bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stochastic population dynamics and time to extinction of a declining population of barn swallows
Authors: Engen Steinar , Sæther Bernt‐erik , Møller Anders Pape 
Published: March 2003

SummaryTime to extinction was predicted for a declining population of barn swallowHirundo rusticain Denmark, using a model that includes demographic as well as environmental stochasticity and that takes the uncertainties in the parameter estimates into account.We apply the concept of population prediction interval (PPI), which is a stochastic interval that includes the unknown variable that may be the extinction time or the population size at some future point of time, with a given probability (1 − α).The lower bound of the upper one‐sided prediction interval for the extinction time for α = 0·10 was 22 years.Ignoring uncertainties in the parameter estimates led to a 41% increase in this statistic.Although the estimate of the demographic variance was small compared to other passerines (σd2 = 0·180), a sensitivity analysis showed that it strongly influenced the predicted time to extinction compared to the model ignoring demographic stochasticity. A similar effect on the prediction of the time to extinction was found for the environmental variance σe2. In addition, choosing σe2 = 0 strongly reduced the width of the prediction interval.This demonstrates that reliable population projections require modelling of the environmental as well as the demographic stochasticity, and that the uncertainty in the estimates of the model parameters must be taken into account.
Disease persistence and apparent competition in a three‐host community: an empirical and analytical study of large‐scale, wild populations
Authors: Gilbert L. , Norman R. , Laurenson K. M. , Reid H. W. , Hudson P. J. 
Published: March 2003

Summary We investigated the effects of three types of host on the persistence of a tick‐borne virus, using the grouse–hare–deer–tick–louping ill virus system of upland Britain. Each host differed in its interaction with the vector and pathogen. Grouse amplify virus only, deer amplify vector only and hares amplify both. Grouse alone suffer high virus‐induced mortality. An analytical model of the system was parameterized using empirical data from two wild populations with different community structures. By changing relative host densities we examined the conditions under which the virus would persist and considered the possibility of parasite‐mediated competition between hosts. Although deer alone and grouse alone were unable to maintain louping ill virus, a deer–grouse community usually allowed virus persistence because grouse transmitted virus while deer maintained the tick population. Since virus reduces grouse populations this is a type of apparent competition, and is unusual because deer do not amplify the virus. At very high deer densities, the opposite effect could occur, whereby virus died out because of ‘wasted’ infected tick bites on deer, that do not transmit virus (the dilution effect). In a hare–grouse two‐host system virus usually persisted because hares amplified both the vector and virus (through non‐viraemic transmission). Thus, apparent competition may occur between mountain hares and grouse. The addition of a third host type increased the likelihood of disease persistence. Hares added to the deer–grouse system rendered the dilution effect unlikely because of additional virus amplifiers. Deer added to the hare–grouse system meant virus almost always persisted because they amplified the vector.
The geographical range structure of the holly leaf‐miner. I. Population density
Authors: Brewer Andrew M. , Gaston Kevin J. 
Published: March 2003

Summary

The local population density structure of a phytophagous insect, the holly leaf‐miner Phytomyza ilicis Curtis, was examined across its natural geographical range in Europe.
The frequency distribution of the number of sample sites at which the leaf‐miner attained different densities per tree was strongly right‐skewed, with the species being absent from a large number of sites at which its host occurred, particularly in southern regions.
There was a decline in the spatial autocorrelation of leaf‐miner densities with increasing distance between sample sites, with negative autocorrelation at long lags resulting in part from high densities being attained at the north‐eastern range limits and low densities at the southern range limits.
Partial regression analysis was used to model leaf‐miner densities in terms of spatial position within the geographical range and the broad climate experienced at the sample localities. This accounted for between 40 and 65% of the variation in densities, dependent upon how the leaf‐miner’s geographical range was defined.
While overall these results are at odds with common and intuitively appealing assertions about the abundance structure of geographical ranges, they can readily be interpreted in terms of a simple modification of a general model of such structures.
Cross‐scale vegetation patterns of Flooding Pampa grasslands
Authors: Perelman S. B. , León R. J. C. , Oesterheld M. 
Published: March 2003

Summary The spatial pattern of vegetation heterogeneity across different scales may indicate the major environmental controls on vegetation structure, and thus guide strategies for the conservation of biodiversity. We analysed cross‐scale vegetation patterns in a 90 000‐km2area of natural grasslands in the Flooding Pampa, Argentina. We assessed the contribution of regional (latitudinal) and landscape (topographic) patterns of species turnover to overall heterogeneity using data from 749 censuses. A large proportion of the variation in species composition across the entire region was observed at very fine spatial scales (0.1–10 km2), associated with subtle topographic features and soil salinity gradients. Latitudinal variation played a secondary role. Species turnover among stands occupying different landscape positions at the same latitude was 50% greater than among inventories encompassing two degrees of latitude. The fine‐grain heterogeneity determined that an area of 10 km2was often sufficient to include 50% of all vascular plant species of the region. Although a large proportion (nearly 70%) of the vascular flora was composed of rare (satellite) species that occurred in less than 10% of the samples, few core species (i.e. those occurring in more than 90% of the samples) were seen at any scale of analysis. Latitude contributed most clearly to variation in species composition among the zonal communities located in well‐drained soils where differences in relative cover of C3and C4grasses were seen. However, photosynthetic pathways still varied more along salinity or topographic gradients than across regions. A latitudinal pattern in abundance of the differentPoaceaetribes was also consistent with their climatic classification. Alpha diversity showed a threefold variation among different stands within a landscape: it increased from low topographic positions and high soil salinity to high topographic positions and low salinity. However, it was constant among surveys within the region. Nearly 25% of the species were exotic, mostly European, whose invasion was promoted by livestock grazing. Compared with the native species, exotics were consistently enriched in annuals, particularly forbs, across latitude, and were less tightly associated with landscape heterogeneity than native species.
Periodic spotted patterns in semi‐arid vegetation explained by a propagation‐inhibition model
Authors: Couteron P. , Lejeune O. 
Published: March 2003

Summary Vegetation cover regularly punctuated by spots of bare soil is a frequent feature of certain semi‐arid African landscapes, which are also characterized by banded vegetation patterns (i.e. tiger bush). The propagation‐inhibition (PI) model suggests that a periodic pattern characterized by a dominant wavelength can theoretically establish itself through a Turing‐like spatial instability depending only on a trade‐off between facilitative and competitive interactions among plants. Under strictly isotropic conditions, spotted and banded patterns are distinct outcomes of a unique process, whereas anisotropy leads to a banded structure. The model predicts that spotted patterns will have a lower dominant wavelength than bands. We test some outcomes of the PI model against vegetation patterns observable in aerial photographs from West Africa. Two sites with rainfall ofc.500–600 mm year−1were studied: a 525‐ha plain in north‐west Burkina Faso and a 300‐ha plateau in southern Niger. Digitized photographs were subjected to spectral analysis by Fourier transform in order to quantify vegetation patterns in terms of dominant wavelengths and orientations. Spotted vegetation proved highly periodic. The characteristic range of dominant wavelengths (30–50 m) was similar at two sites more than 500 km apart. The PI model suggests that spots may occur as a hexagonal lattice but there is little evidence of such patterning in the field. A dominant wavelength was far quicker to establish in simulations (c. 102–103 years for annual grasses) than a hexagonal symmetry (c. 105 years), and observed patterns are therefore likely to be far from the asymptotic structure. Elongated and smudged spots that locally became flexuous bands have been observed in southern Niger. This pattern that had a dominant wavelength of 50 m but lacked any dominant orientation can be interpreted as a transition from spots to bands under fairly isotropic conditions. The PI model provides a framework for further investigation of patterns in semi‐arid vegetation and may be of a broader ecological application.
Effect of shrubs on tree seedling establishment in an abandoned tropical pasture
Authors: Holl Karen D. 
Published: March 2003

Summary

The objective of this study was to evaluate whether early successional shrubs facilitate, tolerate or inhibit different stages of tree establishment in abandoned tropical pasture.
Seed rain, seed predation, seed germination and seedling survival of tropical forest trees in pasture grass, below small (< 25 m2) shrub patches and below large (> 40 m2) shrub patches, were compared in one abandoned pasture in Costa Rica over 2 years.
Seed rain of animal‐dispersed trees was higher below both large and small shrub patches than below grass. Seed rain of wind‐dispersed trees did not differ in the three patch types.
Predation of all animal‐dispersed seeds combined and of three individual species was significantly higher below large and small shrub patches compared with below grass; predation of five species did not differ significantly in the three patch types.
Germination did not differ significantly in the three patch types for any of the species.
Seedling survival was highest below large shrub patches for three of four species.
Computer simulations of probabilities of seeds arriving in the pasture and surviving to the seedling stage suggest that early successional shrubs have a net facilitative effect on the early stages of forest tree seedling establishment compared with areas without shrubs in the pasture studied, although variance was high. Shrubs may facilitate, inhibit and tolerate different stages of tree seedling establishment.
Fine‐scale spatial structure in a grassland community: quantifying the plant’s‐eye view
Authors: Purves D. W. , Law R. 
Published: March 2003

Summary

The fine‐scale spatial patterns of Agrostis stolonifera, Holcus lanatus and Lolium perenne were recorded in an English lowland grassland as presence/absence maps from 400‐cell quadrats at two different scales (2 × 2 cm or 8 × 8 cm cells).
Local spatial structure in these patterns was quantified using spatial covariance functions. Distance‐ and direction‐dependent components were examined separately for both intra‐ and interspecific patterns. The significance of departures from randomness was determined using Monte Carlo techniques.
The smaller‐scale data showed that all three species were significantly aggregated, Agrostis to a greater distance (8 cm) than Holcus or Lolium(4 cm). The intensity of aggregation decreased in the order Lolium > Holcus > Agrostis. The larger‐scale data suggested that this aggregation extended to greater distances, and that it was most intense in Agrostis.
Despite the lack of visual directionality in the environment, Agrostis showed a directional pattern at both scales, with Lolium varying in the same direction at the larger scale.
Only Agrostis and Lolium showed a significant interspecific relationship (segregated to 2 cm at the small scale, but aggregated to 8 cm at the larger scale). There was no evidence of directionality in the interspecific components of pattern.
The nature of spatial structure appears to depend on the scale of observation, but the smaller‐scale data are more likely to provide a biologically interpretable measure of local spatial structure in this community.
A method for quantifying cell size from differential interference contrast images: validation and application to osmotically stressed chondrocytes
Authors: Alexopoulos L. G. , Erickson G. R. , Guilak F. 
Published: March 2003

SummaryAn automatic image analysis method was developed to determine the shape and size of spheroidal cells from a time series of differential interference contrast (DIC) images. The program incorporates an edge detection algorithm and dynamic programming for edge linking. To assess the accuracy and working range of the method, results from DIC images of different focal planes and resolutions were compared to confocal images in which the cell membrane was fluorescently labelled. The results indicate that a 1‐µm focal drift from the in‐focus plane can lead to an overestimation of cell volume up to 14.1%, mostly due to shadowing effects of DIC microscopy. DIC images allow for accurate measurements when the focal plane lies in a zone slightly above the centre of a spherical cell. In this range the method performs with 1.9% overall volume error without taking into account the error introduced by the representation of the cell as a sphere. As a test case, the method was applied to quantify volume changes due to acute changes of osmotic stress.
β‐Amyloid inhibits integrated mitochondrial respiration  and key enzyme activities
Authors: Casley C. S. , Canevari L. , Land J. M. , Clark J. B. , Sharpe M. A. 
Published: March 2003

AbstractDisrupted energy metabolism, in particular reduced activity of cytochrome oxidase (EC 1.9.3.1), α‐ketoglutarate dehydrogenase (EC 1.2.4.2) and pyruvate dehydrogenase (EC 1.2.4.1) have been reported in post‐mortem Alzheimer's disease brain. β‐Amyloid is strongly implicated in Alzheimer's pathology and can be formed intracellularly in neurones. We have investigated the possibility that β‐amyloid itself disrupts mitochondrial function. Isolated rat brain mitochondria have been incubated with the β‐amyloid alone or together with nitric oxide, which is known to be elevated in Alzheimer's brain. Mitochondrial respiration, electron transport chain complex activities, α‐ketoglutarate dehydrogenase activity and pyruvate dehydrogenase activity have been measured. β‐Amyloid caused a significant reduction in state 3 and state 4 mitochondrial respiration that was further diminished by the addition of nitric oxide. Cytochrome oxidase, α‐ketoglutarate dehydrogenase and pyruvate dehydrogenase activities were inhibited by β‐amyloid. The Km of cytochrome oxidase for reduced cytochrome c was raised by β‐amyloid. We conclude that β‐amyloid can directly disrupt mitochondrial function, inhibits key enzymes and may contribute to the deficiency of energy metabolism seen in Alzheimer's disease.
Phosphate limited cultures of the cyanobacterium Synechococcus are capable of very rapid, opportunistic uptake of phosphate
Authors: Ritchie Raymond J. , Trautman Donelle A. , Larkum A. W. D. 
Published: March 2003

Summary

Phosphate uptake rates were measured in Synechococcus R‐2 incubated in artificial secondary‐ and tertiary‐treated sewage.
Phosphate uptake was measured using chemical assay and 32P incorporation. Intracellular pH was measured using accumulation of 14C‐labelled weak acids and bases and membrane potentials using 86Rb+/valinomycin.
Synechococcus cells are capable of very rapid, opportunistic uptake of phosphate (10–30 nmol m−2 s−1) even though net uptake by growing cultures was < 0.5 nmol m−2 s−1. Km and Vmax in the light were not significantly different at pHo 7.5 and 10. The mean Km values were 1.91 ± 0.41 mmol m−3 and 0.304 ± 0.055 mmol m−3 for P‐sufficient (secondary‐treated sewage) and P‐deficient (tertiary‐treated sewage) cells, respectively. The transport systems probably recognize both H2PO4− and HPO42−. Intracellular inorganic phosphate is +28 to +56 kJ mol−1 from electrochemical equilibrium. In P‐sufficient cells uptake is very slow in the dark (c. 0.1 nmol m−2 s−1) but phosphate‐starved cells can opportunistically take up P about 100 times faster.
Two separate ATP‐driven phosphate uptake mechanisms (1 PO4 in per ATP) appear to be responsible for phosphate uptake by the cells. They have different Km values, different light/dark responses and electrical behaviour.
Taxonomic basis for variation in the colonization strategy of arbuscular mycorrhizal fungi
Authors: Hart Miranda M. , Reader Richard J. 
Published: March 2003

Summary

Arbuscular mycorrhizal fungi (AMF) are important components of terrestrial communities but the basic ecology of individual AMF, including their colonization strategy, remains unclear. The colonizing behaviours of 21 AMF isolates from three families (Acaulosporaceae, Gigasporaceae and Glomaceae) were compared to test for a relationship between AMF taxonomy and colonization strategy.
Both the rate and extent of colonization were considered by measuring percentage root colonization, root fungal biomass, soil hyphal length and soil fungal biomass over 12 wk.
Most Glomaceae isolates colonized roots before Acaulosporaceae and Gigasporaceae isolates. The fastest colonizers were also often the most extensive. Taxonomic differences were apparent in the amount and proportion of fungal biomass found in roots vs in soil. Glomaceae isolates had high root colonization but low soil colonization, Gigasporaceae isolates showed the opposite trend whereas Acaulosporaceae isolates had low root and soil colonization. These results were similar for four different host plants.
The results indicate that the colonizing strategies of AM fungi differ considerably and that this variation is taxonomically based at the family level. Arbuscular mycorrhizal fungal taxonomy therefore has a functional basis.
Mineral content of excreta from the spittlebug Philaenus spumarius closely matches that of xylem sap
Authors: Ponder K. L. , Watson R. J. , Malone M. , Pritchard J. 
Published: March 2003

Summary

Analysis of xylem sap from intact transpiring plants is possible using excreta of xylem feeding insects. However, the extent to which the inorganic composition of ingested sap is altered by its passage through the insect has not been fully evaluated. The impact of the xylem feeding insect Philaenus spumarius (meadow spittlebug) on the inorganic composition of ingested sap from tomato (Lycopersicon esculentum) is reported here.
Two nondestructive experimental approaches were used. Sap exuding due to root pressure from a cut tomato stem was compared with excreta from a caged insect feeding just below the cut surface. A second method involved pumping a solution of inorganic ions through a section of tomato stem upon which an adult insect was caged. The composition of insect excreta was compared with that of the input and output solutions.
Overall, the concentrations of inorganic ions were similar in ingested fluid and excreta except for ammonium, which was generally elevated in excreta samples.
It is demonstrated that excreta of P. spumarius can be used for continual monitoring of most inorganic ions in xylem sap of intact transpiring plants, and that stem perfusion is a useful tool for quantifying the effect of the insect on solutions of known composition.
Monoterpene emissions in relation to foliar photosynthetic and structural variables in Mediterranean evergreen Quercus species
Authors: Niinemets Ülo , Hauff Karin , Bertin Nadia , Tenhunen John D. , Steinbrecher Rainer , Seufert Günther 
Published: March 2003

Summary

The relationships between foliar photosynthetic variables and monoterpene emission rates (E) are reported here for two sclerophylls, Quercus coccifera L. and Q. ilex L., which lack specialized foliar monoterpene‐storage structures.
Photosynthesis rates and E were measured within a forest canopy, and in leaves of different exposure in a garrigue ecosystem. Leaf dry mass per unit area (LMA) was used as a proxy of long‐term leaf light availability.
Maximum photosynthetic electron transport rate (Jmax), Rubisco carboxylase activity (Vcmax) and light‐saturated E (Emax) per unit area increased strongly with increasing LMA. The positive effect of LMA on area‐based Emax resulted from an accumulation of enzymes responsible for monoterpenoid synthesis per unit area. Positive relationships were observed between both area‐ and mass‐based Emax and Jmax, but not with photosynthesis rates, suggesting a possible control of terpene synthesis by foliar electron transport rates. Results suggested that monoterpene volatility limitations at low temperatures might also constrain E. Nonspecific storage may explain bursts of monoterpene emission on warm days following cold days, and monoterpene emission in low quantities during night.
Although the correlations between Emax and Jmax do not necessarily give mechanistic insight into the physiology of monoterpene emission, they provide a basis for scaling up leaf emissions to a canopy level. However, both the leaf‐to‐leaf differences in monoterpene synthase activities, and monoterpene storage may perplex the relations between the electron transport rates and E.
Hepatitis B core antibodies in Danish blood donors: a surrogate marker of risk behaviour
Authors: Christensen P. B. , Titlestad I. L. , Homburg K. M. , Georgsen J. , Kristensen T. 
Published: March 2003

Background and ObjectivesThe aim of this work was to determine the prevalence of antibodies to hepatitis B core antigen (anti‐HBc) among Danish blood donors and to correlate this with risk factors for blood‐borne and sexually transmitted diseases.Materials and MethodsDuring a 5‐month period, 10 862 consecutive donors in the County of Funen were screened for anti‐HBc, and repeat‐reactive samples were confirmed by supplementary testing. Information on risk factors was assessed by questionnaire in 585 consecutive anti‐HBc‐negative blood donors and compared with information obtained from confirmed positive donors.ResultsThe prevalence of confirmed positive anti‐HBc among donors was 0·70% (76/10 862, 95% confidence interval [CI]: 0·55–0·87). One donor was positive for anti‐HBc immunoglobulin M (IgM); none tested positive for hepatitis B virus (HBV) DNA. In a logistic regression analysis, age, female gender, tattoos and commercial sexual relations, were independent predictive factors for the presence of anti‐HBc.ConclusionAnti‐HBc is a surrogate marker for previous risk behaviour in the Danish blood donor population. We suggest that screening for anti‐HBc may be used among new donors to supplement interviews on risk behaviour.