Effect of tetanus toxoid inoculation on mortality in the Cayo Santiago macaque population

Tetanus was a major cause of mortality in the free-ranging population of rhesus monkeys (Macaca mulatta) on Cayo Santiago. From 1977 to 1984 the mean (±1 SD) annual total mortality rate (excluding neonatal deaths within 48 h postpartum, abortions, and stillbirths) was 6.39% ± .94%, and the mean annual tetanus mortality rate was 1.33% ± .45%. Tetanus deaths accounted for 19.5% of the total mortality in the colony. In 1985, all monkeys on the island, except infants and six adult monkeys, were given primary inoculations of tetanus toxoid. The following year, boosters were administered, and yearlings received primary inoculations. One fatal case of tetanus and one recovery from mild disease occurred in uninoculated adult monkeys in 1985, but no additional cases have been observed since. For 1985–1986 the mean annual total mortality rate was 3.69% ± .05%, and the mean annual tetanus mortality rate was .08% ± .08%. Thus, during the 2 years after inoculation against tetanus, the mean annual total mortality rate and the mean annual tetanus mortality rate declined by 42.2% and 94.0%, respectively, when compared to the 8-year period (1977–1984) prior to inoculation. These differences were significant [(χ² = 12.48; P < .005), (χ² = 16.94; P < .005)]. The elimination of tetanus infections through mass inoculation of the Cayo Santiago colony is expected to have a profound impact on the demography of the population by increasing the rate of population growth, by decreasing the differential rates of increase of the component social groups, and by changing the age distribution of the population.     

Effect of plant population on the production and distribution of dry matter in maize

Grain and total dry weight of maize (Zea mays L.) at final harvest increased by about 50% and 30%, respectively, when the population was increased from 23000 to 48000 plants/ha, but did not increase further with population up to 74000/ha. Between 6 and 10 weeks after sowing, crop growth rate (C) increased by about 40% with increase in leaf-area index (L) from 2 to 5·5, despite a concomitant increase in moisture stress. Immediately after flowering, which occurred at 10–11 weeks, C increased up to the greatest L attained, 6·7, and would probably have reached a maximum, with L about 7, when nearly all the incident light would be intercepted by foliage. Later, the L at which C was maximal decreased, probably because the photosyhthetic efficiency of leaves decreased faster with time as L increased; 18 weeks after sowing, when L varied between 3 and 5·5, C was maximal when L was 5. The proportion of grain in the dry matter accumulated after flowering increased from 73% when population was 23000/ha to 82% when it was 60000–74000/ha. Stem weight increased until 3–4 weeks after flowering, then remained constant when population was 23000/ha, but decreased in denser populations. The fraction of the ear consisting of grain increased with population.     

Quantitative studies on the regeneration of myelinated nerve fibers. II. Variations of the number and size of regenerating nerve fibers after localized crushing or total section]

1. The number and size of myelinated nerve fibers have been determined at standard levels, in the nerve to medial head of right and left gastrocnemius muscles of 112 rats in which the left sciatic nerve had suffered an experimental lesion according one of the following four modalities: localized crushing, total section followed or not by suture and resection of a nervous segment of about 1 cm. 2. In the nerve to medial head of right gastrocnemius muscle (contralateral nerve used as control), the number of myelinated fibers decreased in average to 10% after crushing, 5% or 4% after section followed or not by suture. However, an increase of 6% was observed after resection. The mean values of the mean diameters showed a decrease of 8% after crushing and 5% after section without suture. This value did not seem to be affected by section followed by immediate suture and after resection, it increased of 11%. On the whole, male rats appeared to be more sensitive than female to the effects of the operation. 3. The nerves of 12 rats have been observed from 15 to 334 days after resection of about 1 cm of sciatic nerve. The 20% of the regenerating myelinated nerve fibers which have succeeded to cross over such a distance had a distribution which remained unimodal; the diameter of the large fibres did not exceed 8 micronm. 4. 34 rats have been sacrificed from 15 to 715 days after sciatic nerve section which was not followed by suture. The number of myelinated nerve fibers became normal again during the 4th month and reached afterwards a mean value of 130%, with very marked variations. The nerve fibre distribution was most frequently unimodal, but may came bimodal one year after the operation in certain nerves. Their mean diameter never exceeded 60% of the normal. 5. The nerves of 34 rats have been examined from 15 to 720 days after section and immediate suture. The number of myelinated nerve fibers returned to normal during the second month and increased afterwards to an average of 150% with very important variations. The nerve fiber distribution was generally unimodal, but may become bimodal 7 months after the operation. Their mean diameter reached only 50 to 55% of the normal. 6. 32 rats have been sacrificed from 10 to 720 rats after a localized crushing. The number of myelinated nerve fibers come back to normal during the 4th week and later increased up to a mean of 115%. Their distribution became early bimodal from the 97th day onwards. Although, their mean diameter nerver exceeded 80% of the normal, the histograms of the regenerating nerve and of the control nerve could be almost superposed during the second year.     

Interactions between Operophtera brumata and Tortrix viridana on oak: new evidence from time-series analysis

1. Yearly population estimates of the green oak leaf roller moth, Tortrix viridana (Lepidoptera: Tortricidae), and the winter moth, Operophtera brumata (Lepidoptera: Geometridae), were taken from the pedunculate oak, Quercus robur, from 1951 to 1966 in Wytham Woods, Oxfordshire, U.K. Larval mortality from parasitoids was recorded for both species over the same period. 2. Operophtera brumata population density fluctuated around a constant mean, whereas T. viridana population density exhibited a linear decline over the sampling period. Population counts were subjected to time-series analysis after the linear decline was removed from the T. viridana data by detrending. Multiple regression models were built so that variation in the per capita rate of increase of each population could be partitioned among (a) current and previous population density, (b) current and previous population density of the second moth species, and (c) rates of larval parasitism. 3. Multiple regression analysis suggested that variation in the per capita rate of increase of O. brumata could be explained by negative feedback from O. brumata density at T–2 (32%), a negative relationship with T. viridana density at T–1 (18%), and a positive relationship with parasitism suffered by T. viridana at T–1 (24%). 4. The T. viridana population time-series was dominated by a rapid feedback process such that per capita rate of increase at time T was negatively related to population density at time T–1, explaining 53% of variance in population growth rate. Per capita rate of increase was unrelated to larval parasitism or densities of O. brumata. 5. In light of previous life-table and experimental studies on O. brumata and T. viridana, the current analyses suggest that pupal predation (data not presented) and interspecific competition are significant determinants of O. brumata population growth rates in Wytham Woods. In contrast, T. viridana population growth rates appear to be dominated by rapid negative feedback consistent with intraspecific competition.     

Changes of Islet Size and Islet Size Distribution Resulting from Protein-Malnutrition in Lean (Fa/Fa) and Obese (fa/fa) Zucker Rats

TSE, ELIZABETH O, FRANCINE M GREGOIRE, BRIGITTE REUSENS, CLAUDE REMACLE, JOSEPH J HOET, PATRICIA R JOHNSON, JUDITH S STERN. Changes of islet size and islet size distribution resulting from protein malnutrition in lean (Fa/Fa) and obese (fa/fa) Zucker rats. Potential alterations in islet size and islet size distribution resulting from protein malnutrition were studied in lean (Fa/Fa) and obese (fa/fa) Zucker rats. The purpose was to investigate whether the distribution of enlarged islets in obese rats was altered by low-protein feeding. Four-week-old, male, lean and obese Zucker rats were fed either a diet containing 20% (w/w) protein (control diet) or a diet containing 5% (w/w) protein (low-protein diet) for 3 weeks. Pancreata were dissected at autopsy and immunostained for insulin. Islet size and distribution were determined by morphometric analysis. Body-weight gain, food intake, and serum insulin and glucose were also measured. After 3 weeks on the diets, serum insulin was significantly lower in both lean (-75%) and obese (-54%) rats fed low protein compared with that in controls. However, obese rats were still hyperinsulinemic compared with lean rats. Protein malnutrition resulted in a shift in distribution of islets to smaller size both in lean and in obese rats, with an increase in the population of small islets (100 μm²) and a decrease in the population of large islets (>20,000 μ;m²). In lean and obese rats fed low protein, β-cell weight was significantly lower, B cell volume fraction tended to decrease, and islet number per section area was significantly elevated when compared with controls. Taken together, these results show that protein deficiency alters the endocrine pancreas in both lean and obese Zucker rats. Although the decrease in islet size and the shift in distribution to smaller islets most likely contribute to the decrease in serum insulin concentration, these changes appear insufficient to normalize hyperinsulinemia in the obese Zucker rat.     

Flow-induced modulation of the permeability of endothelial cells cultured on microcarrier beads

The maintenance of endothelial barrier function is important in the regulation of fluid and solute balance between the vascular space and the surrounding tissue. Since fluid flow across endothelial cells stimulates a wide variety of endothelial responses, the effect of shear stress on barrier function was investigated. Bovine pulmonary artery endothelial cells were cultured on permeable microcarrier beads, placed in a chromatography column, and perfused. Indicator-dilution techniques were used to estimate the permeability of the cell-covered beads to low molecular weight tracers (sodium fluorescein—NaFlsc; cyanocobalamin—B12) as a function of flow rate through the column. Permeability values for both tracers were significantly increased (9.3 ± 0.6 to 19.3 ± 1.7 for NaFlsc; 8.2 ± 0.5 to 20.4 ± 3.1 for B12; mean ± SEM, × 10⁻⁵ cm/s, P < .05) when the flow rate was increased from 0.9 ml/min to 3.2 ml/min (corresponding to average shear stresses of 4.7 and 16.8 dynes/cm²). The permeability increase occurred within minutes of the flow increase, and was reversed by decreasing the flow rate to 0.9 ml/min. In the presence of cytochalasin D, the flow-induced permeability increase was not reversible. Neither inhibition of nitric oxide synthase (with N^G-monomethyl-L -arginine) nor inhibition of cyclooxygenase (with indomethacin) was capable of blocking the flow-induced permeability increase. These results indicate that the rapid modulation of endothelial barrier by flow in vitro is probably not due to prostacyclin or nitric oxide. © 1996 Wiley-Liss, Inc.     

The speciation of coliform genera from above and below a sewer outfall and their susceptibilities to antimicrobial agents

The occurrence of coliforms in a small water course was shown to increase by a factor of thirty six below the outfall of a sewage treatment plant. Speciation of the bacteria from above and below the sewer outfall showed thatEscherichia coli andEnterobacter species predominated. Drug resistance levels were significant in microorganisms from both sampling sites and the occurrence of a significant number of multiple-resistant microorganisms, particularlyE. coli, is reported. BothE. coli andEnterobacter species from below the sewer outfall show a statistically significant increase in resistance to ampicillin andE. coli from below the outfall also shows a statistically significant increase in resistance to sulphamethoxazole as compared with isolates from above the outfall.     

The ecology and production of copepods, particularly Thermocyclops hyalinus, in the tropical Lake George, Uganda

(1) Total copepod numbers per litre in Lake George, Uganda, vary slightly during the course of a year. Numbers increase during the dry seasons June-August and January-February, and decrease during the rains. This variation is more marked in the mid-lake area, where copepod numbers are higher, but the change in numbers is not more than two-fold. By comparison with populations in temperate regions the population in Lake George is stable in size and shows little seasonal variation. (2) The carnivorous cyclopoid Mesocyclops leuckarti forms only 4–21 %, by num-bers, of the total copepod population and does not occur in numbers large enough to be analysed profitably. (3) The bulk of the population is the much smaller Thermoeyclops hyalinus and analysis of this population into the main development stages of its life history shows an almost constant age structure over the lake as a whole, throughout the year. (4) The development time of the eggs, as determined previously (Burgis, 1970), is 1–5 days. This, plus mean development-times of 6 and 11 days for nauplii and copepodites respectively, has been used to determine the daily rate of recruitment from each stage of the life history to the next. The recruitment from nauplii to copepodites and from copepodites to adults are very similar and almost constant. Recruitment from eggs to nauplii is. however, mueh greater, and it thus appears that the major loss to the population is at this eariy stage in the life history. This may well be due to predation by the larvae of Chaohorus. (5) Using this recruitment rate from eggs to nauplii, the potential turnover time of population numbers was calculated to have a mean value of 4–2 days for the lake as a whole. This is similar to maximum summer turnover times found by authors such as Hall (1964) for Daphnia populations in temperate waters. (6) The biomass of individuals at various stages in their life history was deter-mined as carbon, and application of these determinations to the numerical counts was used to calculate the standing-crop biomass in the lake. With a constant population of stable age-structure it is possible to estimate the daily biomass production from the turnover time of numbers. For the population of Thermocyelops hyalinus in Lake George this gives a mean figure of 211 μg C/I/day during 1969–70. (7) Instantaneous birth rates and instantaneous rates of change in population numbers (b and r) have also been calculated for the population of Thermocyclops. The values of r show some variation at one centre site, fluctuating around zero, but when calculated for lake mean figures r is hardly different from zero throughout the year. The values of b also show fluctuations when calculated for one site but when calculated for lake mean figures show no more than two-fold variations, being higher at the beginning of the two rainy seasons. The values of b are not as high as those attained during the summer in some temperate Daplmia populations such as those described by Wright (1965). The values of ZJ tend to be higher inshore than in mid-lake areas of the lake. (8) The instantaneous mortality rate r/has been calculated from b-r. With such very small values of r, d is always very similar to b. Most, if not all, recruitment to the population is counterbalanced by loss, probably predation. (9) At one inshore site the population was analysed and counted in detailed cate-gories, over a period of 10 days. This allowed a more accurate estimation of the standing-crop biomass and thus of production. A mean value of 103 μg C/l/day was obtained for these samples. (10) The mean daily level of production, c. 211 μg C/l/day, o(Thermocyclops hyalinus in Lake George, is among the higher values of zooplankton production available for comparison most of which are from temperate regions of the world. The fact that it is applicable throughout the year gives an annual value of 77g C/m³/year which is probably high.     

Population ecology of rhesus monkeys (Macaca mulatta) at Nanwan Nature Reserve,Hainan, China

Rhesus monkeys (Macaca mulatta) at the Nanwan peninsula of Hainan Island, China, have been observed in the field for 25 years, and have been studied intensively for eight years, beginning in 1981. There were about 115 monkeys in 5 natural groups when the Nanwan Reserve was founded in 1965. From 1965 to 1984, the number of groups increased from 5 to 19, and the total population increased from 115 to 930 individuals, an annual population increase of 12.7%. From 1984 to 1987, the population continued a slower rate of increase (8.9%) to approximately 1200 monkeys in 20 groups. The home ranges of each monkey group at Nanwan varied from 0.16 to 0.72 km², with a mean and standard deviation of 0.37 ± 0.18. The size of the home range of the rhesus was affected by the quality of vegetation and rhesus population density. The average birth rate per year was 77.8 ± 13.9%, varying from 53.8% to 100% since 1978. From 1978, the birth rates of two food provisioned groups were higher in alternate years (x? = 91.7%) and lower in intervening years (x? = 68.8%; P = <0.01). A minority (26.5%) of females have given birth at 4 years of age, most at 5 years. The sex ratio of newborns in any one year varied from 0.3 to 3.5 males to females, with a mean and standard error of 1.09 ± 0.37 males to females.     

Long-term L-NAME treatment potentiates the blood-brain barrier disruption during pentylenetetrazole-induced seizures in rats

We investigated whether the severity of blood-brain barrier disruption caused by pentylenetetrazole-induced seizures is modified by long-term nitric oxide synthase inhibition in rats. Rats were given N-omega-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, in drinking water for 4 weeks, and then treated with pentylenetetrazole to induce seizures. Damage to the blood-brain barrier was investigated using Evans blue dye extravasation. Serum nitric oxide concentration was decreased in L-NAME-treated rats (P<0.01). L-NAME and/or pentylenetetrazole treatments elevated systolic blood pressure of animals (P<0.01). L-NAME caused an increase in the mortality rate after pentylenetetrazole injection leading to the death of animals at about 15 min after the onset of the seizure. Pentylenetetrazole-induced seizures in rats treated with L-NAME caused a significant increase in Evans blue dye extravasation into cerebral cortex, diencephalon and cerebellum, as compared with seizures evoked by pentylenetetrazole injection to L-NAME-untreated rats (P<0.01). Data presented here suggest that the degree of blood-brain barrier disruption induced by seizures is more pronounced in long-term nitric oxide deficiency.     

Characterising spatial and temporal variation in the finite rate of population increase across the northern range boundary of the annual grass <Emphasis Type="Italic">Vulpia fasciculata</Emphasis>

Understanding the factors that influence plant distributions is a considerable challenge for ecologists in the face of environmental change. Here, we quantify spatial and temporal variation in the finite rate of population increase of the annual grass Vulpia fasciculata. Specifically, we test the hypothesis that the northern range boundary is associated with finite rates of population increase of less than one. Seeds of three ecotypes of the annual grass V. fasciculata were introduced annually across a range of sites in Great Britain both within (11) and to the north (4) of its current range boundary in each of 4 years. Populations failed to establish at 17% of target sites due to disturbance. At the remaining target sites, the finite rate of population increase, λ, varied from 0.06 to 33.3 with a geometric mean of 1.88. Of the total variance in the rate of population growth, site and year effects accounted independently for 40% of the variation and in interaction for 50%; ecotype accounted for less than 5% of the variation. Variation in the weather between sites and years had little impact on plant performance, and there was no indication that the rate of population growth was lower to the north of the current range boundary. We conclude that current climatic conditions on the coast of Great Britain are not limiting the distribution of V. fasciculata and that seeds from across its current range have roughly equivalent colonising potential.     

Development of central and peripheral serotonin-producing systems in rats in ontogenesis

The work deals with study of development of central and peripheral serotonin-producing systems in rat ontogenesis before and after formation of the blood-brain barrier. By the method of highly efficient liquid chromatography it has been shown that the serotonin level in peripheral blood before formation of the blood-brain barrier (in fetuses and neonatal rats) is sufficiently high for realization of physiological effect on target cells and organs. At the period of formation of the blood-brain barrier the serotonin level in brain sharply rises, whereas the serotonin concentration and amount in blood plasma and duodenum increase insignificantly. Completion of formation of the blood-brain barrier is accompanied by a significant increase of the serotonin content in duodenum, probably for maintenance of the high serotonin level in blood. To evaluate secretory activity, the mean rate of daily serotonin increment in the studied tissues was calculated. In brain, this parameter was maximal at the period of formation of the blood-brain barrier-from the 4th to the 16th postnatal days. This allows thinking hat brain before formation of the blood-brain barrier is the most important source of serotonin in peripheral blood.     

A simple population viability analysis of Tancho (Grus japonensis) in southeastern Hokkaido, Japan

We employed population viability analysis to estimate future population trends and extinction risk of Tancho, the Japanese or red-crowned crane (Grus japonensis). The stage matrix was based on 15 years of data collected by counting the number of wintering cranes and following the survivorship of banded cranes. The accidental death rate was estimated from the number of dead or seriously injured cranes collected in the Kushiro municipal zoo. Consequently, the accidental death rate was found to increase each year at 0.072% per year during the recent 14 years and 0.132% per year during final 6 years. The carrying capacity (K) was estimated from the mean mire area within the territory of a breeding pair and the geographic information system data in southeastern Hokkaido. Accordingly, K was estimated to be 1,659 in this area. Using the stage matrix, accidental death rates and K (and 20%-lower K), the simulation was conducted under three conditions: (1) the increase of accidental death, (2) the limitation of carrying capacity, and (3) the concurrent occurrence of carrying capacity limitation and accidental death increase. As a result, the extinction probability during 100 years was zero, although the accidental death rate increased at the current rate of 0.132% per year. Therefore, by artificial feeding in winter, the Japanese population of Tancho reached the adequate level, which seems sustainable unless some catastrophic factors seriously damage the population. To raise the tolerance to catastrophic factors, we discuss the probability of their distributional expansion to western or northern Hokkaido and even to Honshu.     

Effects of age and starvation on the gastrointestinal microflora and the heat resistance of fecal bacteria in rats.

The microflora of the gastrointestinal tract in rats 1 day to 100 weeks old, of the cecal contents and wall in starved rats, and of heat-treated feces of normal rats was determined by cultural examination. Streptococci, staphylococci, lactobacilli, actinobacilli, and coliforms colonized the tract during the 1st week of life. Bacteroidaceae, veillonellae, catenabacteria (composed of eubacteria and anaerobic lactobacilli), clostridia, bifidobacteria, anaerobic gram-positive cocci, fusiform bacteria, curved rods, and spirochetes appeared when the rats were 2 to 4 weeks old. Yeasts were slower in colonizing the tract than any other organism. Dramatic changes occurred in the microflora of rats 2 to 4 weeks of age. There was a time lag between the changes in enterococcal and coliform populations. The enterococcal population was depressed over a period from 2 to 6 weeks of age. Bifidobacteria showed a larger population at 4 to 9 weeks than at any other age. The microflora of the stomach was the same as that of the small intestine, with some exceptions. It differed markedly from that of the cecum. The ratio of total aerobic count to anaerobic count gradually increased in the stomach, but decreased in the cecum, with advance in age. The microorganisms distributed in the tract could be divided roughly into 3 types. The population of each organism, except spirochetes, in the cecal wall was approximately 1/1,000 of that in the cecal contents. One of the 2 types of spirochetes was found only in the cecal wall and in a high incidence, forming a large population. In rats starved for 48 hr, coliforms, Proteus spp., anaerobic gram-positive cocci, Clostridia, and some bacteroidaceae showed an increase in population in the cecum, but lactobacilli, veillonellae, and spirochetes decreased. The major organisms cultured from the heat-treated feces were fusiform and curved bacteria, some members of Bacillus, minor anaerobic cocci, and straight rods.     

TRYPTOPHAN TRANSPORT ACROSS THE BLOOD-BRAIN BARRIER DURING ACUTE HEPATIC FAILURE

Abstract— Tryptophan transport across the blood-brain barrier was studied using a single injection dual isotope label technique, in the following three conditions: normal rats, rats with portacaval shunts, and rats with portacaval shunts followed 65 h later by hepatic artery ligation. In both normal rats and those with acute hepatic failure the tryptophan transport system was found to be comprised of two kinetically distinct components. One component was saturable and obeyed Michaelis-Menten kinetics (normal: V_max= 19.5 nmol.min^?1.g^?1. K_m= 113 μM; hepatic failure: V_max, = 33.8 nmol.min^?1.g^?1, K_m= 108 μM), and the second was a high capacity system which transported tryptophan in direct proportion to concentration over the range tested (normal: K= 0.026 ml.min^?1.g^?1; hepatic failure: K= 0.067 ml.min^?1.g^?1). Since the saturable low capacity component transports several neutral amino acids, and their collective plasma concentration is high in relation to the individual K_ms, tryptophan transport by this component is reduced by competitive inhibition under physiological conditions. Thus it was calculated that in normal rats approx 40% of tryptophan influx occurs via the high capacity system. During acute hepatic failure transport via both components was increased substantially, approximately doubling the rate of tryptophan penetration of the blood-brain barrier at all concentrations tested. The contribution by the high capacity component became even more significant than in normal rats, accounting for about 75% of all tryptophan passage from plasma to brain. Brain tryptophan content was 29.9 nmol/g in normal rats and rose to 45.2 nmol/g in rats with portacaval shunts and 50.5 nmol/g in those with acute hepatic failure, correlating with the increased rate of tryptophan transport. In a previous study we found that plasma competing amino acids were greatly increased during acute hepatic failure. Calculations predict that these increased concentrations would cause a reduction in tryptophan transport by the low capacity system. However, because of the increase in the rate of transport by the high capacity component, net tryptophan entry across the blood-brain barrier was actually increased. This increased rate of transport clearly contributes to the increased content of brain tryptophan found during hepatic failure.     

施氮及不同根系分隔模式对尾叶桉和降香黄檀幼苗生长及叶片生理特性的影响

豆科与非豆科树种混交作为一种人工林培育可持续发展模式,在保证木材产量和维持氮素平衡中发挥了重要作用。该研究通过大棚盆栽试验,设计3个施氮水平(0、3、6g·株-1)及3种根系分隔方式(不隔、网隔、膜隔),分析了不同氮素水平及根系分隔模式对尾叶桉(Eucalyptus urophylla)与降香黄檀(Dalbergia odorifera)植株幼苗生长、叶片生理特性、根系形态及生物量的影响。结果表明:(1)随施氮量的增加,尾叶桉与降香黄檀的苗高、地径均呈递增趋势;不同根系分隔模式下,尾叶桉苗高、地径均在不隔模式下生长最好,降香黄檀则在膜隔模式下生长最好。(2)与不施氮处理相比,施氮3、6g·株-1水平下尾叶桉与降香黄檀叶片的超氧化物歧化酶(SOD)活性、过氧化物酶(POD)活性及可溶性蛋白含量均显著升高,而丙二醛(MDA)含量则呈降低趋势。(3)尾叶桉与降香黄檀根系的总长度、总表面积、总体积、平均直径、根尖数和比根长均随氮素水平的增加而增加,且各氮素水平间的差异显著;同一氮素水平下,尾叶桉的根系生长总体表现为不隔网隔膜隔,而降香黄檀根系生长则表现为膜隔不隔网隔,且两树种不同分隔模式间差异显著。(4)尾叶桉与降香黄檀各器官生物量及总生物量均随施氮量的增加而增加,并在6g·株-1施氮水平下生物量最大;各器官生物量分配中,尾叶桉各器官生物量所占比例大小依次为:茎(40.59%)叶(32.37%)根(27.04%),降香黄檀各器官生物量所占比例大小依次为:根(47.67%)茎(40.08%)叶(12.25%)。研究表明,尾叶桉与降香黄檀混交一定程度上扩展了根系横向和纵向水平的养分生态位,扩大了根系吸收土壤养分的空间,同时根系互作提高了降香黄檀的固氮能力,对土壤有效氮的产生有较大影响。     

Estimate of population extinction risk and its application to ecological risk management

Environmental threats, such as habitat size reduction or environmental pollution, may not cause immediate extinction of a population but may shorten the expected time to extinction. We developed a method to estimate the mean time to extinction for a density-dependent population with environmental fluctuation and to compare the impacts of different risk factors. We first derived a formula of the mean extinction time for a population with logistic growth and environmental and demographic stochasticities expressed as a stochastic differential equation model (canonical model). The relative importance of different risk factors is evaluated by the decrease in the mean extinction time. We studied an approximated formula for the reduction in habitat size that enhances extinction risk by the same magnitude as a given decrease in survivorship caused by toxic chemical exposure. In a large population (large K) or in a slowly growing population (small r), a small decrease in survivorship can cause the extinction risk to increase, corresponding to a significant reduction in the habitat size. Finally, we studied an approximate maximum likelihood estimate of three parameters (intrinsic growth rate r, carrying capacity K, and environmental stochasticity σ ² _e ) from time series data. By Monte Carlo sampling, we can remove the bias very effectively and determine the confidence interval. We discuss here how the reliability of the estimate changes with the length of time series. If we know the intrinsic rate of population growth r, the mean extinction time is estimated quite accurately even when only a short time series is available for parameter estimation. Received: March 31, 1999 / Accepted: November 9, 1999     

Incorporating urban infrastructure into biogeochemical assessment of urban tropical streams in Puerto Rico

The influence of built urban infrastructure on stream chemistry was quantified throughout the drainage network of the tropical Río Piedras watershed, San Juan metropolitan area, Puerto Rico. Urbanization and failing domestic wastewater infrastructure appeared to drive changes in surface water chemistry throughout the watershed. Mean baseflow concentrations of chloride (Cl), ammonium (NH₄), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and phosphate (PO₄) all increased with urban infrastructure, while nitrate (NO₃) and dissolved oxygen (DO) decreased. These patterns in stream chemistry suggest that sewage effluent from failing or illegally connected sewer pipes has a major impact on surface water quality. Concentrations of Cl, DO, and NH₄ in stream water were most strongly related to sewer pipe volume, demonstrating the tight connection between urban infrastructure and stream chemistry. The loading and transformation of NO₃ and NH₄ were modeled along the river network and NH₄ loading rates from the landscape were strongly related to urban infrastructure, whereas NO₃ loading rates showed only weak relationships, highlighting the importance for incorporating NH₄ dynamics into river network models in urban environments. Water quality appears to be severely impacted by sewage in this tropical basin, despite large investments in built infrastructure. The high temperatures in the Río Piedras exacerbate water quality problems by reducing saturation DO levels in streams, and intense rainstorms tax the ability of built infrastructure to adequately manage overland flows. These problems are likely typical of much of the urbanized humid tropics.     

A stochastic population approach to the problem of stable recruitment hierarchies in spiking neural networks

Synchrony-driven recruitment learning addresses the question of how arbitrary concepts, represented by synchronously active ensembles, may be acquired within a randomly connected static graph of neuron-like elements. Recruitment learning in hierarchies is an inherently unstable process. This paper presents conditions on parameters for a feedforward network to ensure stable recruitment hierarchies. The parameter analysis is conducted by using a stochastic population approach to model a spiking neural network. The resulting network converges to activate a desired number of units at each stage of the hierarchy. The original recruitment method is modified first by increasing feedforward connection density for ensuring sufficient activation, then by incorporating temporally distributed feedforward delays for separating inputs temporally, and finally by limiting excess activation via lateral inhibition. The task of activating a desired number of units from a population is performed similarly to a temporal k-winners-take-all network.