bluuebunny/crossref_metadata_2025_split huggingface dataset:
Density effects on life‐history traits in a wild population of the great titParus major: analyses of long‐term data with GIS techniques
Authors: WILKIN TEDDY A. , GARANT DANY , GOSLER ANDREW G. , SHELDON BEN C. 
Published: March 2006

SummaryPopulation density often has strong effects on the population dynamics and reproductive processes of territorial animals. However, most estimates of density‐dependent effects use the number of breeding pairs per unit area in a given season and look for correlations across seasons, a technique that assigns the same density score to each breeding pair, irrespective of local spatial variation.In this study, we employed GIS techniques to estimate individual breeding densities for great tits breeding in Wytham Woods UK, between 1965 and 1996. We then used linear mixed modelling to analyse the effect of density on reproductive processes.The areas of Thiessen polygons formed around occupied nestboxes were used to approximate territory size (necessarily inverse of breeding density). There were significant, independent and positive relationships between clutch size, fledging mass and the number of offspring recruited to the population, and territory size (allP < 0·001), but no effect of territory size on lay‐date or egg mass.Thiessen polygons are contiguous and cover all of the available area. Therefore, at low nest densities territory polygons were excessively oversized. Using a novel procedure to address this limitation, territory sizes were systematically capped through a range of maxima, with the greatest effect in the models when territories were capped at 0·9–2·3 ha. This figure approximates to the maximum effective territory size in our population and is in close agreement with several field‐based studies. This capping refinement also revealed a significant negative relationship between lay‐date and territory size capped at 0·9 ha (P < 0·001).These density‐dependent effects were also detected when analyses were restricted to changes within individual females, suggesting that density effects do not merely result from either increased proportions of low‐quality individuals, or increased occupation of poor sites, when population density is high.Overall, these results suggest that, in the current population, great tits with territories smaller thanc.2 ha independently lay smaller and later clutches, have lighter fledglings, and recruit fewer offspring to the breeding population. These analyses thus suggest a pervasive and causal role of local population density in explaining individual reproductive processes.
Relationship between host abundance and parasite distribution: inferring regulating mechanisms from census data
Authors: STANKO MICHAL , KRASNOV BORIS R. , MORAND SERGE 
Published: March 2006

SummaryWe studied the effect of host abundance on parasite abundance and prevalence using data on 57 associations of fleas (Siphonaptera) and their mammalian hosts from Slovakia.We assumed that flea‐induced host mortality could be inferred from the relationship between flea aggregation and flea abundance, whereas host‐induced flea mortality could be inferred from the relationship between flea abundance or aggregation and host abundance.Relationships between flea abundance or prevalence and host abundance were either negative (in 23 flea–host associations) or absent (in 34 flea–host associations). Negative relationships between flea abundance and host abundance were always accompanied by negative relationships between flea prevalence and host abundance.The link between flea abundance/prevalence and host abundance was evaluated as the coefficient of determination of the respective regressions. Across flea–host associations, this link decreased with an increase in the degree of flea aggregation (measured as a parameterbof Taylor's power law).Mean crowding of fleas decreased with an increase of host abundance in eight flea–host associations, being asymptotic in four of them. On the other hand, mean crowding of fleas increased with an increase in flea abundance in 49 flea–host associations, being asymptotic in 15 of them.Results of this study suggest that different flea–host associations are governed by different regulating mechanisms, but different regulation mechanisms may act simultaneously within the same flea–host associations.
Self‐regulation within outbreak populations of feral house mice: a test of alternative models
Authors: SUTHERLAND DUNCAN R. , SINGLETON GRANT R. 
Published: March 2006

Summary

Outbreaks of feral house mice, Mus domesticus, in Australia represent a fundamental failure of the behavioural control mechanisms of population density, as proposed in the hypothesis of self‐regulation.
Mice have the potential to keep numbers in check via a suite of spacing behaviours; however, the self‐regulation hypothesis implies that some social change occurs that permits the population to erupt. It also suggests that at different phases of an outbreak, distinct patterns of social activity are evident.
We compare predictions from two models encapsulating the self‐regulation hypothesis as applied to feral house mice in south‐eastern Australia. Each model may be distinguished by the timing of aggressiveness between mice that leads to a closed social system. We compare individual turnover, residency and territoriality in each sex and age cohort during the increase, peak and low phases of a population outbreak that peaked in 2001.
The activity of 438 mice was monitored via intensive mark–recapture trapping and an automated event recording system that detected the activity of 300 marked individuals at burrow entrances.
Our findings support the second model, which suggests that mice switch from an almost asocial structure at low densities to a territorial system as abundance increases. Adult females appear more likely than males or juveniles to make the significant social shift. The trigger for this change remains unclear and several alternative mechanisms are proposed.
Silica in grasses as a defence against insect herbivores: contrasting effects on folivores and a phloem feeder
Authors: MASSEY FERGUS P. , ENNOS A. ROLAND , HARTLEY SUE E. 
Published: March 2006

Summary

Silica, deposited as opaline phytoliths in the leaves of grasses, constitutes 2–5% of dry leaf mass, yet its function remains unclear. It has been proposed that silica may act as an antiherbivore defence by increasing the abrasiveness and reducing the digestibility of grass leaves, although there is little direct experimental evidence to support this.
We investigated the effects of manipulated silica levels on the abrasiveness of the leaves of five grass species. We also examined the effects of silica levels on the feeding preferences, growth performance and digestion efficiency of two folivorous insects and one phloem‐feeding insect.
Silica addition resulted in increases to leaf abrasiveness in four of the five grass species studied. Silica addition also deterred feeding by both folivores and reduced their growth rates and digestion efficiency.
These effects resulted in lower pupal mass of the lepidopteron larvae Spodoptera exempta and compensatory feeding by the orthopteran, Schistocerca gregaria. In contrast, silica had no effects on the feeding preference or the population growth of the phloem feeder, Sitobion avenae.
Our results demonstrate that silica is an effective defence against folivorous insects, both as a feeding deterrent, possibly mediated by increased abrasiveness, and as a digestibility reducer. The effects of silica on pupal mass and development time may impact on herbivore fitness and exposure to natural enemies.
These results are the first demonstration of a direct effect of silica on the abrasiveness of grasses and the adverse impact of silica on herbivore preference and performance.
How moles contribute to colonization success of water voles in grassland: implications for control
Authors: DELATTRE P. , CLARAC R. , MELIS J. P. , PLEYDELL D. R. J. , GIRAUDOUX P. 
Published: March 2006

Summary

Outbreaks of the water vole Arvicola terrestris cause severe damage in grasslands of upland regions of Europe. The sheer speed of this phenomenon is a challenge to effective pest control measures. While there has been some research into factors that promote outbreaks, especially landscape composition, little is yet known of the biological mechanisms underlying the speed of colonization of grasslands during the population growth stage. Like A. terrestris, the mole Talpa europaea digs vast tunnel networks that may be used by A. terrestris. The availability of extensive networks of this sort might greatly boost the colonization potential of A. terrestris and so explain the speed of onset of its outbreaks.
Areas occupied by networks of A. terrestris and T. europaea tunnels, and their respective locations, were evaluated and mapped with a global positioning system (GPS) during low‐density, growth and the first weeks of abundance phases of cyclic fluctuations of A. terrestris.
During the growth phase and the first weeks of the abundance phase nearly 80% of new A. terrestris colonies were found in T. europaea tunnel networks, although these occupied just 20% of the area under study.
The regulating influence of wooded areas on the risk of outbreaks, which can be identified at larger spatial scales (region, sector), was imperceptible at local scales, probably because of the influence and relative extent of T. europaea tunnel networks.
There was a threshold level of occupancy of grassland by T. europaea, as indexed by surface indices, below which A. terrestris populations were regulated naturally and over long periods.
Synthesis and applications. These findings have implications for controlling outbreaks of A. terrestris. Temporally, chemical pest control of A. terrestris can be reduced by taking action during the A. terrestris low‐density phases, preferably in the autumn. Spatially, control operations should be targeted at T. europaea and early A. terrestris networks. Mechanical destruction of tunnels (e.g. ploughing) and trapping should be considered as an alternative to chemical pest control.
Captive breeding and the fitness of reintroduced species: a test of the responses to predators in a threatened amphibian
Authors: KRAAIJEVELD‐SMIT FEMMIE J. L. , GRIFFITHS RICHARD A. , MOORE ROBIN D. , BEEBEE TREVOR J. C. 
Published: March 2006

Summary

Although captive breeding and reintroduction is a high‐profile management tool for many threatened species, it is unclear how long‐term captive breeding can influence fitness attributes such as natural defences to predators.
Induced defences that have evolved in the Mallorcan midwife toad Alytes muletensis in response to introduced predators were compared in natural and reintroduced populations that had a common ancestry, and in short‐term and long‐term captive populations that differed in ancestry.
Defences against predators were maintained in a reintroduced population derived from stock that had passed through three to eight generations of captive breeding prior to release into a predator‐free area. Heterozygosity did not differ between natural and reintroduced populations, but the reintroduced population displayed lower allelic richness.
A comparison between populations maintained for different lengths of time in captivity revealed a significant reduction in one defensive trait in stock maintained for more than eight generations. Neutral genetic variation (i.e. heterozygosity and allelic richness) did not differ between the short‐term captive population and a natural population, but there was a significant loss of genetic variation in the long‐term captive population.
Synthesis and applications. The results suggest that relatively high levels of heterozygosity and important fitness attributes can be maintained for a few generations in breeding programmes for threatened species despite small numbers of founders and the absence of natural selection. Nevertheless, both fitness and heterozygosity may eventually start to deteriorate in the long term, and this may have implications for reintroduction strategies.
Potential for multiple lag phases during biotic invasions: reconstructing an invasion of the exotic tree Acer platanoides
Authors: WANGEN STEVEN R. , WEBSTER CHRISTOPHER R. 
Published: March 2006

Summary

Perennial woody invaders often form persistent patches that significantly alter the structure and composition of native plant communities. Given their long generation times compared with ruderal invaders, these species may experience prolonged establishment phases between successful introduction and spread. Gap dynamics of shade‐tolerant invaders could lead to multiple lag phases during the invasion process.
In order to investigate the potential for long or multiple lag phases, we reconstructed the invasion of Acer platanoides (a shade‐tolerant, invasive, exotic tree) on an 1130‐ha temperate forested island in Lake Huron, USA. We measured and mapped the spatial location of every A. platanoides≥ 0·5 m in height that had successfully established within a 728‐ha forested park on the island. A simple age–diameter relationship, developed from a randomly selected subsample of the population, was used to assign an establishment date to each individual.
Following a 34‐year establishment phase, the area occupied by ≥ 1 A. platanoides ha−1 increased linearly at a rate of 5·6 ha year−1 for 35 years, after which range expansion slowed. Population growth lagged behind range expansion, with rapid population growth associated with infill between parents. During the expansion phase, numerous satellite populations established but contributed little to population growth and spatial expansion because of the long time required for them to become reproductive. These satellite populations will most probably accelerate population growth and spread once they reach reproductive age.
Roads and trails provided important corridors for propagule movement away from developed areas over the course of the invasion. They also appeared to facilitate longer distance dispersals than would be expected given the biology of the species.
Synthesis and application. Our results suggest that shade‐tolerant invaders with long generation times may undergo long establishment phases as well as periodic lags during the expansion phase. These lags may provide windows of opportunity for control but could easily be misinterpreted as signs that the population has reached an equilibrium density or the geographical extent of its spread. Additionally, roads and trails may provide important corridors for movement of propagules via non‐standard means of dispersal.
Predicting time‐specific changes in demographic processes using remote‐sensing data
Authors: RASMUSSEN HENRIK B. , WITTEMYER GEORGE , DOUGLAS‐HAMILTON IAIN 
Published: March 2006

Summary

Models of wildlife population dynamics are crucial for sustainable utilization and management strategies. Fluctuating ecological conditions are often key factors influencing both carrying capacity, mortality and reproductive rates in ungulates. To be reliable, demographic models should preferably rely on easily obtainable variables that are directly linked to the ecological processes regulating a population.
We compared the explanatory power of rainfall, a commonly used proxy for variability in ecological conditions, with normalized differential vegetation index (NDVI), a remote‐sensing index value that is a more direct measure of vegetation productivity, to predict time‐specific conception rates of an elephant population in northern Kenya. Season‐specific conception rates were correlated with both quality measures. However, generalized linear logistic models compared using Akaike's information criteria showed that a model based on the NDVI measure outperformed models based on rainfall measures.
A predictive model based on coarse demographic data and the maximum seasonal NDVI value was able to trace the large variation in observed season‐specific conception rates (Range 0–0·4), with a low median deviation from observed values of 0·07.
By combining the model of season‐specific conception rates with the average seasonal distribution of conception dates, the monthly number of conceptions (range 0–22) could be predicted within ±3 with 80% confidence.
Synthesis and applications. The strong predictive power of the normalized differential vegetation index on time‐specific variation in a demographic variable is likely to be generally applicable to resource‐limited ungulate species occurring in ecologically variable ecosystems, and could potentially be a powerful factor in demographic population modelling.
The modelled effects of differing fire management strategies on the conifer Callitris verrucosa within semi‐arid mallee vegetation in Australia
Authors: BRADSTOCK R. A. , BEDWARD M. , COHN J. S. 
Published: March 2006

Summary

Callitris verrucosa is an obligate‐seeder co‐dominant within flammable semi‐arid mallee shrublands in southern Australia. We sought to determine if there is an optimal management solution that provides both a significant reduction in wildfire size and maintains a viable population of C. verrucosa. Using a spatial model, the effects of alternative fire management strategies (pattern and level of prescribed burning) on populations of C. verrucosa were simulated.
Plant dynamics and fire propagation were simulated in a gridded landscape model (104 cells, c. 1 ha) incorporating topographic variation typical of dune landscapes. Fire propagation was governed by fuel age, and topographic and weather effects, incorporating potential for spot fires and the influence of wind direction. Plant dynamics were determined by known demographic parameters for C. verrucosa.
The size of unplanned fires declined significantly as a function of increasing prescribed burning level. Mean fire interval was longer at zero or 1% per year of landscape treated with prescribed fire than at higher levels of prescribed burning. Mean fire intervals were shorter under the highest probability of unplanned ignition in the flat landscape and in the dune landscape on the slopes.
The responses of C. verrucosa populations reflected trends in fire intervals as affected by prescribed burning levels/patterns and topography. The lowest population sizes resulted from either high (20% per year) or zero prescribed burning. The highest population sizes occurred consistently at an intermediate level of prescribed ignition (5% per year). Population sizes were significantly larger in dune vs. flat landscapes and under random vs. non‐random prescribed ignition patterns.
Synthesis and applications. The use of prescribed fire to achieve management objectives concerning ‘wildfire control’ and conservation will involve trade‐offs that are affected by landscape characteristics. Prescribed fire is predicted to achieve both a diminution of wildfire size and maintenance of C. verrucosa population in the landscape. The trade‐off required to achieve these objectives concerns the appropriate level and pattern of prescribed fire (strategy).
Is management of an invasive grassAgropyron cristatumcontingent on environmental variation?
Authors: HANSEN MALIN J. , WILSON SCOTT D. 
Published: March 2006

SummaryThe control of invasive species is a challenge heightened by the dependency of management outcomes on environmental variation. This is especially true for plants invading semi‐arid habitats, where growing season precipitation varies greatly among years.Agropyron cristatumis an invasive grass widely introduced in the Great Plains of North America. We studied its demographic responses to management using field experiments and matrix population models. Plants were clipped to simulate grazing, treated with herbicide or left unmanaged, at three levels of water availability, for 2 years.Growth rates (λ) were high in unmanaged populations. Clipped populations were mainly stable, whereas λ for herbicide‐treated populations varied greatly with water availability and between years. Low λ in clipped and herbicide‐treated populations was mainly the result of low seed production, and these populations were the most sensitive to water availability.Clipped populations produced no seeds in the second year, indicating a cumulative negative effect of defoliation. In general, seed production, germination and juvenile survival all increased with water supply, suggesting that invasion may increase under wet conditions.Population projections revealed a steady increase in population size for unmanaged populations, whereas clipped populations were more stable. Herbicide‐treated populations mainly decreased. Life‐cycle stages associated with recruitment contributed the most to the rapid growth of unmanaged populations, whilst the persistence of managed populations relied on the survival of established tussocks.Synthesis and applications.These results demonstrate strong management effects onA. cristatuminvasion in spite of significant population responses to water availability. Management can therefore have large effects on the invasion of native grasslands regardless of among‐year variability in precipitation.
Unpalatable plants facilitate tree sapling survival in wooded pastures
Authors: SMIT CHRISTIAN , DEN OUDEN JAN , MÜLLER‐SCHÄRER HEINZ 
Published: March 2006

Summary

In endangered wooded pasture ecosystems established tree saplings are frequently found in spatial association with protective structures, suggesting nurse effects. This associational resistance is thought to be a driving force behind tree regeneration in wooded pastures. Experimental evidence for associational resistance is, however, scarce. We studied the effects of unpalatable plants on tree sapling performance in a wooded pasture ecosystem in Switzerland and tested the associational resistance hypothesis experimentally.
We transplanted 600 Picea abies saplings of two size classes (5 cm and 10 cm) in plots: (i) with unpalatable plants (either Gentiana lutea or Cirsium acaule, representing a chemically and mechanically defended species, respectively); (ii) with one of both unpalatable plants clipped to the ground; and (iii) without any unpalatable plants. Grazing intensity was measured once in each plot and tree sapling performance was followed for 2 consecutive years.
Grazing intensity was significantly higher in plots without unpalatable plants, and significantly higher in plots with Cirsium than with Gentiana. Correspondingly, sapling survival (21·5% and 10·6% after 1 and 2 grazing years, respectively) was significantly higher near unpalatable plants, and significantly higher in plots with Gentiana than with Cirsium. Large tree saplings survived significantly better than small ones, and depended less on the unpalatable plants for survival.
Except for saplings planted near Gentiana, sapling removal by cows and horses was the major cause of death, the extent varying between unpalatable species and treatments.
The growth in height of the saplings was unaffected by the treatments.
Synthesis and applications. This study shows that unpalatable plants can enhance tree regeneration in wooded pastures. Sapling survival was significantly higher near unpalatable plants where grazing intensity was significantly lower. Protection against cattle is particularly important for small saplings. These results have important management implications for the endangered and disappearing wooded pastures in western Europe. Transplanting tree saplings near unpalatable plants could be an alternative reforestation technique in intensively grazed wooded pastures. On the other hand, removal of unpalatable plants and shrubs might prevent or slow down undesired tree encroachment in less intensively grazed areas.