Determinants of human population growth

The 20th century has seen unprecedented growth of the human population on this planet. While at the beginning of the century the Earth had an estimated 1.6 billion inhabitants, this number grew to 6.1 billion by the end of the century, and further significant growth is a near certainty. This paper tries to summarize what factors lie behind this extraordinary expansion of the human population and what population growth we can expect for the future. It discusses the concept of demographic transition and the preconditions for a lasting secular fertility decline. Recent fertility declines in all parts of the world now make it likely that human population growth will come to an end over the course of this century, but in parts of the developing world significant population growth is still to be expected over the coming decades. The slowing of population growth through declining birth rates, together with still increasing life expectancy, will result in a strong ageing of population age structure. Finally, this paper presents a global level systematic analysis of the relationship between population density on the one hand, and growth and fertility rates on the other. This analysis indicates that in addition to the well-studied social and economic determinants, population density also presents a significant factor for the levels and trends of human birth rates.     

水体中氮素对水葫芦生长发育的影响

本文通过对水体中氮素营养条件的研究,分析了水体中不同氮素对水葫芦生长的影响,从而为福建省水葫芦的综合治理提供科学依据。结果表明：在水体氮浓度为25～30mg·L^1的范围内,水葫芦生长情况最佳,高于或低于这个范围,水葫芦的生长发育均受到不同程度的抑制。     

The growth analysis of water hyacinth,Eichhornia crasipes solms,in different water temperature conditions

A 40-day culture experiment of water hyacinth was made in 4 different water temperatures, 15, 20, 25 and 30°C, which were combined with 4 levels of concentration of culture solution, 1/3, 1, 3 and 9-fold of the standard solution containing 28 ppm of totalN and 7.7 ppm of totalP. The optimum condition for obtaining the maximum plant growth shifted from 30°C: 3-fold condition in the early stage to 20–25°C: 3-fold condition in the later stages of the experiment. The relation between the fresh weight biomass per 100-l tank,w, and the concentration of culture solution,f, was expressed successfully by a reciprocal equation,1/w=A F/f+A F ^′f/(1-f/C F)+B F, in whichA f,A f′, andB f are time dependent coefficients andC f is the upper limit of the concentration to permit plant growth which can change with time. The relation betweenw and water temperature,T, was expressed by another reciprocal equation,1/w=A T/e ^aT+A T^′e^bT+B T, in whicha andb are constants andAt At′ andB t are time dependent coefficients. The latter formulation shows that the temperature can be breated as an exponential factor, and it suggests the possibility of the growth coefficient of the logistic growth equation, ψ, being affected by temperature.     

Decomposition of young water hyacinth leaves in lake water

Rates of weight loss and release of nutrients during different phases of decomposition in young water hyacinth leaves were determined under laboratory conditions. The leaves decomposed solely by physical leaching during the initial 4-day phase and later by microbial processes. The largest part of weight loss and nutrient release by physical leaching took place within the first 4 h of incubation and thereafter the decomposition rate declined. Microbial processes decayed leaves at a significantly higher rate than that by physical leaching. The overall decay rate constants were related inversely and the release of nutrients directly to the levels of leaf additions in the lake water. The dissolved inorganic and organic nutrients were released chiefly by abiotic processes during the initial as well as later phases of decay. The release was significantly higher during the initial phase in comparison with that during the later phase. Microbes utilized only a small amount of nutrients that were released during decomposition of water hyacinth leaves. The % release of various elements from the decaying leaves was in the order of K > P > C > Na > N.     

Decomposition of water hyacinth detritus in eutrophic lake water

A study was conducted to determine the seasonal production of detritus by water hyacinths [Eichhornia crassipes (Mart.) Sohns] cultured in eutrophic Lake Apopka water, and the decomposition of detritus in situ and under laboratory conditions. Annual averages for C, N and P deposited through detritus production at the sediment-water interface were 2870, 176 and 19 kg ha^-1 yr^-1, respectively.Decomposition rates were faster in the root zone of hyacinth mats than at the sediment-water interface. Approximately 92% of the detritus C deposited at the sediment-water interface was decomposed in one year, while only 11% of the detrital organic N was mineralized. Detrital tissue gained P during decomposition, suggesting P limitation for the system. Dry-weight loss of detrital tissue was significantly correlated with the mass of C lost (r ² = 0.947^**), C/N ratio (r ² = 0.644^**) and C/P ratio (r ² = 0.428^**).Florida Agricultural Experiment Stations Journal Series No. R-00348.     

Pretreatment and enzymic saccharification of water hyacinth cellulose

Water hyacinth was pretreated, under variable conditions, with NaOH, alkaline H₂O₂, peracetic acid and sodium chlorite. Combined pretreatments included sodium chlorite with each of NaOH, alkaline H₂O₂ and peracetic acid. Combined pretreatment with 0.1% NaClO₂ for 1 h at 100 °C and peracetic acid at 100 °C for 15 min afforded the most promising sample. The recovered lignin, cellulose and hemicellulose of this sample was 2.56%, 96.69%, and 81.38%, respectively. The same sample, by cellulase hydrolysis showed the highest cellulose conversion (80.8%) and 90% saccharification using 200 FPU/g substrate. Some ambient factors affecting saccharification of pretreated water hyacinth were investigated. Enzymic saccharification after 6 h was about 50% of that at 48 h, indicating a slow hydrolysis rate by time. Addition of 8% glucose at the beginning of the enzymic hydrolysis decreased the saccharification to about its half while addition of 8% ethanol brought about complete inhibition of the enzyme. Addition of cellobiase to the reaction mixture increased cellulose conversion and saccharification by 10%.     

Nematode growth patterns and the moulting cycle: the population growth profile

Population growth profiles of Caenorhabditis elegans and Panagrellus redivivus constructed from length frequencies have a number of steps in them coinciding with the number of extrauterine moults. Each step has a constant size relationship with that of one of the midmoults measured directly.
The profiles could only have the shape they do if there are corresponding steps in the true growth curve of individual worms: the fact that previous workers have been unable to detect these steps being due to the limitations of techniques available for the study of synchronous and individual growth curves. Nevertheless, a synchronous system with Trichostrongylus retortaeformis gives qualitative support to the findings from population profiles.
The population growth profile is a new tool in the study of environmental effects on moulting, though there are theoretical reasons why the true growth curve cannot be derived from it.
Abandonment of the "continuous growth" model for post-embryonic development simplifies the framing of hypotheses to explain ecdysis in nematodes.     

Allelopathic effects of water hyacinth [Eichhornia crassipes

Eichhornia crassipes (Mart) Solms is an invasive weed known to out-compete native plants and negatively affect microbes including phytoplankton. The spread and population density of E. crassipes will be favored by global warming. The aim here was to identify compounds that underlie the effects on microbes. The entire plant of E. crassipes was collected from El Zomor canal, River Nile (Egypt), washed clean, then air dried. Plant tissue was extracted three times with methanol and fractionated by thin layer chromatography (TLC). The crude methanolic extract and five fractions from TLC (A-E) were tested for antimicrobial (bacteria and fungal) and anti-algal activities (green microalgae and cyanobacteria) using paper disc diffusion bioassay. The crude extract as well as all five TLC fractions exhibited antibacterial activities against both the gram positive bacteria; Bacillus subtilis and Streptococcus faecalis; and the gram negative bacteria; Escherichia coli and Staphylococcus aureus. Growth of Aspergillus flavus and Aspergillus niger were not inhibited by either E. crassipes crude extract nor its five fractions. In contrast, Candida albicans (yeast) was inhibited by all. Some antialgal activity of the crude extract and its fractions was manifest against the green microalgae; Chlorella vulgaris and Dictyochloropsis splendida as well as the cyanobacteria; Spirulina platensis and Nostoc piscinale. High antialgal activity was only recorded against Chlorella vulgaris. Identifications of the active antimicrobial and antialgal compounds of the crude extract as well as the five TLC fractions were carried out using gas chromatography combined with mass spectroscopy. The analyses showed the presence of an alkaloid (fraction A) and four phthalate derivatives (Fractions B-E) that exhibited the antimicrobial and antialgal activities.     

Polyamine oxidase from water hyacinth: purification and properties

Polyamine oxidase was purified to homogeneity from leaves of water hyacinth by the criterion of sodium dodecyl sulfate gel electrophoresis (SDS disc PAGE). The enzyme showed a high specificity for spermidine and spermine (K_m values 28 micromolar and 20 micromolar, respectively). The optimal pH of the enzyme for both spermidine and spermine was 6.5. The molecular weight of the enzyme estimated by Sephadex G-200 gel filtration was 87,000, while SDS disc PAGE gave a single band at the molecular weight of 60,000. Octamethylenediamine and quinacrine were strong inhibitors of the enzyme, but p-chloromercuribenzoate was without effect. A prosthetic group in the enzyme was identified as flavin adenine dinucleotide.     

Kinetics of the pyrolytic and hydrothermal decomposition of water hyacinth

The kinetics of water hyacinth decomposition using pyrolysis and hydrothermal treatment was compared. With pyrolysis, initial vaporization occurred at 453 K as determined by thermogravimetric analysis, while initial solubilisation occurred at 433 K with subcritical hydrothermal treatment. The “kinetic triplet” was determined for the ranges of 423-483 K (range I) and 473-553 K (range II) using the Coats-Redfern method for both treatments. The calculated activation energies for ranges I and II were 110 and 116 kJ/mol for conventional pyrolysis and 145 and 90 kJ/mol for hydrothermal treatment. The similar activation energies for the two temperature ranges observed for pyrolysis implied that only hemicellulose decomposition occurred. For hydrothermal treatment, both hemicellulose and cellulose decomposition occurred in temperature range II, in which a notable lower activation energy was observed. This implied hydrothermal treatment was more suitable for conversion lignocellulosic biomass under these conditions.     

Acarine regulators of water hyacinth in Kerala (India)

Screening of the mite fauna on water hyacinth, Eichhornia crassipes, at 25 localities throughout Kerala (India) yielded a list of 21 phytophagous species from nine families. Distribution of mites at these localities and type of injury produced suggested that Oligonychus biharensis and Orthogalumna terebrantis warrant further study. In an experimental set-up the injury caused by these mites was quantified. Infestation by Ol. biharensis lead to reduced photosynthesis by the host (significant loss in chlorophyll content). Injurious effects of Or. terebrantis involved leaf mining by the developing instars leading to wilting of the leaves resulting in reduced dry weight. The relative merits of the two species as potential control agents of water hyacinth are discussed.     

除草剂与水葫芦象甲的协同控制作用

在室内采用水葫芦象甲与除草剂控制水葫芦的试验中发现象甲2头／株＋200mg·L^-1草甘膦对水葫芦的控制效果较显著,药后15d死亡率达到100％;而象甲4头／株＋46mg·L^-12,4-D二甲胺盐对水葫芦的防治效果显著,药后10d死亡率达到90％以上,对照无死亡且有开花现象。     

Sulphate uptake and metabolism in water hyacinth and salvinia during cadmium stress

Water hyacinth (Eichhornia crassipes (Mart.) Solms) and salvinia (Salvinia auriculata Aubl.) were exposed to toxic levels of Cd with the objective of evaluating its effect on sulphate uptake and metabolism. Plants were treated with 0 and 5 μmol L⁻¹ Cd for 3 days and, then sulphate uptake, ATP sulfurylase activity, soluble thiol content and Cd-binding complexes were determined. Water hyacinth showed a lower sulphate uptake, but its kinetic parameters were not affected by Cd. In salvinia, however, both V_max and affinity to sulphate (1/K_m) decreased with Cd treatment. The ATP sulfurylase activity increased in Cd-treated plant of both species, except in the roots of salvinia. In the presence of Cd water hyacinth always exhibited higher activity of this enzyme. The total soluble thiol content was always higher in water hyacinth. In Cd treated plants it increased in the leaves of water hyacinth, but decreased in salvinia. Cysteine content increased only in water hyacinth leaves, while γ-glutamylcysteine content increased in the two parts of the plants of both species after Cd treatment, especially in water hyacinth. Glutathione contents, on the contrary, after Cd treatment, reduced in both parts of the plants of water hyacinth but only in the leaves of salvinia. The unidentified thiol fraction content increased with Cd treatment in both species, especially in water hyacinth. Root and leaf extracts of both species showed peaks with maxima at A₂₆₅/A₂₈₀. In treated plants these peaks coincided with Cd content peaks indicating the formation of Cd-binding peptides. It was estimated that in the presence of Cd about 97% of Cd was associated with these complexes and water hyacinth had 28% more Cd-binding peptides than salvinia. Despite its lower sulphate uptake, water hyacinth showed higher rates of sulfur reduction and assimilation into soluble thiols. Possibly, glutathione is used in water hyacinth roots to synthesize hitherto unidentified Cd-binding peptides.     

Metal localization in water hyacinth roots from an urban wetland

PCA, principal components analysis
MDS, multidimensional scaling
STEM, scanning transmission electron microscopy

Metal localization within and around roots of water hyacinth ( Eichhornia crassipes ) growing in a wetland receiving urban run-off was studied by energy dispersive X-ray microanalysis of sections from freeze-substituted roots. Sampling randomly from an order of magnitude gradient in metal concentrations (Cu and Pb) allowed us the opportunity to identify general patterns of metal localization. Iron was present at high levels at the root surface, and this may have been a root plaque as described for wetland plants with roots anchored in flooded soils. Iron levels decreased centripetally across the root and were higher in cell walls than within cells. Trace metals (Cu, Zn and Pb) were not localized at the root surface. In contrast with iron, trace metal levels increased centripetally across the root, tended to be higher inside cells and were highest within cells in the stele. Variability of localization was high for all metals analysed. Multivariate statistical analyses (principal components analysis and multidimensional scaling) were useful for identifying overall patterns in elemental distribution.     

Effect of temperature and water activity on spore germination and mycelial growth of three fungal biocontrol agents against water hyacinth (Eichhornia crassipes)

Aims: To determine the effect of water activity (a_w = 0·880–0·960) and temperature (15–35°C) on the percentage of viable conidia and mycelial growth of three biocontrol agents effective against water hyacinth in Mali: Alternaria sp. isolate Mlb684, Fusarium sacchari isolate Mln799 and Cadophora malorum isolate Mln715. Methods and Results: The fungi were grown in vitro on plates containing potato dextrose agar medium at different a_w values (glycerol being added to adjust the a_w). The percentage of viable conidia and radial growth rate decreased with decreasing water activity. Statistical analysis showed a significant effect of a_w, temperature and the a_w × temperature interaction on mycelial growth (P < 0·0001). Water activity emerged as the factor exerting the greatest influence. Differences were observed between the fungi tested, the C. malorum appearing more tolerant to low a_w and the F. sacchari more tolerant to high temperature (35°C). Growth models predicting the combined effect of a_w and temperature were developed and response surfaces generated, showing fairly good agreement with the experimental values. Conclusions: Our results confirm the previous finding that a_w has a greater influence than temperature on fungal growth. Under most conditions, variation of environmental factors has a detrimental influence on the percentage of viable conidia and mycelial growth rate of fungal isolates. Significance and Impact of the Study: The developed models may contribute to predicting the best environmental conditions for use of these fungi as effective biocontrol agents against water hyacinth.     

Undesirable side-effects of water hyacinth control in a shallow tropical reservoir

1. Based on a comprehensive data set collected monthly during 8 years (1997–2004), we evaluated the effects of mechanical removal of Eichhornia crassipes on the limnological characteristics and algal biomass of a polymictic shallow tropical reservoir. 2. Interrupted time series analyses indicated that the limnological responses to macrophyte removal can be classified as an ‘abrupt permanent impact’ implying that the overall mean of the time‐series shifted promptly after intervention. These analyses indicated a significant increase for pH, total phosphorus, total phytoplankton and cyanobacterial biomass, and a decrease in water transparency and CO₂ concentrations in the surface water; also, the increase in water stability, increase of bottom soluble reactive phosphorus (SRP) and decrease in bottom oxygen levels. 3. Cyclic anoxic periods previously observed during springs and summers were replaced by a persistent period of anoxic conditions in the sediment overlying water. Anoxic conditions were suitable for SRP release from sediments. Heavy cyanobacterial blooms became more persistent, maximum biomass (4229 mm³ L⁻¹) was 30 times larger, the blooms frequently reached 2 m and sometimes the bottom of the reservoir, contrasting to the preremoval period in which it reached at most 1 m deep. 4. The long‐term P dynamics in the system, initially driven by allochthonous nutrient loadings were replaced by internal ecological processes. Water hyacinth removal markedly accelerated the process of eutrophication due to internal feedback mechanisms, leading to a switch to a more turbid state. Biological feedback mechanisms were driven by cyanobacterial blooms by enhancing water stability, oxygen anoxia at the bottom and by increasing suitable conditions for P internal loading. These data support the hypothesis of the role of cyanobacterial blooms as an important factor impairing water quality and driving the ecosystem towards a stable degraded state. 5. These findings have important implications for the restoration of shallow stratifying eutrophic lakes, as the alternative degraded state is most likely to occur when compared with their non‐stratifying counterparts. Moreover, feedback mechanisms in tropical and subtropical shallow lakes seem to be stronger than in temperate ones, as stratification events are more likely to occur over the year, intensifying system resilience to restorative strategies.