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101.
Russell Dinnage Marc W. Cadotte Nick M. Haddad Gregory M. Crutsinger David Tilman 《Ecology letters》2012,15(11):1308-1317
Large‐scale habitat destruction and climate change result in the non‐random loss of evolutionary lineages, reducing the amount of evolutionary history represented in ecological communities. Yet, we have limited understanding of the consequences of evolutionary history on the structure of food webs and the services provided by biological communities. Drawing on 11 years of data from a long‐term plant diversity experiment, we show that evolutionary history of plant communities – measured as phylogenetic diversity – strongly predicts diversity and abundance of herbivorous and predatory arthropods. Effects of plant species richness on arthropods become stronger when phylogenetic diversity is high. Plant phylogenetic diversity explains predator and parasitoid richness as strongly as it does herbivore richness. Our findings indicate that accounting for evolutionary relationships is critical to understanding the severity of species loss for food webs and ecosystems, and for developing conservation and restoration policies. 相似文献
102.
M. L. CHAVES M. RIERADEVALL P. CHAINHO J. L. COSTA M. J. COSTA N. PRAT 《Freshwater Biology》2008,53(1):55-76
1. Macroinvertebrate assemblages of five non‐glacial intermittent high altitude headwater streams (above 1400 m – Serra da Estrela, Portugal), with dry periods of different lengths (0–3 months), were investigated in nearly undisturbed conditions to (i) examine spatial differences and identify environmental variables responsible for the observed invertebrate patterns, (ii) assess the association of dry period length with invertebrate community structure and (iii) determine the influence of using different taxonomic identification levels (order, family and genus) to assess invertebrate community patterns. 2. More than 100 macroinvertebrate genera were identified. Insects clearly dominated these communities with more than 95% of total captures and around 95% of the total richness. Diptera were the most rich and abundant group with chironomid occurrences comprising over 70% of macroinvertebrate captures. 3. The highest taxon richness, diversity, EPT (Ephemeroptera + Plecoptera + Trichoptera) and OCH (Odonata + Coleoptera + Heteroptera) genus richness, the greatest number of exclusive and characteristic taxa identified by the Indicator Value (IndVal), and a distinct community structure shown by Canonical Correspondence Analyses (CCA), were found in the only stream that was never totally dry, with pools lasting over summer. Environmental gradients that spatially structured the macroinvertebrate communities were always related to flow variations. 4. Over time, the highest abundances found in these systems were also related to flow variations and maximum genus richness occurred in the connected pools or in isolated pools. Streams with longer dry periods presented a distinct recolonization phase, with higher abundance of the stonefly larvae Nemoura sp. and the presence of the chironomid larvae Krenosmittia sp., possibly arriving from the hyporheos. 5. Taxonomic level of invertebrate identification was vital for recognizing the characteristic taxa (IndVal) of streams yet was not critical for identifying streams with the highest macroinvertebrate richness/diversity or structuring environmental gradients. 6. Overall, this study emphasizes the variability of high altitude intermittent streams macroinvertebrate communities, despite spatial proximity. This variability was probably related to flow intermittency and hydrologic permanence, different vegetation covers and riverbed substrata. Consequently, the establishment of reference conditions should involve long‐term data collections and more detailed physical characterization. Also, these findings have significant implications for accurately predicting the ecological consequences of future climate change in high altitude scenarios. 相似文献
103.
This paper combines the data sets available today for 14C-age calibration of the last 60 ka. By stepwise synchronization of paleoclimate signatures, each of these sets of 14C-ages is compared with the U/Th-dated Chinese Hulu Cave speleothem records, which shows global paleoclimate change in high temporal resolution. By this synchronization we have established an absolute-dated Greenland-Hulu chronological framework, against which global paleoclimate data can be referenced, extending the 14C-age calibration curve back to the limits of the radiocarbon method. Based on this new, U/Th-based GreenlandHulu chronology, we confirm that the radiocarbon timescale underestimates calendar ages by several thousand years during most of Oxygen Isotope Stage 3. Major atmospheric 14C variations are observed for the period of the Middle to Upper Paleolithic transition, which has significant implications for dating the demise of the last Neandertals. The early part of “the transition” (with 14C ages > 35.0 ka 14C BP) coincides with the Laschamp geomagnetic excursion. This period is characterized by highly-elevated atmospheric 14C levels. The following period ca. 35.0-32.5 ka 14C BP shows a series of distinct large-scale 14C age inversions and extended plateaus. In consequence, individual archaeological 14C dates older than 35.0 ka 14C BP can be age-calibrated with relatively high precision, while individual dates in the interval 35.0-32.5 ka 14C BP are subject to large systematic age-‘distortions,’ and chronologies based on large data sets will show apparent age-overlaps of up to ca. 5,000 cal years. Nevertheless, the observed variations in past 14C levels are not as extreme as previously proposed (“Middle to Upper Paleolithic dating anomaly”), and the new chronological framework leaves ample room for application of radiocarbon dating in the age-range 45.0-25.0 ka 14C BP at high temporal resolution. 相似文献
104.
Joshua Ladau 《Theoretical Ecology》2008,1(4):241-248
A long-standing question in ecology is whether interspecific competition affects co-occurrence patterns of species. Null model
tests of presence–absence data (NMTPAs) constitute an important approach to address the question, but different tests often
give conflicting results when applied to the same data. Neyman–Pearson hypothesis testing theory provides a rigorous and well
accepted framework for assessing the validity and optimality of statistical tests. Here, I treat NMTPAs within this framework,
and measure the robustness and bias of 72 representative tests. My results indicate that, when restrictive assumptions are
met, existing NMTPAs are adequate, but for general testing situations, the use of all existing NMTPAs is unjustified — the
tests are nonrobust or biased. For many current applications of NMTPAs, restrictive assumptions appear unmet, so these results
illustrate an area in which existing NMTPAs can be improved. In addition to highlighting useful improvements to existing NMTPAs,
the results here provide a rigorous framework for developing improved methods. 相似文献
105.
C. M. Gómez 《Journal of biosciences》2008,33(5):743-753
The relationship between stimulus intensity and the probability of detecting the presence of the stimulus is described by
the psychometrical function. The probabilistic nature of this relationship is based on the stochastic behaviour of sensory
neural channels and sensory networks involved in perceptual processing (Kiang 1968). This study tries to establish a continuum
of variability across different levels of integration in the central nervous system. Once the opening and closing times of
ionic channels was simulated, a threshold to the collective behaviour of voltage-gated ionic channels was imposed in order
to generate the spike train of a single neuron. Afterwards, the trains of spikes of different neurons were added up, simulating
the activity of a sensory nerve. By adding the activity due to the stimulus to the spontaneous neural behaviour, the psychometric
function was simulated using a thresholding approach. The results can replicate the stochastic resonance phenomenon, but also
open up the possibility that attentional phenomena can be mediated not only by increasing neural activity (bursting or oscillatory),
but also by increasing noise at the neural level. 相似文献
106.
Rapid induction of pluripotency genes after exposure of human somatic cells to mouse ES cell extracts 总被引:5,自引:0,他引:5
Bru T Clarke C McGrew MJ Sang HM Wilmut I Blow JJ 《Experimental cell research》2008,314(14):2634-2642
The expression of 4 pluripotency genes (Oct4, Sox2, c-Myc and Klf4) in mouse embryonic fibroblasts can reprogramme them to a pluripotent state. We have investigated the expression of these pluripotency genes when human somatic 293T cells are permeabilized and incubated in extracts of mouse embryonic stem (ES) cells. Expression of all 4 genes was induced over 1–8 h. Gene expression was associated with loss of repressive histone H3 modifications and increased recruitment of RNA polymerase II at the promoters. Lamin A/C, which is typically found only in differentiated cells, was also removed from the nuclei. When 293T cells were returned to culture after exposure to ES cell extract, the expression of the pluripotency genes continued to rise over the following 48 h of culture, suggesting that long-term reprogramming of gene expression had been induced. This provides a methodology for studying the de-differentiation of somatic cells that can potentially lead to an efficient way of reprogramming somatic cells to a pluripotent state without genetically altering them. 相似文献
107.
Prolactin (PRL) promotes maternal behavior (MB), a complex pattern of behavior aimed at maximizing offspring survival. 3α,5α-reduced
neurosteroids may also regulate MB. Indeed, PRL, 3α,5α-reduced neurosteroids, and 5α-reductase (5α-R), the key enzyme in the
biosynthesis of these neuroactive steroids, are all increased in stress situations These facts led us to hypothesize a possible
interrelation between PRL levels and 5α-R. In the present study we quantified mRNA levels of both 5α-R isozymes in prefrontal
cortex of male and female rats after administration of sulpiride, an inductor of PRL secretion. Our results demonstrated that
mRNA levels of both 5α-R isozymes were significantly increased in male and female rats by sulpiride, directly or via sulpiride-induced
hyperprolactinemia. Since 3α,5α-reduced neurosteroids and PRL exert anxiolytic effects in response to stress, these molecules
and 5α-R may possibly participate in a common pathway of significant adaptation to stress situations. 相似文献
108.
Jean‐Pierre Flatt 《Obesity (Silver Spring, Md.)》2004,12(12):2013-2022
Objective: A systems dynamics computer model was developed to examine how the interactions between carbohydrate and fat metabolism influence body weight regulation. It reflects the operation of a two reservoir‐system: one representing the body's limited glycogen, and the other, its large fat reserves. The outflows from the reservoirs correspond to the oxidation of glucose and fat, whose relative contributions are affected by the size of the prevailing glycogen and fat reserves. Together, they meet the body's energy expenditure. Replenishments occur three times per day, in portions restoring total glycogen content to specific levels. A parameter mimicking the action of insulin is necessary to create realistic responses. Research Methods and Procedures: The model was run for 125‐day periods to establish the degree of adiposity for which rates of fat oxidation become commensurate with fat intake and the influence thereon of various dietary, environmental, lifestyle, and inherited variables. Results: Equivalent degrees of adiposity can be sustained under a variety of conditions. For instance, the impact on steady‐state body fat contents of a 10% increase or decrease in the energy provided by dietary fat is offset by a 26‐gram decrease or increase in mean glycogen levels. Discussion: Environmental factors such as food diversity, palatability, and availability can be expected to raise the range within which glycogen levels are habitually maintained. This restrains fat oxidation, until expansion of the fat mass is sufficient to promote fat oxidation to a rate commensurate with dietary fat intake. This metabolic leverage can explain why increased food offerings tend to raise the prevalence of obesity. 相似文献
109.
中华芦荟组培苗与正常苗某些理化特性的比较研究 总被引:3,自引:0,他引:3
对中华芦荟(Aloe vera L.var.chinensis) 组培苗与正常苗的干重,含水量,可溶性糖含量,叶绿素及类胡萝卜素含量,蛋白质含量及蛋白质电泳,光合速率,呼吸速率,超氧化物歧化酶活性等理化指标进行了比较研究,结果表明,中华芦荟组培苗与正常苗的上述各项指标均差异不显著,证明植物组织培养方法是中华芦荟快繁的一条有效途径,可用组培苗来代替正常的扦插苗。 相似文献
110.
Beyond genomics 总被引:5,自引:0,他引:5
David A. Fell 《Trends in genetics : TIG》2001,17(12):78-682
The added value obtained by combining genomic analysis with proteomic analysis and supplementary information on mechanisms of interaction has been well illustrated in a recent paper by Ideker et al. This gives a far more meaningful interpretation of metabolic responses to a mutation or change in environment than could be obtained from correlation of expression profiles alone, and it provides evidence that addition of metabolomics data would give even further gains. The results also hint at competition between mRNA molecules for translation, so that responses at the protein level are attenuated relative to changes in gene expression. 相似文献