Oscillations in a system with material cycling

We study a system of two integrodifierential equations which models the evolution of a biotic species feeding on an abiotic resource. We also consider nutrient recycling with time delay. By Hopf bifurcation theory we prove the existence of stable oscillations for a range of values of the input of nutrients.Work performed within the activity of the research group Evolution Equations and Physico-Mathematical Applications, M.P.I. (Italy), and under the auspices of G.N.F.M., C.N.R. (Italy)     

Interactions of piperidine derivatives with the nicotinic cholinergic receptor complex from Torpedo electric organ

The interactions of eight piperidine derivatives with nicotinic receptor complexes fromTorpedo californica electric organ were studied using [¹²⁵I]alpha-bungarotoxin ([¹²⁵I]BGT) as a probe for the acetylcholine binding site and [³H]perhydrohistrionicotoxin ([³H]H₁₂-HTX) as a probe for a site associated with the receptor-gated ion channel.Cis- andtrans-2-methyl-6-n-undecanyl piperidines (MUP), major constituents of fire ant venom, had a high-affinity for [³H]H₁₂-HTX binding sites (K_i=0.08–0.24 M), but had no affect on receptor binding. MUP affinity for [³H]H₁₂-HTX binding sites was approximately doubled in the presence of 1 M carbamylcholine. Introduction of a 2-hydroxyl group to the undecanyl side channel had little effect on activity of the alkaloid. The analog 2,6- (but not 3,5-) dimethylpiperidine was a moderately active inhibitor of [³H]H₁₂-HTX binding (K _i-8.8 M). 2-Methylpiperidine was considerably less active (K _i=600 M), although it was more potent than either 3- or 4-methylpiperidine. The affinities of 2,6-dimethylpiperidine and 2-methylpiperidine for [³H]H₁₂-HTX binding sites were decreased in the presence of 1 M carbamylcholine. Carbamylcholine affinity for the receptor was increased by up to 7 fold in the presence of 10 and 32 M MUP, but was decreased in the presence of 2,6-dimethylpiperidine and 2-methylpiperidine. Thecis- andtrans-isomers of MUP were equipotent in producing each of its effects. In these actions, MUP resembles a variety of other compounds derived from 2,6-disubstituted piperidines, including histrionicotoxins, gephyrotoxins and pumiliotoxins. These studies establish the importance of alkyl substitutions in theortho position of the piperidine ring in conferring ion channel specificity, and the importance of substantial alkyl side chains in conferring the ability of channel blockers to stabilize the nicotinic receptor complex in high affinity, desensitized conformations.     

Effects of simulated fire on vesicular-arbuscular mycorrhizae in pinyon-juniper woodland soil

Effects of fire on vesicular-arbuscular mycorrhizal fungi were tested using microcosms constructed from soil, litter, and duff collected beneath canopies of pinyon pine, Utah juniper, and in the open space (interspace). Burning was conducted over wet and dry soils. Soil temperatures were monitored continuously throughout the microcosms during burning. Plants grown in soils burned when dry had a lower VAM colonization than soils burned when wet. Juniper soils demonstrated the greatest reduction, over 95%, compared to their respective controls. Plants grown in interspace soils burned when wet were least affected. There was a positive correlation (r²=0.90) between the decrease in VAM colonization and the soil temperature as a result of the fire. Temperature effects, and associated reductions in VAM, were related to amount of litter burned in each microcosm and the moisture content of the soils.     

The effect of fluctuations in tidal inundation frequency on a salt-marsh vegetation

Over a period of 15 years recordings were made of the species cover in permanent plots on the salt marsh of one of the West Frisian Islands, Schiermonnikoog (The Netherlands). Correlations between annual changes in the cover of the major species, and fluctuations in the monthly frequency of inundation by seawater were studied. First, a spectral analysis was carried out on the inundation frequency data to look for predictable patterns. Subsequently, fluctuations were defined as deviations from these predictable patterns. In a repeated multiple regression model, the effects of the season in which the fluctuations occurred, and the elevational position of the plots on the salt marsh were studied as factors influencing the correlation patterns. The behaviour of various species is discussed in relation to their seed bank characteristics and their salt tolerance.     

Response to grazing after nine years of cattle exlusion in a Flooding Pampa grassland,Argentina

This paper reports on changes induced by the introduction of cattle in a grassland that had remained ungrazed for 9 yr, in comparison with two adjacent grasslands: one that remained enclosed and one that has been continuously subject to grazing. Basal cover was measured on 25 interception lines, each 1 m long, three times during one year. The variables studied were: total cover, cover of grasses and dicots, cover of creeping grasses, floristic composition, and dissimilarity among sites. At the first sampling, 2 yr after cattle re-introduction, the newly grazed site was more similar to the ungrazed than to the grazed site. The newly grazed site had very low cover of dicots; the species of dicots present were different from those found in the continuously grazed area. Creeping grasses had higher cover in the newly grazed site than in the other sites, and continued to increase. At the last sampling, one year later, the newly grazed site had become more similar to the contiuously grazed site. Only after 5 yr of cattle grazing the exotic dicots that were dominant in the continuously grazed site, were recorded in the re-opened site. The absence of propagules of these species or the absence of safe sites may account for this delayed invasion.     

美洲蜚蠊机械感受器输出信号的计算机分析

对蜚蠊单个机械感受器诱发反应峰电位的历程及幅度,使用系列分析、栅分析、时序分布、累加密度函数、栅—频分析、特征参数的伪三维隐线显示、峰电位幅度概率密度函数、峰电位间隔概率密度函数等分析方法,获得较多的神经信号间隔编码信息,以揭示刺激和反应之间的复杂关系。峰电位间隔和栅—频分析图由函数经曲线拟合后,分别求得描述其动态过程的时间常数τ_1,τ_2和τ_3,应用上述多种分析显示方法,使研究者更为直观地观察和定量描述刺激—反应间的动态关系。     

Some epidemiological models with nonlinear incidence

Epidemiological models with nonlinear incidence rates can have very different dynamic behaviors than those with the usual bilinear incidence rate. The first model considered here includes vital dynamics and a disease process where susceptibles become exposed, then infectious, then removed with temporary immunity and then susceptible again. When the equilibria and stability are investigated, it is found that multiple equilibria exist for some parameter values and periodic solutions can arise by Hopf bifurcation from the larger endemic equilibrium. Many results analogous to those in the first model are obtained for the second model which has a delay in the removed class but no exposed class.Research supported in part by Centers for Disease Control Contract 200-87-0515. Support services provided at University House Research Center at the University of IowaResearch supported in part by NSERC A-8965 and the University of Victoria President's Committee on Faculty Research and Travel     

A fluorescent sterol probe study of cholesterol/phospholipid membranes

The behavior of dehydroergosterol in -α-dimyristoylphosphatidylcholine (DMPC) unsonicated multilamellar liposomes was characterized by absorption spectroscopy and fluorescence measurements. Dehydroergosterol exhibited a lowered absorption coefficient in multilamellar liposomes whiel the steady-state fluorescence anisotropy of dehydroergosterol in these membranes decreased significantly with increasing dehydroergosterol concentration, suggesting membrane sterol-sterol interactions. The comparative steady-state anisotropy of 0.9 mole percent dehydroergosterol in multilamellar liposomes was lower than in small unilamellar vesicles suggesting different sterol environments for dehydroergosterol. Dehydroergosterol fluorescence lifetime was relatively independent of membrane sterol content and yielded similar values in sonicated and unsonicated model membranes. In multilamellar liposomes containing 5 mole percent cholesterol, the gel-to-liqui crystalline phase transition of DMPC detected by 0.9 mole percent dehydroergosterol was significantly broadened when compared to the phase transition detected by dehydroergosterol in the absence of membrane cholesterol (Smutzer, G. et al. (1986) Biochim. Biophys. Acta 862, 361–371). In multilamellar liposomes containing 10 mole percent cholesterol, the major fluorescence lifetime of dehydroergosterol did not detect the gel-to-liquid crystalline phase transition of DMPC. Time-correlated fluorescence anisotropy decays of dehydroergosterol in DMPC multilamellar liposomes in the absence and presence of 5 mole percent cholesterol exhibited a single rotational correlation time near one nanosecond that was relatively independent of temperature and low concentrations of membrane cholesterol. The limiting anisotropy of 0.9 mole percent dehydroergosterol decreased above the gel-to-liquid crystalline phase transition in membranes without cholesterol and was not significantly affected by the phase transition in membranes containing 5 mole percent cholesterol. These results suggested hindered rotational diffusion of dehydroergosterol in multilamellar liposomes. Lifetime and time-correlated fluorescence measurements of 0.9 mole percent dehydroergosterol in multilamellar liposomes further suggested this fluorophore was detecting physical properties of the bulk membrane phospholipids in membranes devoid of cholesterol and was detecting sterol-rich regions in membranes of low sterol concentration.     

论生态平衡和林火烈度

 本文探讨林火对森林生态系统的干扰,提出火烈度指标作为衡量标准。建议采用林木蓄积量作为森林生态系统的状态指数。根据林火蔓延过程的物理分析,导出一种模型化的燃烧蔓延方程,并对兴安落叶松(Larix gmelinii)在不同蔓延速度下计算了火烈度。结合火烧迹地调查资料指出在不同火烈度下,林火对森林生态系统的影响及其对策。     

Techniques for studying the execution of foreign queens by temporal subcastes in fire ants (Solenopsis invicta)

Using monogynous and polygynous colonies of the fire ant, Solenopsis invicta Buren, we developed methodology to determine which behavioural subcaste of workers was responsible for executing unfamiliar queens that were introduced to them. Separation of subcastes and introduction of queens to each of these separately was partly effective, but more accurate results were obtained by removing workers that were in the act of executing queens, marking them individually, and returning them to the colony. Their subsequent behaviour was then recorded to determine whether they behaved like nurses, reserves, or foragers. The results showed that foragers are dominant in execution behaviour and that all morphological subcastes (minors, medias, and majors) are involved. This methodology may be applied to other social insect species.

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Résumé A l'aide de colonies monogynes de S. invicta Buren, nous avons testé la sensibilité de deux méthodes: Premièrement, nous avons isolé les trois sous-castes temporelles des ouvrières (nurses, réserves et fourrageuses) auxquelles nous avons introduit des reines étrangères. Nous avons trouvé que les fourrageuses étaient le groupe le plus agressif, exécutant un pourcentage élevé des reines qui leur étaient présentées. Le degré de physogastrie de la reine étrangère, sa colonie d'origine et la présence de couvain n'avaient pas d'effet visible sur sa destinée. La sensibilité de cette méthode était limitée par la difficulté d'une séparation complète des souscastes temporelles.Deuxièmement, nous avons capturé des ouvrières exécutant activement des reines étrangères, après marquage nous les avons remises dans leur colonie pour observations ultérieures. Nous avons remarqué que leur localisation dans le nid et leur comportement correspondaient aux séquences trouvées pour les fourrageuses bien plus qu'à celles des ouvrières réserves ou nurses. Cette méthode est plus sensible. Des mesures de la largeur de la capsule céphalique de ces ouvrières ont montré qu'aucune des sous-caste morphologiques (mineurs, moyennes et majeurs) étaient dominantes dans le comportement d'exécution. Nos résultats indiquent que la sensibilité des ouvrières à certaines phéromones de la reine et odeur de la colonie augmente avec leur âge.