bluuebunny/crossref_metadata_2025_split huggingface dataset:
Survival‐variation within and between functional categories of the African multimammate rat
Authors: Julliard Romain , Leirs Herwig , Stenseth Nils. Chr. , Yoccoz Nigel. G. , Prévot‐Julliard Anne‐Caroline , Verhagen Ron , Verheyen Walter 
Published: March 2003

1.By identifying ecological factors specific to functional categories of individuals, it may be possible to understand the mechanisms underlying life‐history evolution and population dynamics. While empirical analyses within the field of population biology have focused on changes in population size, theoretical models assuming differential sensitivities of population growth rate or fitness to demographic parameters have mostly been untested, particularly against data on small mammals.2.Statistical modelling of capture–mark–recapture data on the multimammate rat (Mastomys natalensis) from Tanzania shows that: (i) females survive slightly better than males and subadults survive much better than adults; (ii) temporal variation of survival of all individuals is similarly related to the rainfall of the month; (iii) subadults exhibit a strongly density‐dependent low persistence rate in the population immediately after their first capture; (iv) subadults survival in later months is, however, positively related to density; and (v) adult survival shows negative density‐dependence.3.Both density‐dependent and density‐independent factors simultaneously determine stage‐dependent survival variation of the multimammate rat. Whereas environmental factors in this population seem to affect survival rates of all individuals in a similar manner, density‐dependent relationships are more complex.4.The patterns of survival variation in small mammals may be different from those observed in large mammals.5.Further studies of demography in small mammals should aim at understanding how much of the variability in population growth rate is accounted for by the variability of the demographic rates resulting from limiting (density‐independent) and regulating (density‐dependent) factors, respectively. This study emphasizes the use of robust and accurate statistical methods as well as stage‐ or age‐structured population modelling.
Polyandry and competition for territories in bronze‐winged jacanas
Authors: Butchart Stuart H. M. , Seddon Nathalie , Ekstrom Jonathan M. M. 
Published: March 2003

Summary1. We tested the hypothesis that in territorial, sex‐role reversed birds the distribution of resources may determine the dispersion of males, which may, in turn, determine the dispersion of females (i.e. the converse of the classical view of breeding systems in most species).2. Bronze‐winged jacana (Metopidius indicus) territories were exclusive within the sexes, and female territories encompassed 1–4 male territories. The breeding density was limited by both habitat availability (floaters settled when new habitat flooded) and competition for territories (floaters filled natural and experimental vacancies).3. Territory vacancies created by removal experiments were filled quickly either by floaters, neighbours, or male co‐mates that expanded their territories. Changes in male territories had a significantly greater impact on female territories than vice versa, in terms of size and overlap before and after the experiment. Female dispersion was therefore influenced by the dispersion of males. However, males also played an active part in mate changes through occasional territory adjustments, and sexual interactions with females.4. There was no evidence for a relationship between dispersion of resources and male territory size, but heavier males defended larger territories. Both neighbouring males and co‐mates constrained the size of male territories. There was no evidence that polyandry arose through males settling on and subdividing female territories, but rather males competed for space, and female territories were superimposed on the mosaic of male territories.5. Female territory size was positively correlated with harem size, but seemed to be unrelated to habitat quality. Females may have attempted to maximize territory size to encompass as many males as possible. Males in larger harems defended smaller territories. The degree of polyandry therefore depended on both male and female territory sizes.
Demographic estimates from radio‐tagging: models of age‐specific survival and breeding in the goshawk
Authors: Kenward R. E. , Marcström V. , Karlbom M. 
Published: March 2003

Summary 1. Age‐specific survival and breeding (ASSAB) models were developed with data from 318 goshawks (Accipiter gentilis L.) fitted during 1980–7 on the Baltic island of Gotland with tail‐mounted radio‐tags. 2. Comparisons with recaptures and recoveries of 238 ringed juveniles without radio‐tags detected no marking bias among hawks radio‐tagged at nests, nor survival bias from tag impacts or signal‐loss effects, but disproportionate adult deaths within a month of trapping. 3. Among 63 dead hawks found by radio‐tagging, 35% were killed by humans and 65% died from natural causes, mainly starvation. Ring recoveries over‐represented killing by humans. 4. Radio‐tagging estimated first‐year male survival rates of 0·49–0·54 to the next spring, lower than estimates of 0·69–0·71 for females; combined survival was higher than in contemporary ringing records. Excluding first‐month data from trapped adults, second‐year survival was 0·59 for males, 0·71 for females, and 0·79–0·83 for both sexes thereafter. 5. No hawks were observed to build nests or lay eggs in their first year. Only 8% of radio‐tagged females laid eggs in their second year, but 47% of older females. Second‐year males were as often as older males in egg‐laying pairs (70%), but were less likely to rear young. 6. Mark‐recapture of 351 goshawks ringed in nests estimated that 262 fledged on Gotland annually, from 151 nests with eggs. Emigration was 3% (SE ± 2%), with 1 ringed immigrant. 7. An ASSAB model with staggered‐entry survival estimates predicted no population change when based on males, with breeding success of 75% from laying to fledging, and a brood size of 2·27; independently recorded values for 165 nests were 75% (± 3%) and 2·24 (± 0·08). 8. The model required a 0·5% increase in female survival rate for equal numbers of hawks to pair, and an increase in fledging success from 73% to 75% for a balanced population with a brood size matching the 2·27 of males. The model estimated a male : female ratio of 1·0 : 1·78 in the adult population, with 71% of males breeding annually but only 40% of females.
Direct and indirect competitive effects of foliage feeding guilds on the performance of the birch leaf‐miner Eriocrania
Authors: Fisher A. E. I. , Hartley S. E. , Young M. 
Published: March 2003

Summary 1. Competitive interactions between Eriocrania spp. Zeller (Lepidoptera: Eriocraniidae) leaf‐miners and other folivorous guilds (free‐living Lepidoptera and phloem feeding aphids) of silver birch (Betula pendula Roth) were examined experimentally. 2. Measurements of the foliage quality were taken from the experimental trees in order to discover whether variation in nitrogen and secondary compounds (specifically total phenolics), had an effect on the strength of any competitive interactions between the guilds. 3. There was evidence for indirect interactions within the leaf‐mining guild mediated by the host plant: miners in aggregations had double the mortality of single miners. Interactions with leaf‐chewing Lepidoptera were limited to interference and resource removal since only damage to the leaf being mined had an effect on the miner performance. There were no interactions between Eriocrania and aphids, despite the presence of aphids reducing the quantity of foliage available by 33%. 4. The strength of the competitive interactions, either within or between guilds, was not modified by the quality of the host trees. However, biomass of emerging larvae was negatively related to increased phenolic content of host trees and positively related to increased nitrogen content. Furthermore, larvae emerged later from trees where foliage had higher phenolic content. 5. The longevity of feeding events by competitive folivore guilds and the proximity of the feeding to the miners were identified as the key factors influencing the effects of foliage damage on the performance of Eriocrania.
On the misuse of residuals in ecology: testing regression residuals vs. the analysis of covariance
Authors: García‐Berthou Emili 
Published: March 2003

Summary

An analysis of variance (anova) or other linear models of the residuals of a simple linear regression is being increasingly used in ecology to compare two or more groups. Such a procedure (hereafter, ‘residual index’) was used in 8% and 2% of the papers published during 1999 in the Journal of Animal Ecology and in Ecology, respectively, and has been recently recommended for studying condition.
Although the residual index is similar to an analysis of covariance (ancova), it is not identical and is incorrect for at least four reasons:
the regression coefficient used by the residual index differs from the one used in ancova and is not the least‐squares estimator of the model.
in contrast to the ancova, the error d.f. in the residual index are overestimated because of the estimation of the regression coefficient.
the residual index also assumes the homogeneity of regression coefficients (parallelism assumption), which should be tested with a special ancova design.
even if the assumptions of the linear model hold for the original variables, they will not hold for the residuals.
More importantly, the residual index is an ad hoc sequential procedure with no statistical justification, unlike the well‐known ancova. For these reasons, I suggest that a t‐test or an anova of the residuals should never be used in place of an ancova to study condition or any other variable.
Statistical modelling of the population dynamics of a raptor community in a semi‐desert environment
Authors: Krüger Oliver , Liversidge Richard , Lindström Jan 
Published: March 2003

SummaryWe performed an extensive statistical modelling study on the population fluctuations and population growth rates of 15 raptor species in the Kalahari desert in South Africa.The correlation pattern between rainfall and population abundance changed systematically with raptor body weight and diet type. The abundance of heavier raptors feeding on larger prey‐items had lower correlations with rainfall than lighter raptors feeding on small prey‐items. Whereas raptor species feeding on small prey‐items were more affected by immediate rainfall, species feeding on large prey‐items were more affected by rainfall in the previous year.Population abundances were explained most parsimoniously by direct and delayed density dependence and rainfall during the current and previous breeding season. Interspecific competition was never a predictor variable. Population abundances of species best described by rainfall fed on larger prey‐items than population abundances of species best described by density dependence.Population growth rates were always best described by direct density dependence. The strength of density dependence was positively correlated with reproduction rate, due mainly to Falconiform species having higher reproduction rates than Accipitrid species.Shifting from the species to the guild level, we found that abundance and biomass shares of feeding guilds did not vary significantly over time, supporting the hypothesis of guild constancy.
Resource availability and goshawk offspring sex ratio variation: a large‐scale ecological phenomenon
Authors: Byholm Patrik , Ranta Esa , Kaitala Veijo , Lindén Harto , Saurola Pertti , Wikman Marcus 
Published: March 2003

Summary

Local population studies have shown that sex allocation among many birds and mammals seems to be partly non‐random and in connection to surrounding factors, such as environmental or parental quality. In this scenario, if environmental quality varies in space and time, it is feasible that environmental quality also comes to influence offspring sex ratio on larger geographical scales.
Investigating this idea – using nation‐wide data sets on size‐dimorphic Finnish northern goshawks Accipiter gentilis from 1989 to 1998 – we found that offspring sex ratio is related to spatial and temporal variation in availability of their main prey, woodland grouse species.
In a majority of locations (n = 73), broods are large and male‐biased when grouse density is high, while small and female‐biased broods are found in areas and times with low grouse density. When adjusted for grouse density, the correlation between brood size and sex ratio disappears, but the correlation between grouse density and sex ratio remains.
We also found evidence indicating a mechanism balancing local offspring sex ratios towards the previously rarer sex with a time lag of 1 year. That is, local sex ratios in subsequent years are negatively correlated.
Our study highlights the importance of inclusion of both time and space in sex ratio studies. Ignorance of these dimensions could explain a great deal of the inconsistency in many earlier local sex ratio studies.
Irrespective of whether the observed pattern is the result of facultative manipulation, sex biased mortality, or both, it is suggested that the male‐biased sex ratio during periods of high grouse availability maximizes goshawk reproductive success.
Seasonal patterns of growth, expenditure and assimilation in juvenile Atlantic salmon
Authors: Jones Wayne , Gurney William S. C. , Speirs Douglas C. , Bacon Philip J. , Youngson Alan F. 
Published: March 2003

Summary

We report a modelling study of a data‐set describing the growth of individual Atlantic salmon (Salmo salar L.) parr in the Girnock Burn (Scotland). A development of the compensatory growth model due to Broekhusien et al. (1994) was fitted to these data by numerical optimization.
The model uses carbon mass as a surrogate for an energy currency. This mass is divided into structure and reserve components, so as to describe decoupled changes in length and wet‐weight.
Using the same parameters for all fish, our model explained 83% of the variability in length and weight at age. Adding a single additional parameter for each individual enabled the model to explain over 96% of length and weight variability.
Weak negative correlation between size at first capture and within‐study growth argues against genetic causality of observed growth variability.
The energetic basis of our model enables us to infer time‐series of net assimilation and basal maintenance rates for the observed individuals. Maximal growth occurs early in the season when high assimilation is accompanied by low temperatures and maintenance rates. In late season, continuing high assimilation is balanced by high maintenance rates consequent on summer temperatures.
Winter management of Californian rice fields for waterbirds
Authors: Elphick Chris S. , Oring Lewis W. 
Published: March 2003

1.  Recent legislation designed to reduce air pollution has restricted Californian rice‐farmers from burning rice stubble after harvest. Intentional flooding of fields during winter to speed straw decomposition is becoming increasingly common as growers seek alternatives to burning residual straw. The potential for flooded fields to act as a surrogate for destroyed wetland habitat may be an additional benefit in a region that hosts a large proportion of North America’s wintering waterbirds. We investigated the degree to which waterbirds use flooded fields and whether the method of flooding affects their use. Specifically, we tested whether waterbird use (a) was greater in intentionally flooded fields than in unflooded fields, (b) differed among flooded fields receiving different straw manipulations and (c) varied with water depth. 2.  Intentionally flooded rice fields received significantly greater use by 24 of 31 species studied. Only great blue herons Ardea herodias and sandhill cranes Grus canadensis were significantly more common in unflooded fields. Geese densities did not differ between flooded and unflooded fields. 3.  We found no differences in the densities of most species in flooded fields that received different straw manipulations to improve decomposition rates. Exceptions included several small shorebirds which occurred at highest densities in fields where straw was incorporated into the soil. 4.  Species differed in their use of different water depths. For 14 species we tested whether preferred depths, suggested in the literature, received disproportionately higher use. Most of these species were more likely to be encountered within the suggested depth ranges. Depth, however, was a poor predictor of bird density. Depths of 15–20 cm resulted in frequent use by the greatest number of species. 5.  We conclude that flooding rice fields increased suitable habitat for most, but not all, species studied. Different straw manipulation methods had little effect on most species. Water depth, however, was important in determining species occurrence. During the first half of the winter, water depths were greater than the median depths used by most species.
Modelling Chlamydia–koala interactions: coexistence, population dynamics and conservation implications
Authors: Augustine David J. 
Published: March 2003

1.  Considerable research has been conducted on koala Phascolarctos cinereus population dynamics and the epidemiology of Chlamydia psittaci infection in koalas, but the impact of Chlamydia on koala populations has been difficult to assess. 2.  I developed a model of koala and Chlamydia population dynamics to examine interactions between Chlamydia transmission and pathogenicity, koala mating behaviour and demography, and koala population persistence. 3.  Simulations based on sexual and parent–offspring parasite transmission demonstrate that stable Chlamydia–koala coexistence is possible in a small population for a broad range of demographic, behavioural, pathogenicity and transmission parameter estimations. Koala population persistence was most sensitive to reduced annual survivorship of adults (4–10‐year‐old males and 2–12‐year‐old females), highlighting the need for accurate field estimates of adult survivorship in order to assess Chlamydia’s impact on specific populations. 4.  If koalas become less resistant to disease in fragmented, high‐stress habitats (i.e. experience increased Chlamydia‐induced mortality and sterility rates), Chlamydia is not predicted to cause koala extinctions under most conditions. Extinctions are only predicted if Chlamydia transmission rates also increase (e.g. due to new transmission pathways or increased mating frequency), or other non‐disease factors change birth and mortality rates to reduce the koala population’s intrinsic rate of increase below 0·1. 5.  The most important predicted effect of habitat fragmentation and other forms of human disturbance on this unique host–parasite relationship is the extinction of Chlamydia in populations where koala resistance to disease decreases.
Soil seed bank of an upland calcareous grassland after 6 years of climate and mangement manipulations
Authors: AKINOLA M. OLATUNDE , THOMPSON KEN , BUCKLAND SARAH M. 
Published: March 2003

Summary

The soil seed bank of limestone grassland plots was investigated after 6 years of manipulation of temperature, rainfall, fertility and disturbance. An inoculum of seeds of species alien to the site was also added to the experiment.
The seed bank was largely made up of species that were native to the site and had small, persistent seeds. Large‐seeded species, whether native or introduced, were rare or absent in the seed bank. Small‐seeded species were generally more deeply buried than large‐seeded species. Total seed density was comparable with that of other limestone grasslands.
Significant climate effects were few, although the seed bank of the winter annual Arabidopsis thaliana was increased by summer drought and that of Cerastium fontanum was increased by winter heating.
The density of seeds of native species declined in response to disturbance and increased fertility treatments, while that of introduced species increased. Changes in the seed bank lagged behind changes in the vegetation, with introduced species making up a higher proportion of the vegetation than of the seed bank after 6 years of manipulation.
Root vole movement patterns: do ditches function as habitat corridors?
Authors: Mauritzen Mette , Bergers Piet. J. M. , Andreassen Harry P. , Bussink Harry , Barendse Rutger 
Published: March 2003

1. Ditches are often connected to root vole habitat patches (i.e. moist reed patches) in the Netherlands. Due to the linear structure of ditches and because ditch habitat is qualitatively similar to root vole habitat patches, we hypothesized that ditches could function as habitat corridors facilitating dispersal movement of root voles. In order to test this hypothesis, we radiotracked root voles released in a landscape novel to them, consisting of ditches and agricultural meadows. 2. Agricultural meadows often surround the marsh patches inhabited by root voles. As the meadows are mowed regularly, we included the length of the meadow vegetation as an experimental factor in the study. 3. Assuming that ditches function as habitat corridors, we expected root voles in the ditches to move faster and more unidirectionally than root voles in the meadows, and to prefer the ditches to meadows. 4. We found that the ditches did not facilitate faster movements than the meadows. Although the root voles moved back and forth within the ditches, they showed a more directional movement pattern than the root voles in the meadows. Furthermore, the root voles preferred the ditch habitat irrespective of the vegetative cover in the meadow. 5. We conclude that ditches could function as habitat corridors for root voles, as they preferred to move in ditches when in unfamiliar areas.