bluuebunny/crossref_metadata_2025_split huggingface dataset:
An alternative approach to the measurement of seasonal trends in bird breeding success: a case study of the bullfinch Pyrrhula pyrrhula
Authors: Newton I. 
Published: March 2003

1. On the basis of juvenile:adult ratios among netted birds and studies of moult, inferences are drawn about seasonal and annual variation in the breeding output of a bullfinch population studied near Oxford, in southern England. 2. In most years during 1962–66, the juvenile:adult ratio among netted birds increased from July or August to October, and then declined. The ratio in October, taken as a measure of annual productivity, varied between 2·8 and 5·5 in different years. 3. In the Oxford area, egg‐laying among bullfinches occurred between mid‐April and late August, so that the last young of the year left the nest in late September/early October. However, not all pairs continued breeding this late in the year. Individual bullfinches started to moult when they finished breeding. The proportion of adults which started moult after 20 August (implying successful breeding from eggs laid after mid‐July) varied between 7 and 68% in different years (or between 19 and 63% using a different method of estimation). Juveniles from these late nests, identified as those starting moult after 31 August, formed 24–61% of the total juvenile population in different years, the rest having been produced from clutches begun before mid‐July. 4. In the 2 years when the greatest proportions of adults started to moult after 20 August and the greatest proportion of juveniles started after 31 August, the juvenile:adult ratios in October were highest. By implication, the two‐fold variations in overall productivity between years were associated primarily with variations in the amount of late breeding. 5. This study, based on moults and age‐ratios among netted birds, provided information on seasonal productivity that was practically impossible to obtain in this species by conventional nest finding. An implication was that an interaction between the seasonal pattern of breeding in particular years and a seasonal trend in predation on nest contents, had major effects on overall annual productivity.
Density‐dependent growth in brown trout: effects of introducing wild and hatchery fish
Authors: Bohlin T. , Sundström L. F. , Johnsson J. I. , Höjesjö J. , Pettersson J. 
Published: March 2003

SummaryAlthough it is not clear to what extent density dependence acts on the survival, emigration or growth of organisms, experiments testing alternative explanations are rare. A field experiment on 1‐year‐old brown trout (Salmo truttaL.) was undertaken to address the following questions: are the mortality, movement and growth of wild stream‐living trout affected by population density? If so, are the density‐dependent effects of released hatchery trout different from the effects of wild fish?In each of two small streams, two replicate treatment blocks were used, each with four treatments assigned to stream sections 50–70 m in length: (1) control, no fish was introduced and population density was kept at its original level. (2) Trout biomass was doubled by introducing additional wild fish. (3) Trout biomass was doubled by introducing additional hatchery fish. (4) Hatchery trout were introduced, but biomass was kept at its original level by the removal of some resident wild fish.We found no treatment effects on the recapture rates of resident trout, which suggests that survival was not strongly affected by competition. They were also remarkably stationary, regardless of treatment. However, trout growth rate was reduced to the same extent in both treatments with increased density, suggesting that growth was negatively density‐dependent, and that the density‐dependent effects of hatchery trout and introduced wild fish were similar.Wild resident fish grew faster than introduced wild trout, which in turn grew faster than hatchery trout. Hatchery fish and introduced wild fish moved more than wild resident fish.The results show that population density affected growth in trout parr. We conclude that competition is not limited to the underyearlings, as has previously been suggested, and that density‐dependent growth is the main density‐dependent response in yearling trout. Furthermore, this effect was the same for wild and hatchery‐reared competitors, suggesting that stocking of hatchery fish may affect natural populations negatively through density dependence.
Consumer–resource interactions and cyclic population dynamics of Tanytarsus gracilentus (Diptera: Chironomidae)
Authors: Einarsson Árni , Gardarsson Arnthor , Gíslason Gísli Már , Ives Anthony R. 
Published: March 2003

Summary

Tanytarsus gracilentus population dynamics in Lake Myvatn show a tendency to cycle, with three oscillations occurring between 1977 and 1999 having periods of roughly 7 years. The population abundance fluctuated over four orders of magnitude.
A partial autocorrelation function (PACF) accounting for measurement error revealed a strong positive lag‐1 autocorrelation and a moderate negative lag‐2 partial autocorrelation. This suggests that the dynamics can be explained by a simple second‐order autoregressive process.
We tested the alternative hypotheses that the cyclic dynamics of T. gracilentus
were driven by consumer–resource interactions in which
T. gracilentus
is the consumer, or predator–prey interactions in which
T. gracilentus is the prey. We analysed autoregressive models including both consumer–resource interactions and predator–prey interactions.
Wing length of T. gracilentus
was used as a surrogate for resource abundance and/or quality, because body size is known to fluctuate with resource abundance and quality in dipterans. Furthermore, the wing lengths of
Micropsectra lindrothi
, a species ecologically similar to
T. gracilentus
, fluctuated synchronously with
T. gracilentus wing lengths, thereby indicating that the shared resources of these two species were indeed cycling. Wing lengths of other chironomid species were not synchronized.
The predators of T. gracilentus
included midges in the genera
Procladius
and
Macropelopia
, and the fish
Gasterosteus aculeatus (three‐spined stickleback).
The autoregressive models supported the hypothesis that T. gracilentus dynamics were driven by consumer–resource interactions, and rejected the hypothesis that the dynamics were driven by predator–prey interactions.
The models also revealed the consequences of consumer–resource interactions for the magnitude of fluctuations in T. gracilentus
abundance. Consumer–resource interactions amplified the exogenous variability affecting
T. gracilentus per capita population growth rates (e.g. temperature, rainfall, etc.), leading to variability in abundance more than two orders of magnitude greater than the exogenous variability.
The importance of biotic interactions in abundance–occupancy relationships
Authors: Holt Alison R. , Warren Philip H. , Gaston Kevin J. 
Published: March 2003

SummaryMetapopulation dynamics have been postulated as a possible mechanism giving rise to positive interspecific relationships between local abundance and regional occupancy. These may operate through the carrying capacity or the rescue effect hypotheses. However, both are based on single species models that sum independent occurrences of species to form an assemblage, ignoring interspecific interactions.Here we test experimentally whether interspecific interactions and dispersal influence the formation of the abundance–occupancy relationship in metapopulation systems in laboratory microcosms containing protists and bacteria.The effect of species interactions was tested by comparison of the abundance–occupancy relationship in multiple habitat patch systems containing all species together, with the relationship formed by combining data from equivalent systems containing each protist species alone.Abundance–occupancy relationships in interacting communities were better defined than those in non‐interacting communities. The inclusion of interspecific interactions was found to cause a reduction in the abundance and occupancy of the majority of species, as a consequence of which the position of species within the relationship changed drastically.
The role of males in the dynamics of ungulate populations
Authors: Mysterud Atle , Coulson Tim , Stenseth Nils Chr. 
Published: March 2003

Summary

In this review, we focus on how males can affect the population dynamics of ungulates (i) by being a component of population density (and thereby affecting interpretation of log‐linear models), and (ii) by considering the mechanisms by which males can actively affect the demographic rates of females.
We argue that the choice of measure of density is important, and that the inclusion or exclusion of males into models can influence results. For example, we demonstrate that if the dynamics of a population can be described with a first‐order auto‐regressive process in a log‐linear framework, the asymmetry between the effects of females on the male dynamics and vice versa can introduce a second order process, much in the same way that the interaction between disease and host or predator and prey can. It would be useful for researchers with sufficient data to explore the affects of using different density measures.
In general, even in harvested populations with highly skewed sex ratios, males are usually able to fertilize all females, though detailed studies document a lower proportion of younger females breeding when sex ratios are heavily female biased. It is well documented that the presence of males can induce oestrus in females, and that male age may also be a factor. In populations with both a skewed sex ratio and a young male age structure, calving is delayed and less synchronous. We identify several mechanisms that may be responsible for this.
Delayed calving may lower summer survival and autumn masses, which may lead to higher winter mortality. If females are born light, they may require another year of growth before they start reproducing. Delayed calving can reduce future fertility of the mother. As the proportion of calves predated during the first few weeks of life is often very high, calving synchrony may also be an important strategy to lower predation rates.
We argue that the effects of males on population dynamics of ungulates are likely to be non‐trivial, and that their potential effects should not be ignored. The mechanisms we discuss may be important – though much more research is required before we can demonstrate they are.
The effect of an environmental change on mobile butterfly larvae and the nutritional quality of their hosts
Authors: Hellmann Jessica J. 
Published: March 2003

Summary

Herbivores that forage on resources that change over time may be strongly affected by environmental factors that alter their temporal overlap with host plants. The magnitude of these effects may be mediated by the availability of alternative hosts and by behavioural adaptations for foraging on temporal food resources.
This study examines the temporal interaction of a butterfly species and its two host plants to determine how larvae utilize their host resources and are affected by conditions of accelerated host senescence. By changing host use over time, this butterfly may track temporal declines in host quality and buffer the impacts of environmental variation and change. At the same time, it is hypothesized that host declines and changes in the host environment affect larval survivorship and hence butterfly population size.
With three sets of field and greenhouse experiments, the following were examined: (i) larval host plant use and the dependence of larval diet on oviposition, (ii) nutritional differences between hosts, and (iii) the impact of conditions that accelerate host plant death (i.e. temperature) on larval survivorship and growth.
Larvae were observed to forage widely, vary their diet through time, and use hosts independently of their natal plant. Larvae tracked changes in host quality by steadily increasing their use of the longer‐lasting, but nutritionally variable, host plant. Temperature conditions that accelerated host death actually conferred a survivorship advantage when larvae were able to utilize this host.
These results suggest that larval diet choice and movement may be an adaptive strategy for foraging on declining food resources. It also suggests that the net effect of environmental extremes on larvae is strongly mediated by host plant use. Differences in larval survivorship when one or both hosts are available suggest that the long‐lasting host is essential to butterfly population persistence. By comparing this mobile taxa to species with fewer host‐switching opportunities, we better understand the diversity of foraging strategies of food‐limited insects and the effects of environmental change.
Habitat switching by Bewick's swans: maximization of average long‐term energy gain?
Authors: Nolet B. A. , Bevan R. M. , Klaassen M. , Langevoord O. , Van Der Heijden Y. G. J. T. 
Published: March 2003

Summary

In a system where depletion drives a habitat shift, the hypothesis was tested that animals switch habitat as soon as the average daily net energy intake (or gain) drops below that attainable in the alternative habitat.
The study was performed in the Lauwersmeer area. Upon arrival during the autumn migration, Bewick's swans first feed on below‐ground tubers of fennel pondweed on the lake, but subsequently switched to feeding on harvest remains in sugar beet fields.
The daily energy intake was estimated by multiplying the average time spent foraging per day with the instantaneous energy intake rate while foraging. In the case of pondweed feeding, the latter was estimated from the functional response and the depletion of tuber biomass. In the case of beet feeding, it was estimated from dropping production rate. Gross energy intake was converted to metabolizable energy intake using the assimilation as determined in digestion trials. The daily energy expenditure was estimated by the time–energy budget method. Energetic costs were determined using heart rate.
The daily gain of pondweed feeding at the median date of the habitat switch (i.e. when 50% of the swans had switched) was compared with that of beet feeding. The daily gain of beet feeding was calculated for two strategies depending on the night activity on the lake: additional pondweed feeding (mixed feeding) or sleeping (pure beet feeding).
The majority of the swans switched when the daily gain they could achieve by staying on the pondweed bed fell just below the average daily gain of pure beet feeders. However, mixed feeders would attain an average daily gain considerably above that of pondweed feeders. A sensitivity analysis showed that this result was robust.
We therefore reject the hypothesis that the habitat switch by swans can be explained by simple long‐term energy rate maximization. State‐dependency, predation risk, and protein requirements are put forward as explanations for the delay in habitat switch.
A demographic comparison of two southern elephant seal populations
Authors: Mcmahon Clive R. , Burton Harry R. , Bester Marthán N. 
Published: March 2003

Summary

To estimate concurrent age‐specific survival for southern elephant seals at Macquarie and Marion islands, seals were marked from 1993 to 1997 in the first 3 weeks of life and resighted (recaptured) on return to their natal islands (1994–2001). These recaptures formed the basis for the survival analysis in the mark–recapture program mark. Weaning masses were collected at each location.
Recapture probabilities were (= 376·480, P < 0·0001) higher at Marion Island than at Macquarie Island. There are two possible reasons: (1) the population at Marion Island is smaller and less dense than at Macquarie Island and (2) seals hauled out along a smaller section of the coast at Marion Island than at Macquarie Island, which: (1) facilitates the detection and individual identification of seals and (2) increases access to hauled out seals.
Age‐specific survival estimates (corrected for preweaning mortality and tag loss) differed ( = 22·264, P < 0·05) at the two islands and were consistently higher at Macquarie Island. The survival estimates for male and female seals were different at Macquarie Island (= 34·657, P < 0·0001) and Marion Island (= 20·373, P = 0·002). Female survival estimates were higher than male survival estimates. The combined survival estimates for juvenile seals (1–3 years) differed between islands but survival of older seals (4–6 years) did not. The inclusion of gender in the survival models did not improve model performance and hence male and female estimates were considered jointly.
The mean wean masses of male and female seals combined from 1993 to 1998 were not different between islands (T6837 = 1·169, P = 0·242). At Macquarie Island the mean annual wean mass was 118·8 kg (SD = 27·2, n= 6504) while at Marion Island it was 120·6 kg (SD = 24·7, n= 335).
The mean age at first breeding was different (P < 0·001) at the two island populations. At Macquarie Island the mean age of first breeding was 4·68 years, and at Marion Island it was 3·95 years. More (= 67·39, P < 0·0001) 3‐year‐old females breed at Marion Island (28·7%) than at Macquarie Island (1·2%) and the proportion of seals that had bred at least once by age 7 was greater at Marion Island than at Macquarie Island.
We conclude that the observed decreases in elephant seal numbers between the 1950s and 1990s in the Pacific and Indian Ocean sectors were driven principally by resource limitation in the Southern Ocean. A conglomerate of factors including local predation by killer whales and intraspecific resource competition is postulated as a cause for the inter‐island (regional) differences in population trends. It appears that more resources are available per capita to the Marion Island population than are available to the Macquarie Island population.
An insect (Argyresthia retinella, Lep., Yponomeutidae) outbreak in northern birch forests, released by climatic changes?
Authors: Tenow O. , Nilssen A. C. , Holmgren B. , Elverum F. 
Published: March 2003

1. In the early 1990s, birch Betula pubescens L. forests in north‐western Norway were damaged by the bud‐ and shoot‐mining larvae of Argyresthia retinella not previously known for outbreaks. In 1993–96, the outbreak was mapped and changes in attack intensity and foliage structure were quantified by sampling birch twigs along transects from coast to inland. Results were considered in relation to a variable climate. 2. The outbreak extended 400 km along the coast, mainly within the lowest 100–150 m a.s.l. It started in the late 1980s and the attack intensity culminated in 1993–94. Repeatedly, 30–50% of the leaf‐carrying shoots were damaged or killed. Trees compensated by sprouting clusters of shoots from undamaged shoots and, hence, increased foliage clumping. Eventually, twigs and branches died. 3. The overwintering post‐diapause egg, the larva and the adult stages were considered the most sensitive to changes in temperature climate. The air temperature, averaged for a combination of these stages, was calculated for each year, as well as deviations from the long‐term (1868–1996) average. 4. The 5‐year running average of deviations approached or exceeded + 1 °C three times: in the 1930s, around 1960 and around 1990. The third peak coincided with the outbreak of the 1990s. There are no reports of an A. retinella outbreak in either the 1930s or 1960s. However, in one photographic documentation from 1940, clumped foliage structure of birch suggests an outbreak in the 1930s. 5. Severe A. retinella attacks appear to be a fairly new phenomenon, possibly connected to recent high temperature deviations. If so, with the present air temperature climate, outbreaks may occur at intervals of ≈20–25 years. With a trend of decreasing air temperature, outbreaks may be less frequent. At higher temperatures, natural or from anthropogenic global warming, the outcome is more uncertain although more frequent outbreaks may occur initially. 6. It is recommended that foresters learn to identify damage made by A. retinella and include observations on this insect in their reporting. Monitoring A. retinella damage in the North and elsewhere, should contribute to the understanding of the outbreak ecology of this insect.
The horizontal movement of seeds in arable soil by different soil cultivation methods
Authors: Marshall E. J. P. , Brain P. 
Published: March 2003

1. To assess the impact of soil cultivation on the horizontal movement of seeds in arable soil, plastic beads and barley or triticale seeds were used as seed models. Different coloured beads were introduced in the field immediately before each of five cultivations: ploughing, two tine cultivations, harrowing and seed drilling. Beads were recovered from 20‐cm soil cores divided into four 5‐cm deep soil horizons. 2. After a typical cultivation sequence of five operations, beads were found up to 15 m from their source, although most beads were found within 2 m. Most beads were recovered from the surface 5 cm of the soil profile, except for those introduced onto the surface or at 20 cm depth before ploughing, which were concentrated below 10 cm. 3. Regression analysis was used to determine the pattern of bead movement by seed drilling. A novel analysis using Fast Fourier Transforms established the probability distribution functions of the remaining cultivation operations for horizontal movement. Using the final seed distributions, the effects of each cultivation were sequentially deconvoluted and the probability distribution functions smoothed. The proportions of beads moved were also calculated. 4. Ploughing and seed drilling moved seed the least distance compared with other cultivations. The mean distances moved were 0·36 m and 0·26 m, respectively. Tine cultivations moved beads 0·71 m and 1·21 m, while harrowing moved seed a mean distance of 1·58 m. Cultivation sequences based on ploughing are likely to limit seed movement in soil. 5. The Fourier deconvolution approach has potential for predicting future seed distributions and thus the spatial behaviour of weed patches within fields.