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491.
M.D. Fraser V.J. TheobaldM.S. Dhanoa O.D. Davies 《Agriculture, ecosystems & environment》2011,144(1):102-106
A comparative experiment was conducted to test the extent to which grazing of rank Molinia caerulea-dominated grassland increases botanical diversity and improves animal performance. The study was carried out on an area of under-grazed Molinia-dominant pasture that had not been stocked for 20 years. Three treatments were compared from 2001 until 2008: (1) no grazing, (2) grazing by Welsh Mountain sheep and (3) grazing by Welsh Black heifers. Patterns of change in sward surface composition across the grazing season were consistent with greater consumption of Molinia by the cattle, but there was little evidence of management regime influencing changes in sward composition year-on-year. There was a significant effect of year on animal performance (P < 0.001). Although the growth rates of the heifers improved over the course of the first three years of the experiment, this performance was not sustained. In contrast, any weight changes for the sheep were positive. The results highlight the slow pace of change within upland swards, and the need to develop site-specific grazing guidelines. 相似文献
492.
Lu Y Beeman RW Campbell JF Park Y Aikins MJ Mori K Akasaka K Tamogami S Phillips TW 《Die Naturwissenschaften》2011,98(9):755-761
We report that the abdominal epidermis and associated tissues are the predominant sources of male-produced pheromones in the
red flour beetle, Tribolium castaneum and, for the first time, describe the stereoisomeric composition of the natural blend of isomers of the aggregation pheromone
4,8-dimethyldecanal (DMD) in this important pest species. Quantitative analyses via gas chromatography–mass spectrometry showed
that the average amount of DMD released daily by single feeding males of T. castaneum was 878 ± 72 ng (SE). Analysis of different body parts identified the abdominal epidermis as the major source of aggregation
pheromone; the thorax was a minor source, while no DMD was detectable in the head. No internal organs or obvious male-specific
glands were associated with pheromone deposition. Complete separation of all four stereoisomers of DMD was achieved following
oxidation to the corresponding acid, derivatization with (1R, 2R)- and (1S, 2S)-2-(anthracene-2,3-dicarboximido)cyclohexanol to diastereomeric esters, and their separation on reversed-phase high-performance
liquid chromatography at −54°C. Analysis of the hexane eluate from Porapak-Q-collected volatiles from feeding males revealed
the presence of all four isomers (4R,8R)/(4R,8S)/(4S,8R)/(4S,8S) at a ratio of approximately 4:4:1:1. A walking orientation bioassay in a wind tunnel with various blends of the four synthetic
isomers further indicated that the attractive potency of the reconstituted natural blend of 4:4:1:1 was equivalent to that
of the natural pheromone and greater than that of the 1:1 blend of (4R,8R)/(4R,8S) used in commercial lures. 相似文献
493.
Achievement of at least “good ecological status” in all waterbodies under the EU Water Framework Directive by 2015 will in some cases be a challenge. The twin challenge is to manage expectations of policy makers for such waterbodies as to a realistic length of time required for improvement in water quality. Hence, understanding the source, transformation processes and residence time of nitrate in a hydrological system is an essential part of meeting such challenges. On a dairy farm with 24 shallow groundwater wells, the dual isotopic composition of nitrate (δ15N and δ18O) was used to clarify nitrate sources, to assess spatial and temporal variability in nitrate concentrations and to determine if and where denitrification was occurring. Vertical travel time was estimated to correlate nitrate concentrations with management practices. Organically derived nitrogen was the predominant source contributing to groundwater nitrate concentrations. Denitrification was identified as prevalent within specific regions of the study site. The distinct low temporal variability in the isotopic data suggests constancy among nitrate sources and processes over time across the study site. Vertical travel times of up to 3 years were estimated on site indicating the influence of recent management practices on nitrate concentrations. Very slow horizontal migration of groundwater (decades) indicates a legacy of older management practices. Stable isotope techniques, together with an understanding of time lag, provide an extra mechanism to test the efficacy of monitoring and mitigation programmes. 相似文献
494.
The sympatric neotropical termites Nasutitermes corniger and Nasutitermes ephratae are clearly distinguishable based on morphology, nest architecture, defensive secretion composition, and molecular markers.
However, given the extensive ecological, geographical, and behavioral overlap of these closely related species, the potential
for interbreeding may exist. To explore this possibility, heterospecific pairs were formed experimentally to examine courtship
and colony-establishment behaviors, and reproductive potential. Courtship and nest construction behavior occurred in heterospecific
pairs in a similar manner to that of conspecific pairs. Survival of pairs depended upon the species of the female partner.
N. ephratae females paired with N. corniger males produced as many offspring as conspecific pairs. N. corniger females mated to N. ephratae males, however, produced significantly fewer offspring at 60 days post-establishment than the reciprocal cross or conspecific
N. ephratae or N. corniger pairs. This was also the only pairing in which any aggression was observed. Heterospecific pairs and groups formed in mate
choice mesocosms, suggesting that species recognition between these two termites is not an important aspect of mate choice.
Overall, species mismatch tolerance and hybrid offspring viability are high. The present data, together with previous evidence
from defensive secretions and isozyme analysis, suggest that hybridization may periodically occur in nature, and that reproductive
barriers between these two species may be incomplete. Hybridization could provide a rare but important source of genetic diversity
and may ensure mating opportunities for the more abundant sex of alates in each species. 相似文献
495.
496.
497.
El Samad O Zahraman K Baydoun R Nasreddine M 《Journal of environmental radioactivity》2007,92(2):72-79
Fallout from the Chernobyl reactor accident due to the transport of a radioactive cloud over Lebanon in the beginning of May 1986 was studied 12 years after the accident for determining the level of (137)Cs concentration in soil. Gamma spectroscopy measurements were performed by using coaxial high sensitivity HPGe detectors. More than 90 soil samples were collected from points uniformly distributed throughout the land of Lebanon in order to evaluate their radioactivity. The data obtained showed a relatively high (137)Cs activity per surface area contamination, up to 6545Bqm(-2) in the top soil layer 0-3cm. The average activity of (137)Cs in the top soil layer 0-3cm in depth was 59.7Bqkg(-1) dry soil ranging from 15 to 119Bqkg(-1) dry soil. The horizontal variability was found to be about 45% between the sampling sites. The depth distribution of total (137)Cs activity in soil showed an exponential decrease. Estimation of the annual effective dose due to external radiation from (137)Cs contaminated soil for selected sites gave values ranging from 19.3 to 91.6 micro Svy(-1). 相似文献
498.
In beaver ponds, the species diversity, abundance, biomass, and production of cladocerans and the amount of food consumed by them have increased and the composition of dominant species has changed as compared to those in the stretches of small rivers with running water or dammed by man. The stimulating effect of water from a beaver pond has been confirmed in laboratory experiments. The average number of juvenile Ceriodaphnia affinis produced by one female was considerably greater in this water than in the water from flowing and dammed (by man) river stretches. 相似文献
499.
An inventory of rock vegetation has been taken in river valleys on the eastern slope of the southern part of the Northern Urals and the northern part of the Middle Urals. New habitats of some rare plant species, relict communities of forest-steppe species at the northern limit of their distribution, and ultrasmall populations of petrophilous species growing in isolation for a long time have been revealed. In studies on a group of similar specially protected areas (e.g., within river valleys), it is proposed to identify key sites distinguished by the highest species richness of vegetation 相似文献
500.
The dependence of the annual increment of vegetative organs on air temperature and humidity and the amount of precipitation per phenophase, as well as the period of their growth in Scots pine, were studied in the forest-steppe zone of Cisbaikalia. The multiple effect of variation in environmental conditions on the annual increment of organs increased when precipitation in September of the previous year was taken into account. These factors proved to have a stronger effect on variation in the annual increment of needles (R 2 = 0.59, p = 0.00005) than that in the annual ring width (R 2 = 0.38, p = 0.0002). 相似文献