共查询到20条相似文献，搜索用时 62 毫秒
1.
为了能够客观地表现种群消长的动态模式,以胸高断面积代替种群生物量,采用洪伟等提出的Logistic改进模型对木荷(Schima superba)种群的增长动态进行研究,运用改进单纯形对模型参数进行优化。拟合结果表明,木荷种群的最大增长速度出现在第4龄级至第5龄级之间,即胸径为32.5~42.5 cm时期。在所调查的林分中木荷种群基面积已达到17.463 7 m2/hm2,而木荷种群的平衡位置是基面积为15.031 1 m2/hm2的点,表明目前木荷种群还处在演替阶段,达到平衡状态还要经过一段时期。 相似文献
2.
Doncaster CP 《Science (New York, N.Y.)》2006,311(5764):1100; author reply 1100
Stochasticity in time series explains concave responses of per capita growth rate to population size. The gradients with the natural log of population size have more biological importance because they measure strength of density compensation. Its weakening with increasing body size across taxa (Sibly et al., Reports, 22 July 2005, p. 607) is consistent with slower responses in ascent than descent toward carrying capacity. Time series therefore suggest that populations of large-bodied animals underfill their environments. 相似文献
3.
P A Morrison 《Science (New York, N.Y.)》1974,185(153):757-762
The population changes in San Jose and St. Louis between 1960 and 1970 exemplify the two broad trends-urban formation followed by metropolitan dispersal-that have shaped 20th-century urbanization in this country. The fact that these developmental trends were expressed through demographic processes found to be common to both cities, despite their contrasting recent experiences, suggests that generalizations can be made about the complex forces underlying urbanization. The formation of metropolitan San Jose's population parallels the traditional process whereby a region's growth comes to be focused, through migration, on a few urban centers. The modern variant is not characterized by a rural-to-urban shift, however, but by migration flows among urban areas, and particularly to a few most-favored areas, such as San Jose. Migratory growth has left a powerful demographic legacy in San Jose. This legacy is also instructive for studying the migratory formation of any new city's population. Its demographic character determines its demographic destiny, whose likely variations we can now perceive with some clarity. San Jose's population is both youthful and chronically migratory. The presence of many prospective parents and relatively few elderly persons lays a broad foundation for the population's continued growth through natural increase, despite the national downturn in fertility (14). Even without further net in-migration, the population of new cities like San Jose would continue to grow at an above-average rate. The hypermobility of San Jose's population (that is, its propensity for further migration) also has an important bearing on the future. With about 21 migrants entering and 17 departing each year per hundred residents, San Jose's rapid migratory growth rests (as it would in other new cities) on a precarious arithmetic balance. A significant dip in local employment growth could easily reduce net migration to a small fraction of its present high level. Even a slight decline would result in the inflow's no longer exceeding the high volume of outflow. Demographic analysis alone cannot foresee such an employment downturn, but if it happened, the migratory downturn probably would be swift. Hypermobility also works the other way; and given San Jose's focal position in California's expanding metropolitan structure (with its virtually endless supply of migratory growth), net migration could resume with equal swiftness. The outward dispersal of population from central cities that has occurred in St. Louis has been accelerating in other cities as well, and will remain a prominent feature of U.S. urban growth. It may seem paradoxical that in a period noted for something called "urban growth" there are so many declining central cities, but that is merely one indication that the "central city" no longer is the real city, except in name. Real city or not, the central city can expect to come into political conflict with other jurisdictions created in the process of dispersion. In cities like St. Louis, where population is dispersing but old political boundaries are fixed, the problems of the central city are separated from the resources in the suburbs. Transitional problems associated with persistent and severe outmigration also arise: accumulation of disadvantaged citizens, declining demand for city housing, and a diminished replacement capacity in the population. Carried far enough, the last of these problems results in natural decrease, and thereafter the population's decline acquires its own dynamic. As noted earlier, the white population in St. Louis has reached this point: The number of persons dying now exceeds the number being born. For two reasons, this natural decrease can do little other than intensify. First, a substantial proportion of whites are either entering or already within the high-mortality age brackets.The white population's crude death rate therefore will continue to rise. Second, prospective parents are becoming scarce among St. Louis's whites, and the national evidence that parents in general will choose to have smaller families continues to mount. The white population's crude birth rate is therefore likely to fall, barring a dramatic increase in fertility or a strong and sustained inflow of childbearing families. Nor is St. Louis's black population likely to grow substantially. It is expanding steadily through natural increase, but black migration out of the city is more than enough to cancel that increase. 相似文献
4.
为合理的种植密度可构建良好的群体结构,优化群体光合生理功能,提高光能利用效率,充分发挥高粱品种的增产潜力。以酿造高粱(Sorghum bicolor L. Moench)‘晋杂23号’为试验材料,在大田试验条件下研究了不同种植密度（4.5万株/hm2、7.5万株/hm2、10.5万株/hm2和13.5万株/hm2）对高粱生长、群体光合生理指标、籽粒产量及其构成因素的影响。结果表明：随着种植密度增加,高粱株高相应增高,而茎粗相应变细;高粱的叶面积指数(LAI)、群体光合势(LAD)和总光合势呈显著性增加,而透光率、群体净同化率(NAR)、群体生长率(CGR)呈现递减的趋势;高粱单株干物质积累量呈现递减的趋势,而群体干物质积累量呈显著性增加。在4.5万株/hm2~10.5万株/hm2密度范围内,籽粒产量随密度增加呈显著性增加,其中密度在10.5万株/hm2的籽粒产量比4.5万株/hm2增产13.8%。随着密度增加,单位面积穗数相应增加,穗粒数相应减少,千粒重没有显著的影响。相关分析表明,种植密度与LAI、LAD、生物产量和籽粒产量呈显著性正相关,而与透光率、NAR和CGR呈显著性负相关。本研究表明,种植密度对高粱群体生理指标、产量和产量构成因素有显著的影响。酿造高粱‘晋杂23号’的种植密度以10.5万株/hm2为宜,在高粱生产中可形成良好的群体结构,协调群体与个体的生长,优化群体光合生理功能,使生物产量与籽粒产量都达到较高水平。 相似文献
5.
Angiosperms are a relatively recent evolutionary innovation, but their genome sizes have diversified remarkably since their origin, at a rate beyond that of most other taxa. Genome size is often correlated with plant growth and ecology, and extremely large genomes may be limited both ecologically and evolutionarily. Yet the relationship between genome size and natural selection remains poorly understood. The manifold cellular and physiological effects of large genomes may be a function of selection on the major components that contribute to genome size, such as transposable elements and gene duplication. To understand the nature of selection on these genomic components, both population-genetic and comparative approaches are needed. 相似文献
6.
Ross JV 《Science (New York, N.Y.)》2006,311(5764):1100; author reply 1100
Sibly et al. (Reports, 22 July 2005, p. 607) recently estimated the relationship between population size and growth rate for 1780 time series of various species. I explain why some aspects of their analysis are questionable and, therefore, why their results and estimation procedure should be used with care. 相似文献
7.
Farming and the fate of wild nature 总被引：4，自引：0，他引：4
World food demand is expected to more than double by 2050. Decisions about how to meet this challenge will have profound effects on wild species and habitats. We show that farming is already the greatest extinction threat to birds (the best known taxon), and its adverse impacts look set to increase, especially in developing countries. Two competing solutions have been proposed: wildlife-friendly farming (which boosts densities of wild populations on farmland but may decrease agricultural yields) and land sparing (which minimizes demand for farmland by increasing yield). We present a model that identifies how to resolve the trade-off between these approaches. This shows that the best type of farming for species persistence depends on the demand for agricultural products and on how the population densities of different species on farmland change with agricultural yield. Empirical data on such density-yield functions are sparse, but evidence from a range of taxa in developing countries suggests that high-yield farming may allow more species to persist. 相似文献
8.
Theories of density-dependent natural selection state that at extreme population densities evolution produces alternative life histories due to trade-offs. The trade-offs are presumed to arise because those genotypes with highest fitness at high population densities will not also have high fitness at low density and vice-versa. These predictions were tested by taking samples from six populations of Drosophila melanogaster kept at low population densities (r-populations) for nearly 200 generations and placing them in crowded cultures (K-populations). After 25 generations in the crowded cultures, the derived K-populations showed growth rate and productivity that at high densities were elevated relative to the controls, but at low density were depressed. 相似文献
9.
研究了密度(12.04、24.08、48.16、144.49、240.81、337.13、433.46、529.78、626.11尾/m2)对凡纳滨对虾(平均初始湿重0.026±0.006g)存活、生长和能量收支的影响。实验周期45d。结果表明:I.12.04、24.08和48.16尾/m2密度水平对虾的存活率显著高于626.11尾/m2密度水平,但与其它处理差异不显著;II.12.04和24.08尾/m2密度水平对虾的特定生长率显著高于其它处理,特定生长率(SGRd)与密度(D)的回归方程为:SGRd=-0.3911Ln(D) 10.068(R2=0.8745)。 相似文献
10.
Experimental evidence is presented supporting a developmental model that explains the genetic basis for brain and body size associations. Evolutionary change in body size causes correlated change in brain size because some genes affect both traits. The commonly observed correlation between brain and body size results from genetic variation in growth determinants affecting both traits simultaneously during fetal and early postnatal growth. Later growth reduces brain-body correlation because of changes in the underlying causal components of growth in each trait. Brain-body size evolution shows a different pattern at higher taxonomic levels from that seen within and between closely related species because body-size evolution among higher taxa occurs primarily by change in early portions of growth, which share more genetic growth determinants with brain size. 相似文献
11.
龙栖山黄山松种群优势度增长规律研究 总被引：16，自引：2，他引：16
本文应用有限空间种群增长的逻辑斯谛模型研究了龙栖山自然保护区黄山松种群基面积增长规律，指出黄山松在不同林型下和同一林型不同密度下的基面积最大增长速度的径级范围． 相似文献
12.
在普通群体中,建立了3个及3个以上复等位基因的基因频率、基因型频率和随机交配率的关系及其数学模型,讨论了复等位基因及其频率,以及由复等位基因所构成的各种基因型频率在世代中的动态规律、遗传趋势、群体平衡及其性质。结果表明,在普通群体遗传中,非平衡群体的数学模型(4)可以转换为随机交配群体的数学模型和非平衡自交群体的数学模型;平衡群体的数学模型(7)可以转换为随机交配群体的数学模型和平衡自交群体的数学模型。在普通群体遗传平衡时,当3个复等位基因频率相等且随机交配率等于3/5时,群体各种基因型频率相等。 相似文献
13.
The authors make the case that population growth can lead to increases in agricultural output per head on a sustainable basis. This argument, originally developed by E. Boserup, is "illustrated by a study of Machakos District, Kenya during 1930-1960, which shows that, if policies are supportive, agricultural and non-farm incomes grow faster than even the rapid population growth rate experienced in Africa. Land use capability is not fixed, but can be transformed by investment, new technologies and good management. Lack of investment and consequent degradation are most likely at low population densities. While the study cannot foretell the future, Java illustrates a similar theme at even higher densities." 相似文献
14.
Hoch I 《Science (New York, N.Y.)》1976,193(4256):856-863
As cities increase in size, so do wage rates for the same work. There is evidence that the wage differential is persistent and stable over time, which suggests that the differential does not arise from a lack of adjustment that is in process of correction. Indeed, there is an inverse relation between size and growth rate. Large metropolitan areas with high wage rates have been losing population in recent years, which is hardly a sign that their higher wage rates are temporary inducements to workers to move into those cities. It is much more plausible that the differential is a more-orless permanent money payment that compensates urban residents for costs they bear as population size increases. This argument does not deny that there are nonwage benefits as well as costs of city size, that city size effects may vary between individuals and groups, or that there may be scope for improved policy on population distribution. Nonetheless, the benefits of size seem to be outweighed by the costs; all types and groups of people generally can and do move about until alternative locations are less attractive than their current location; and solutions to population distribution problems will often emerge as byproducts to the solutions of more basic problems. 相似文献
15.
Bazin et al. (Reports, 28 April, 2006, p. 570) found no relationship between mitochondrial DNA (mtDNA) diversity and population size when comparing across large groups of animals. We show empirically that species with smaller populations, as represented by eutherian mammals, exhibit a positive correlation between mtDNA and allozyme variation, suggesting that mtDNA diversity may correlate with population size in these animals. 相似文献
16.
针对有限空间种群增长的逻辑斯谛模型的缺陷, 本文应用广义的逻辑斯谛曲线研究了龙栖山自然保护区黄山松种群数量动态, 并指出了黄山松在不同林型下和同一林型不同密度下的断面积最大增长速度的径级范围。结果表明, 广义的逻辑斯谛曲线能很好地拟合黄山松种群数量动态规律, 从而为黄山松的生产和经营管理提供了理论依据。 相似文献
17.
采用分期接虫和分期回收的方法，研究常规棉与转基因棉品种棉铃虫寄生性天敌在不同棉花品种间的寄生作用。结果表明：棉铃虫第2～4代发生期以及卵期和幼虫期，转基因棉品种的棉铃虫卵、幼虫寄生率均显著低于常规棉品种。棉铃虫卵期的寄生蜂主要为拟澳洲赤眼蜂(Tricogramma confisum)，幼虫寄生蜂为棉铃虫齿唇姬蜂(Compoletis chlorideae)等。由此可见转基因棉对棉铃虫寄生性天敌存在非亲和效应。通过对转Bt基因棉国抗22及其亲本泗棉3号棉铃虫不同世代间卵、幼虫寄生率的纵向比较，发现同一品种棉花上不同世代间棉铃虫卵寄生率差异不显著，而幼虫寄生率差异显著。 相似文献
18.
19.
Thomas JA Telfer MG Roy DB Preston CD Greenwood JJ Asher J Fox R Clarke RT Lawton JH 《Science (New York, N.Y.)》2004,303(5665):1879-1881
There is growing concern about increased population, regional, and global extinctions of species. A key question is whether extinction rates for one group of organisms are representative of other taxa. We present a comparison at the national scale of population and regional extinctions of birds, butterflies, and vascular plants from Britain in recent decades. Butterflies experienced the greatest net losses, disappearing on average from 13% of their previously occupied 10-kilometer squares. If insects elsewhere in the world are similarly sensitive, the known global extinction rates of vertebrate and plant species have an unrecorded parallel among the invertebrates, strengthening the hypothesis that the natural world is experiencing the sixth major extinction event in its history. 相似文献
20.
World population and world income can grow at any likely rate for the next 50 to 75 years, probably for longer, and mineral supplies will continue to keep pace with demand. Not, however, without environmental costs, without affecting Third World development, and, perhaps most important, without ignoring critical questions of power. In what might be termed the revisionist form of the limits to growth thesis, Aurelio Peccei and Alexander King, cofounders of the Club of Rome, seem to be saying that the forecasts of doom themselves are unimportant but they symbolize critical problems of the nature and uses of power in the modern world (30): . . . the Club of Rome is questioning the quality of growth and its distribution around the world. . . . We know that the present structure of the world is obsolete. . . . Both private and state capitalism are stale . . . we have to develop something else. Surely, continually increasing rates of mineral production are symptoms of this obsolete power structure, a result of the fact that, ultimately, population growth and monetary income growth lead to demands for natural resources that necessitate their being found and produced regardless of the implications. Since such higher rates of production are geologically and economically sustainable, we should choose among alternative paths of growth, and hence among alternative rates of mineral resource development, according to what we like or dislike about these implications. The key information will not be found in tables comparing reserves and consumption but in preferences and ethics. 相似文献