bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dynamics of shrub encroachment in an African savanna: relative influences of fire, herbivory, rainfall and density dependence
Authors: Roques K.G. , O'Connor T.G. , Watkinson A.R. 
Published: March 2003

Summary

Shrub encroachment has been widely observed in savanna regions. This study analysed the causes of shrub encroachment in the lowveld savanna of north‐eastern Swaziland, southern Africa, and highlighted management regimes that can be used to reduce or prevent it.
The rates and dynamics of shrub encroachment were quantified for the period 1947–90 using aerial photographs, and for 1997 using a ground survey. Five similar areas with different land‐use histories were compared to investigate the relative importance of fire, herbivory, rainfall, soil type and shrub density in driving shrub dynamics.
In the study area as a whole, shrub cover increased from a mean of 2% in 1947 to 31% in 1990. Dichrostachys cinerea accounted for most of the increase in cover, contributing 81% to total shrub cover during 1997. Shrub cover was strongly correlated with shrub density and weakly negatively correlated with tree cover.
Shrub encroachment varied across land‐use fence lines. The key determinants of shrub dynamics were grazing, through its negative effect on fire frequency and an interaction between drought frequency and high shrub cover. Browsing pressure had a significant but minor impact on dynamics, while soil type had no significant effect. High grazing pressures through their effect on fire frequency were critical throughout the study period in promoting shrub encroachment, while the interaction between drought and high shrub cover produced declines in the later stages. Browsing had an impact on encroachment only in the early stages.
Frequent fires, facilitated by low grazing pressures, were capable of preventing shrub encroachment. When coupled with drought, frequent fires could reduce high shrub densities.
As cover and density were strongly correlated, it can be inferred from the negative correlation between change in cover (density) and initial cover (density) that the rate of shrub encroachment was cover (density)‐dependent and that there was a shrub equilibrium of 40% cover, approximating to 2400 plants ha−1. Shrub population growth was driven by events (fire, drought) as well as continuous agents (density dependence, mean browsing and grazing pressure).
Bush encroachment can be reversed by a combination of management (frequent fires) and climatic events (drought). The implications for savanna management are discussed.
Local declines in the abundance of harbour seals: implications for the designation and monitoring of protected areas
Authors: Thompson Paul M. , Van Parijs Sofie , Kovacs Kit M. 
Published: March 2003

Summary

Information on the abundance of European pinnipeds is required to assess interactions with fisheries and to meet new conservation obligations under the European Community (EC) Habitats Directive. A detailed time‐series of data has documented increases in grey seal populations, but there remains considerable uncertainty over the current status of UK harbour seal populations.
We present data on changes in the abundance of harbour seals in a study area within Orkney, Scotland, UK, previously considered a stronghold for this species. In 1998, intensive shore‐based counts of both adults and pups were only 16–36% of similar counts made between 1984 and 1987. Furthermore, a significant reduction in the proportion of yearlings present in haul‐out groups indicated a reduction in local recruitment.
Current monitoring of UK harbour seals, using wider‐scale aerial survey techniques, has provided important estimates of site distribution and relative abundance at regional and national scales. However, these larger‐scale surveys lack associated data on within‐year variation in counts at particular sites. Consequently, they cannot currently be used to explore the spatial scale of the changes in seal numbers that we report here. Similarly, current data from wider‐scale monitoring cannot be used to determine whether observed local changes in abundance result from redistribution or a change in survival or reproduction.
Management and monitoring plans are currently being developed for special areas of conservation (SAC) for harbour seals in response to the EC Habitats Directive. We suggest that more intensive research is required to assess whether or not similar changes in the abundance and age structure of seals have occurred in these areas. Without a better understanding of the current status of harbour seals both within and outside proposed SAC, future efforts to monitor the effectiveness of management initiatives will be constrained.
The impact of a root‐crown weevil and pasture competition on the winter annual Echium plantagineum
Authors: A.W. Sheppard , M.J. Smyth , A. Swirepik 
Published: March 2003

Summary

Root‐feeding insect herbivores may suppress their host populations by altering the competitive balances within the plant community of the host‐plant. In this study, repeated field experiments using caged and natural populations of a root‐crown weevil, Mogulones larvatus, on the annual weed Echium plantagineum (Boraginaceae) were conducted with and without pasture competition in its exotic range.
The experiment used a split‐plot design, excluding weevils with insecticide and plant competition through weeding.
The greatest effect of the weevils was observed when natural densities of weevils occurred on naturally growing E. plantagineum rosettes in the field. Weevils reduced plant survival by 43%, and the size and seed weight of survivors by 58% and 74%, respectively.
Pasture competition had a similar impact on E. plantagineum as the weevils for plant size and seed weight, except that there was no impact on rosette survival. There were also no significant interactions between the effects of weevils and competition. Most impact occurred in the second half of the growing season.
The number of weevil eggs and larvae and all plant reproductive parameters measured per unit biomass were independent of plant size.
Such impact experiments help develop simplified impact assessment methodologies for assessing the success of biological control projects in the field.
Modelling landscape‐scale habitat use using GIS and remote sensing: a case study with great bustards
Authors: Osborne P.E. , Alonso J.C. , Bryant R.G. 
Published: March 2003

Summary

Many species are adversely affected by human activities at large spatial scales and their conservation requires detailed information on distributions. Intensive ground surveys cannot keep pace with the rate of land‐use change over large areas and new methods are needed for regional‐scale mapping.
We present predictive models for great bustards in central Spain based on readily available advanced very high resolution radiometer (AVHRR) satellite imagery combined with mapped features in the form of geographic information system (GIS) data layers. As AVHRR imagery is coarse‐grained, we used a 12‐month time series to improve the definition of habitat types. The GIS data comprised measures of proximity to features likely to cause disturbance and a digital terrain model to allow for preference for certain topographies.
We used logistic regression to model the above data, including an autologistic term to account for spatial autocorrelation. The results from models were combined using Bayesian integration, and model performance was assessed using receiver operating characteristics plots.
Sites occupied by bustards had significantly lower densities of roads, buildings, railways and rivers than randomly selected survey points. Bustards also occurred within a narrower range of elevations and at locations with significantly less variable terrain.
Logistic regression analysis showed that roads, buildings, rivers and terrain all contributed significantly to the difference between occupied and random sites. The Bayesian integrated probability model showed an excellent agreement with the original census data and predicted suitable areas not presently occupied.
The great bustard's distribution is highly fragmented and vacant habitat patches may occur for a variety of reasons, including the species’ very strong fidelity to traditional sites through conspecific attraction. This may limit recolonization of previously occupied sites.
We conclude that AVHRR satellite imagery and GIS data sets have potential to map distributions at large spatial scales and could be applied to other species. While models based on imagery alone can provide accurate predictions of bustard habitats at some spatial scales, terrain and human influence are also significant predictors and are needed for finer scale modelling.
Suppression of weeds by spring wheat Triticumaestivum increases with crop density and spatial uniformity
Authors: Weiner Jacob , Griepentrog Hans‐Werner , Kristensen Lars 
Published: March 2003

Summary

Recent advances in our understanding of the advantage of initial size in competition among individual plants (size‐asymmetric competition) suggest that the potential for many crops to suppress weeds is much greater than generally appreciated. We hypothesize that this potential can be realized if: (i) the crop density is increased significantly and (ii) the crop is regularly (uniformly) distributed in two‐dimensional space rather than sown in traditional rows.
We tested these hypotheses by sowing four varieties of spring wheat Triticum aestivum at three densities (200, 400 and 600 m−2) and in two spatial patterns (normal rows and a uniform grid pattern) in the presence of high weed pressure.
There were strong and significant effects of both crop density and spatial distribution on weed growth. Weed biomass decreased with crop density and was 30% lower in the grid pattern.
There was a negative linear relationship between above‐ground weed biomass in early July and crop yield at harvest, so weed suppression translated directly into yield. The treatment with high crop density and the grid sowing pattern contained 60% less weed biomass and produced 60% higher yield than the treatment closest to normal sowing practices (crops sown in rows at 400 m−2).
The results were similar when the experiment was repeated in the following year, even though weed abundance was lower and the weed community was very different. There was 30% less weed biomass and 9% higher yield when the crop was sown in a grid pattern.
While weed biomass decreased monotonically with density for all varieties, a significant variety–density interaction suggested that the attributes resulting in good weed suppression at high crop density may not be the same as those most advantageous at low crop density.
A more crowded, uniform, distribution of some crops could contribute to a strategy to reduce the use of herbicides and energy‐intensive forms of weed control.
Assessment of light and soil conditions in ancient and recent woodlands by Ellenberg indicator values
Authors: Dzwonko Zbigniew 
Published: March 2003

Summary

The system of Ellenberg indicator species is widely accepted and used to characterize climate, light and soil conditions in central Europe and beyond. The aim of this study was to examine the extent to which Ellenberg indicator values can be used to characterize environmental conditions in ancient and recent woodlands.
The study was based on data from 264 plots in ancient deciduous woodlands and adjacent 70‐year‐old recent woodlands located on the same soils in the Carpathian foothills. Vegetation data were related to information on light intensity and a range of soil variables.
Despite a similar number of species on the plots in the ancient and recent woodlands, the correlations between the mean indicator values for light (mL), reaction (mR) and nitrogen (mN) and measured light intensity, soil pH and cation exchange capacity, respectively, were significantly lower in the recent woodlands than in the ancient woodlands.
Due to a very slow and differentiated regeneration rate of the field layer, the species with the lowest indicator values for reaction and nitrogen were significantly less abundant in the recent woodlands than in the ancient woodlands, while the species with higher values of these indicators were more frequent. The change of the reaction of many species to light conditions was the main reason for poor prediction of light intensity in the recent woodlands by the mean indicator values for light.
The mean Ellenberg indicator values for light, reaction and nitrogen were relatively good predictors of the environmental conditions in the ancient woodlands, with stabilized species composition determined mainly by the environmental regime. They were, however, much weaker indicators in the recent woodlands. This implies that the mean indicator values for secondary woodlands should be interpreted with caution and verified by measurements of environmental variables.
Germination and establishment of halophytes on brine‐affected soils
Authors: Keiffer Carolyn Howes , Ungar Irwin A. 
Published: March 2003

Summary

Contamination of soils with oilfield brines is a significant environmental problem in many oil‐production areas. This study tested the feasibility of using different salt‐accumulating halophyte species to remediate brine‐contaminated soils at a site in south‐eastern Ohio, USA, and tested whether planting season affected the germination, yield and sodium uptake of selected species.
Atriplex prostrata, Hordeum jubatum, Salicornia europaea, Spergularia marina and Suaeda calceoliformis were sown in October 1992 (autumn sowing) and March 1993 (spring sowing). For A. prostrata and Spergularia marina, spring‐sowing produced greater germination and survival, while H. jubatum exhibited greater germination and survival in autumn sowings. Germination and survival of Salicornia europaea and Suaeda calceoliformis were not significantly affected by sowing season, but Salicornia europaea germinated and survived poorly in both seasons.
Spring‐sown A. prostrata and Suaeda calceoliformis and autumn‐sown H. jubatum and Spergularia marina produced the greatest yields.
High soil conductivities negatively affected the survival and yield of spring‐sown A. prostrata, H. jubatum and Spergularia marina, but did not significantly affect autumn sowings.
Root biomass allocation was greater in autumn‐sown H. jubatum, Spergularia marina and Suaeda calceoliformis. We hypothesized that autumn‐sown seeds of these species have earlier root development that facilitates avoidance of extreme salinity and moisture fluctuations found at the soil surface.
All species accumulated higher amounts of Na+ and Cl− than other ions. Autumn‐sown A. prostrata and Suaeda calceoliformis accumulated significantly more Na+ than when spring‐sown. Ash content (a measure of accumulated inorganic matter) ranged from 15% to 35% in the various plant tissues. Hordeum jubatum had significantly more Na+ in its roots than in its shoots, making it less suitable for site remediation than A. prostrata or Suaeda calceoliformis.
With the exception of Salicornia europaea, all species reduced soil salinity significantly compared with paired control plots, ranging from 4% in H. jubatum to 17% in A. prostrata. Leaching following precipitation resulted in a 44% reduction in Na+ from control plots over a 4‐year period. However, there was a 59% reduction in Na+ from plots vegetated with halophytes over the same period.
The results indicate that establishment of salt‐accumulating halophytes on salt‐affected sites can sufficiently remediate the soil to the point where it can be returned to agricultural productivity or where native plants can invade and become established. The phytoremediation process can be facilitated by tailoring plant selection to site conditions, using inputs of fertilizer and water to enhance the growth of the halophytes, and by harvesting the plants on a regular basis.
Spectral analysis of intercycle heart fluctuations in the diethyl‐ether‐anaesthetized or pithed rat treated with prazosin, dl‐propranolol, endothelin‐1, α‐r atriopeptin and ACE‐inhibitors
Authors: Bernardi M. , Deslauriers R. , Docherty J. , Rossi C. , Rossini L. , Rossini P. , Tonnini C. 
Published: March 2003

1 Frequency‐domain spectral analysis of stationary ECG R‐R intervals was made by fast Fourier transformation (FFT) in conditions of monitored arterial blood pressure and respiratory activity in diethyl‐ether‐anaesthetized and pithed adult rats. This technique yields a number of parameters which allow quantitative evaluation of the non‐random distribution of the mean values obtained in time‐domain studies. The frequency‐analysis method enables the overall heart rate variability to be broken down into its various constituents, which are differently affected by physiological loading and capable of selective reactivity to pharmacological agents. 2 The low‐frequency spectral component obtained by breaking down the total spectral density power, i.e. the appropriate variability signal (band width < 0.15 Hz) and a higher‐frequency band corresponding to spontaneous (0.80–1.60 Hz) or artificially imposed (0.75 Hz) respiratory activity were estimated and their integrated areas evaluated as absolute powers or normalized fractional values. 3 The power of the high‐frequency spectral component increased in all animal preparations under treatment with prazosin, dl‐propranolol, endothelin‐1 and the angiotensin converting enzyme (ACE) inhibitors captopril, lisinopril, quinapril and ramipril. The power of the low‐frequency band increased under α‐r atriopeptin and ACE inhibitors in the pithed preparations only, and decreased in the anaesthetized animals. 4 The new power spectrum features and trends detected indicate that these time‐independent, model‐dependent cardiovascular and respiratory markers are subject to some form of complex peptidergic control. 5 The relative roles of the various factors operating in the genesis of these short‐term changes in spectral power in the low‐ and high‐frequency bands cannot be interpreted as indicating a reciprocal push–pull relationship between sympathetic and parasympathetic control. 6 The study findings, however, can be interpreted as providing evidence of a different and to some extent alternative form of integrative cardiovascular control persisting in the pithed rats (i.e. in the ‘peripheral’, CNS‐destroyed preparations). 7 New areas of theoretical and applied research are being developed in the (auto)classification of (iso)receptors and drug analogues through exploration of multiple physiological and pharmacokinetic parameters in the frequency‐domain. Furthermore, model‐independent frequency‐domain methods not requiring stationary data will afford scope for even more significant developments by separating the overlapping dynamic processes from a whole series of correlated effects.
Effects of some imidazolidine α2‐adrenoceptor agonists in rat isolated atria
Authors: Thaina P. , Nott M. W. , Rand M. J. 
Published: March 2003

1 The prejunctional α2‐adrenoceptor agonist activity of four imidazolidines (UK14819, UK14304, UK15121 and UK11957) were compared to that of clonidine in rat isolated atrial preparations. 2 The preparations consisted of spontaneously beating left and right atrial pairs that were incubated with [3H]‐noradrenaline. The efflux of radioactivity induced by electrical field stimulation of intramural sympathetic nerves was used as an index of neurotransmitter release and inotropic and chronotropic responses were recorded. 3 Two quinoxalinyl imidazolidines (UK14819 and UK14304 which have chloride and bromide substitution, respectively, in the phenyl moiety) caused decreases in the efflux of radioactivity with stimulation at 2 Hz and 10 Hz but not at 20 Hz. These effects were antagonised by the α2‐adrenoceptor antagonist idazoxan (0.3 μm) but they were not affected by the α1‐adrenoceptor antagonist prazosin (0.1 μm). 4 The third halogenated quinoxalinyl imidazolidine analogue (UK15121, which has an iodide substitution in the phenyl ring) and a quinolinyl imidazolidine (UK11957) have actions similar to clonidine. They decreased the efflux of radioactivity with stimulation at 1 Hz (for UK11957) or 2 Hz (for UK15121) and enhanced it at higher frequencies of stimulation. Both the inhibitory and enhancing effects were antagonised by idazoxan but they were not affected by prazosin. 5 Unlike the other three imidazolidines in the present study, the quinolinyl imidazolidine (UK11957), caused a decrease in resting release of radioactivity and this effect was prevented by the monoamine oxidase inhibitor pargyline (30 μm). 6 These findings suggest that the 5‐chloro‐or 5‐bromo substituted quinoxalinyl imidazolidines (UK14819 and UK14304) are full agonists at prejunctional α2‐adrenoceptors, but the 5‐iodo‐substituted quinoxalinyl imidazolidine (UK15121) and the quinolinyl analogue of UK14304 (UK11957), like clonidine, appear to be partial agonists at these receptors.
Effects of the chronic in vivo administration of L‐NAME on the contractile responses of the rat perfused mesenteric bed
Authors: Mendizabal V. E. , Feleder E. C. , Adler‐Graschinsky E. 
Published: March 2003

1 The effects of the chronic in vivo inhibition of nitric oxide synthase (NOS) with Nω‐nitro‐
l‐arginine methyl ester (L‐NAME) on vascular contractility were studied in the rat perfused mesenteric bed. The chronic treatment with L‐NAME during 4 weeks induced a rise in systolic blood pressure (basal: 115.1 ± 6.5 mmHg; chronic L‐NAME treatment: 171.7 ± 7.7 mmHg, n = 16, P < 0.05). 2 After the chronic NOS inhibition, the potentiation of the maximal vasoconstrictor responses to noradrenaline, phenylephrine and KCl was to the same extent as that observed after the in vitro exposure to 100 μm L‐NAME. 3 No further potentiation of the contractile responses was achieved when the mesenteric beds isolated from L‐NAME treated rats were incubated in vitro with 100 μm L‐NAME. 4 The endothelium removal but not the inhibition of prostanoid synthesis with either 10 μm indomethacin or 10 μm 17‐octadecynoic acid potentiated the contractions to noradrenaline and to KCl both under control conditions as well as after the chronic in vivo administration of L‐NAME. 5 These observations taken together suggest that after chronic L‐NAME maximum inhibition of nitric oxide synthase was achieved and no compensatory mechanisms able to counterbalance the increase in contractile responses were developed. 6 Further studies are necessary to elucidate the nature of the factors, other than nitric oxide, that contribute to the potentiation of contractile responses observed when the endothelium is removed after L‐NAME treatment.
Differential blockade of α,β‐methylene‐ATP, noradrenaline and electrically evoked contractions of the rat vas deferens
Authors: Boselli C. , Grana E. 
Published: March 2003

1 The present study was carried out compare the effects of nifedipine, verapamil, papaverine and chloroethylclonidine (CEC) on the electrically‐induced contractions and on the contractions produced by exogenous noradrenaline and α,β methylene‐ATP in the epidydimal portion of rat vas deferens. 2 Nifedipine inhibited in a concentration‐dependent fashion the purinergic component (phase I) of the electrically evoked response. Verapamil (10−−7 M−10−−5 M) did not inhibit significantly or even potentiated phase I of the contractile response to single‐pulse field stimulation but inhibited the response to α,β methylene‐ATP. Papaverine left unaffected phase I but depressed the response to α,β methylene‐ATP. CEC significantly potentiated the purinergic component of the electrically‐evoked response and the response induced by α,β methylene‐ATP. 3 Nifedipine was devoid of any inhibitory action on the noradrenergic component (phase II) of the response to single shock. Verapamil at the highest concentration used (10−−5 M) was able to partially inhibit phase II. Papaverine abolished in a concentration‐dependent manner the noradrenergic component of the response to single shock. CEC abolished the phase II of single shock while was devoid of any inhibitory action on exogenous noradrenaline. 4 The implications of these differences among the compounds studied in the present work are discussed in relation to roles of calcium channels.