal trends in clutch size, hatching success and hatching asynchrony.
Sex difference in criteria determining fidelity towards breeding sites in the great cormorant
Authors: Schjørring Solveig , Gregersen Jens , Bregnballe Thomas 
Published: March 2003

Summary 1. Many animals choose to breed in sites where they have previously been successful. Such fidelity could arise from the predictability of high quality breeding sites in a temporally stable environment. The quality of a site may be indicated by factors other than an individual’s own success, because it may fail as a result of a random event that is unrelated to the intrinsic quality of the site. In particular, prior experience (familiarity) with the breeding area and the performance of neighbours could give complementary information about the quality of the site. 2. We present results from a long‐term study of colonial great cormorants (Phalacrocorax carbo sinensis), where movement and reproductive success of individually marked birds within a colony was known. 3. Individuals were more likely to return to the same breeding site if they had been successful the previous season. 4. Fidelity of both males and females increased with increasing level of familiarity with the breeding area. Males were more likely to breed again in the same area, and their fidelity was more dependent on familiarity with the area than female fidelity. 5. The success of breeding sites within the colony was spatially autocorrelated, and cueing on neighbour performance should thus be advantageous. Female fidelity increased with increasing success of neighbouring birds, while male fidelity was unaffected by neighbour success. 6. We suggest that the difference between the sexes in the criteria determining fidelity arises because it is mainly the males that are involved in territorial disputes. They may therefore benefit more than females from knowing their neighbours, and this could override the importance of the intrinsic quality of the breeding area (i.e. reproductive success of neighbours). 7. The conflict over preferred breeding sites that arises within breeding pairs because of this sex difference may be an explanation for the high rate of mate change between years (92.5%) observed in this species.
Beetles in fragmented woodlands: a formal framework for dynamics of movement in ecological landscapes
Authors: Murrell David J. , Law Richard 
Published: March 2003

Summary 1. We consider the movement of carabid beetles in a fragmented woodland landscape. Landscape ecology has lacked a formal framework for the analysis of dynamics of this kind, and stochastic spatial simulations have generally been needed to gain insight. 2. We first define an individual‐based stochastic model for movement in a heterogeneous landscape. From this a deterministic approximation is derived which describes the build‐up in association of the beetles with their environment; the deterministic model makes use of a second‐order spatial moment. It is shown that the results of the deterministic approximation match closely those of the underlying stochastic process. On this basis we suggest that the dynamical system provides a formal framework for the dynamics of animal movements in ecological landscapes. 3. The results show that the beetles, starting from an initial random layout, rapidly become associated with the woodlands and, by the time 100 days have elapsed, the association is close to its asymptotic state. The strength of the association depends very much on the spatial auto‐covariance function of the environment. 4. A number of extensions are suggested, including landscapes comprising multiple habitats of different degrees of suitability, landscapes that change through human activity, birth and death processes of single species, and interactions between species. The method of moments provides a flexible basis for future work in this area.
Cycles and synchrony: two historical ‘experiments’ and one experience
Authors: Bjørnstad Ottar N. 
Published: March 2003

Summary 1. Theoretical models predict that spatial synchrony should be enhanced in cyclic populations due to nonlinear phase‐locking. 2. This is supported by Rohani et al.′s (1999) comparison of spatial synchrony of epidemics in two childhood diseases prior to and during the vaccination era. Measles is both more synchronous and more cyclic before vaccination. Whooping cough, in contrast, is more synchronous during the vaccination era, during which multiannual fluctuations are also more conspicuous. 3. Steen et al. (1990) analysed historic records of cyclic rodents, to show that cyclicity was lost during the early part of the 20th century. I reanalyse the data, and find that the loss of cyclicity is associated with loss of regional synchrony. 4. I use a coupled map lattice model to show that imperfect phase‐locking provides an alternative explanation for regionwide synchrony of cyclic populations.
Mate fidelity, senescence in breeding performance and reproductive trade‐offs in the barn swallow
Authors: Saino Nicola , Ambrosini Roberto , Martinelli Roberta , Møller Anders Pape 
Published: March 2003

SummarySenescence is defined as the decline in performance with age, and can be determined by diverse mechanisms such as accumulation of mutations, ‘wear and tear’ and antagonistic pleiotropy. Animals can mitigate the negative effects of senescence on reproductive performance by acquiring experience as they age.We analysed the effect of age on clutch size, breeding date, fledging success and offspring quality (morphology and T cell‐mediated immunity) of barn swallows (Hirundo rustica, Linnaeus) by comparing the progeny of individuals of known age during consecutive years.Reproductive performance had no significant effect on probability of divorce, and adults that re‐mated with each other had no reproductive advantage.Body size, feather development and T cell‐mediated immunity declined with paternal and/or maternal age already after the first breeding season. Body mass declined after paternal age of two years. T cell‐mediated immunity of nestlings in one year covaried negatively with that of their siblings the previous year.Hence, barn swallows suffer an age‐dependent decline in offspring quality, and apparently trade quality of current offspring against that of subsequent reproductive events, although causality of this relationship could not be assessed in this correlational study.
Effects of fat reserves on annual apparent survival of blackbirds Turdus merula
Authors: Miller Mark W. , Aradis Arianna , Landucci Giuseppe 
Published: March 2003

Summary

Fat reserves are stored energy that may help birds survive periods of harsh winter weather. This hypothesis predicts that annual apparent survival is higher for birds with large fat reserves than for birds with few or no fat reserves in winter.
Blackbirds (Turdus merula Linnaeus) were ringed in central Italy from 16 November to 20 February during 1990–2001. Fat scores were recorded for each bird. We used these capture–mark–recapture data for 1703 blackbirds to estimate the effect of large fat reserves on annual apparent survival, while controlling for transients, using computer programs surviv and mark. Probability of birds retaining large fat reserves, or retaining few fat reserves, over 2 successive years was also estimated.
Birds with large fat reserves did not have higher estimated annual apparent survival than birds with few fat reserves (φ̂large= φ̂few= 0·595, SE = 0·043), inconsistent with our prediction. No effects of age, sex or year were detected on annual apparent survival. Birds with few fat reserves in any given year tended to have few fat reserves the following year (, SE = 0·052). Birds with large fat reserves in any given year were unlikely to have large fat reserves the next year (, SE = 0·080).
Large fat reserves may not increase annual survival of blackbirds wintering in central Italy. Winter weather in our study area may be too mild to effect survival. Alternatively, increased predation risk associated with large fat reserves may counteract any benefits of reduced starvation risk.
The geographical range structure of the holly leaf‐miner. II. Demographic rates
Authors: Brewer Andrew M. , Gaston Kevin J. 
Published: March 2003

Summary

Few studies have sought to document the variation in demographic rates exhibited by local populations across the geographical range of a species. None the less, such information has the potential to yield insights into the factors that determine the internal abundance structure of ranges.
Major components of demographic rates are reported here for the holly leaf‐miner Phytomyza ilicis Curtis across its natural geographical range in Europe, including areas at each of the extreme limits and at the core of its occurrence.
Correlograms reveal that a number of these components exhibit significant spatial structure across the geographical range and, in conjunction with interpolated maps, suggest that some of these patterns are, in terms of broad trends, reasonably simple.
Across the whole range, individual mortality components are largely independent of one another and of leaf‐miner population densities, often exhibiting very different spatial patterns. Thus, P. ilicis populations experience markedly different mortality profiles across the range.
While some correlations between broad‐scale environmental variation and the components of demographic rates were found, it was not possible to separate these effects from shared spatial structure.
In sum, the contribution of different components of demographic rates to local densities varies markedly across the geographical range of P. ilicis, and in isolation the pattern observed in any one locality provides limited information on what is occurring elsewhere, but these components none the less often exhibit systematic geographical patterns of variation.