共查询到20条相似文献，搜索用时 15 毫秒
1.
综述了鱼类对晶体氨基酸的利用效率及其影响因素。晶体氨基酸的溶失,晶体氨基酸与饲料中蛋白态氨基酸的吸收不同步主要影响了鱼类对晶体氨基酸的利用;饲料pH、饲料离子平衡、水温和氨基酸构型也会对晶体氨基酸的利用产生一定的影响;不同种类和大小的鱼类对晶体氨基酸的利用也有很大的差异。因此,可通过微胶囊或包被处理、增加投喂频率和调节饲料pH/离子平衡来提高鱼类对晶体氨基酸的利用效率。 相似文献
2.
鱼类对晶体氨基酸利用效率的研究进展 总被引：1，自引：0，他引：1
综述了鱼类对晶体氨基酸的利用效率及其影响因素。晶体氨基酸的溶失,晶体氨基酸与饲料中蛋白态氨基酸的吸收不同步主要影响了鱼类对晶体氨基酸的利用;饲料pH、饲料离子平衡、水温和氨基酸构型也会对晶体氨基酸的利用产生一定的影响;不同种类和大小的鱼类对晶体氨基酸的利用也有很大的差异。因此,可通过微胶囊或包被处理、增加投喂频率和调节饲料pH/离子平衡来提高鱼类对晶体氨基酸的利用效率。 相似文献
3.
应用“相邻格子法”对塔里木河中游不同生境胡杨种群进行调查,研究不同生境胡杨种群径级结构和空间分布格局。结果表明:1）不同生境下胡杨径阶结构不同,河漫滩生境胡杨幼苗充足,为增长型种群;过渡地带各径阶分布合理,为稳定型种群;而沙地生境径阶≤16 cm的胡杨种群未出现,为衰退型种群。2）河漫滩以及过渡地带生境胡杨种群在5种空间尺度下均处于集群分布;沙地生境总体为随机分布。3种生境下胡杨种群均在5 m×5 m的尺度有较强的聚集性。3）河漫滩、过渡带生境下各个发育阶段均为集群分布,沙地生境随着发育进程,空间分布由聚集分布向随机分布转化。 相似文献
4.
Prospects for an invasion: competition between Aedes albopictus and native Aedes triseriatus 总被引：3，自引：0，他引：3
Competition between larval populations of the native North American treehole mosquito Aedes triseriatus and Aedes albopictus, recently introduced from Asia to North America, was assessed by comparing per capita growth rate estimates for experimental cohorts of larvae developing under a variety of initial density combinations in fluid obtained from tires or from treeholes. Estimates of carrying capacities and competition coefficients indicate that competition between the two species will result in stable coexistence in treehole communities but local extinction of A. triseriatus in tire habitats. 相似文献
5.
Bakker RT 《Science (New York, N.Y.)》1982,217(4554):53-55
Among extant large reptiles, juveniles often occupy different habitats from those of adults or subadults and thus avoid competition with and predation from the older animals; small juveniles often choose cryptic habitats because they are vulnerable to a wide variety of predators. Evidence from fossil humeri and femora of Early Permian reptiles collected from sediments of several distinct habitats indicate that similar shifts in habitat occurred. Juvenile Dimetrodon seem to have favored cryptic habitats around swamp and swampy lake margins; adults favored open habitats on the floodplains. Similar patterns of habitat shift seem to be present in the reptile Ophiacodon and the amphibian Eryops and may have been common in fossil tetrapods of the Permian-Triassic. 相似文献
6.
水体中悬浮泥沙对藻类生长的影响 总被引：2，自引：0，他引：2
藻类生长需要适当的光强,水体中悬浮物(如泥沙)的存在,势必对进入水体的光强产生抑制,从而影响藻类生长。通过实验研究了不同浓度和不同粒径的泥沙对藻类生长的影响,结果表明,适量的泥沙浓度反而使水体chla最大浓度比空白水体有所增大,100目粒径的泥沙对光线的阻隔作用最强。 相似文献
7.
The distribution of species body size within taxonomic groups exhibits a heavy right tail extending over many orders of magnitude, where most species are much larger than the smallest species. We provide a simple model of cladogenetic diffusion over evolutionary time that omits explicit mechanisms for interspecific competition and other microevolutionary processes, yet fully explains the shape of this distribution. We estimate the model's parameters from fossil data and find that it robustly reproduces the distribution of 4002 mammal species from the late Quaternary. The observed fit suggests that the asymmetric distribution arises from a fundamental trade-off between the short-term selective advantages (Cope's rule) and long-term selective risks of increased species body size in the presence of a taxon-specific lower limit on body size. 相似文献
8.
在水体中发现大型底栖动物入侵种是一种世界范围普遍的现象。人类移动的同时将物品从一个大陆带到另一个大陆,这使许多水生动物物种能够突破自然的防线侵入到其他的区域和大陆。藉由人类现代的旅行方式,在不同区域间移动的物种数量显著增加,成功入侵种的数量也随之增多。成功入侵种需要具备一些特征,如可动性,小体型,适应性强和繁殖率高等,这些均有利于物种跨区域的入侵。当然,新生活环境也必须满足可适应的标准。对一个受人类活动破坏而丧失其原有物种的水环境来说,它对新物种入侵的抵抗力会很弱甚至缺乏。德国水体受人类活动的胁迫非常严重,许多自然水体受到的破坏已无法挽回,有些甚至已经完全成为了人工环境。这些水体由相互连接着的溪流河川网络组成,这很容易使新物种的入侵范围扩大。在外来物种的入侵面前,受人类活动胁迫和直接破坏的水体,比尚保持着天然环境状况的水体更加脆弱。 相似文献
9.
在水体中发现大型底栖动物入侵种是一种世界范围普遍的现象。人类移动的同时将物品从一个大陆带到另一个大陆,这使许多水生动物物种能够突破自然的防线侵入到其他的区域和大陆。藉由人类现代的旅行方式,在不同区域间移动的物种数量显著增加,成功入侵种的数量也随之增多。成功入侵种需要具备一些特征,如可动性,小体型,适应性强和繁殖率高等,这些均有利于物种跨区域的入侵。当然,新生活环境也必须满足可适应的标准。对一个受人类活动破坏而丧失其原有物种的水环境来说,它对新物种入侵的抵抗力会很弱甚至缺乏。德国水体受人类活动的胁迫非常严重,许多自然水体受到的破坏已无法挽回,有些甚至已经完全成为了人工环境。这些水体由相互连接着的溪流河川网络组成,这很容易使新物种的入侵范围扩大。在外来物种的入侵面前,受人类活动胁迫和直接破坏的水体,比尚保持着天然环境状况的水体更加脆弱。 相似文献
10.
Separate sedimentary populations are not all characterized by lognormal size distributions. Populations of different sizes of material are produced by different mechanisms of abrasion: pebbles and gravel by singlestage crushing; sand by repeated abrasion; and silt and clay by chipping from larger grains. Each mechanism imposes a different size distribution on the product. 相似文献
11.
Adult body size and fecundity of a number of hemimetabolous aquatic insects depend largely on thermal conditions during the larval period. Small adults and reduced fecundity result when temperatures are either warmed or cooled with respect to more optimal thermal conditions. Temperature apparently affects adult size by altering the larval growth rate and the timing and rate of adult tissue development for each larva. The data suggest a new interpretation for the geographic distribution of aquatic insects. 相似文献
12.
Pechmann JH Scott DE Semlitsch RD Caldwell JP Vitt LJ Gibbons JW 《Science (New York, N.Y.)》1991,253(5022):892-895
Reports of declining amphibian populations in many parts of the world are numerous, but supporting long-term census data are generally unavailable. Census data from 1979 to 1990 for three salamander species and one frog species at a breeding pond in South Carolina showed fluctuations of substantial magnitude in both the size of breeding populations and in recruitment of juveniles. Breeding population sizes exhibited no overall trend in three species and increased in the fourth. Recent droughts account satisfactorily for an increase in recruitment failures. These data illustrate that to distinguish between natural population fluctuations and declines with anthropogenic causes may require long-term studies. 相似文献
13.
本研究以国家水禽品种资源基因库（福建）内保护的金定鸭、莆田黑鸭、连城白鸭、绍兴鸭、攸县麻鸭、山麻鸭等6个鸭品种为对象,通过比较6个品种不同世代的生长性能和蛋品质指标,分析“家系等量随机选配法”在水禽基因库保种中的应用效果,为进一步丰富和完善水禽保种技术及理论提供依据。结果表明,6个品种不同世代间不同日龄体重、300日龄体尺、产蛋性能及蛋品质测定结果均无显著性差异（P ＞0.05）,且测定结果均符合国家标准及畜禽遗传资源志对该品种的描述。在闭锁小群体中采用“家系等量随机选配法”能有效地保存水禽品种资源的遗传多样性。 相似文献
14.
不同稳定性群落下飞机草种群格局的分形特征 总被引：1，自引：0，他引：1
为揭示桂西南喀斯特区域不同群落的稳定性对飞机草种群入侵、定居、生存和发展能力的影响,笔者运用分形理论中的计盒维数和信息维数研究了不同稳定性群落下飞机草种群的格局分形特征。结果表明,不同稳定性群落的飞机草种群格局分形特征不同。稳定性强的群落飞机草分布格局计盒维数和信息维数均较小,种群占据空间的能力较弱;稳定性中等的群落飞机草的格局分形特征存在计盒维数较大和较小2种情况,反映了飞机草在剧烈的种间竞争中所处的地位强弱状况,信息维数较小则说明飞机草种群聚集强度较弱,个体分布均匀;稳定性差的群落飞机草格局计盒维数较大,飞机草种群空间占据能力较强,信息维数存在较大和较小2种情况则隐含飞机草种群聚集强度随着群落的演替发展而变小。 相似文献
15.
【目的】了解大旗瓣凤仙花野生种群生长状况和花部特征,以及异质生境对其种群特征和生长的影响,为大旗瓣凤仙花野生资源生长状况评价及保护提供理论参考。【方法】在桂林市阳朔县选择18个大旗瓣凤仙花野生种群作为调查对象,详细调查记录其种群规模、生境基本参数,株高、冠幅、分枝数等植株性状,以及花色、花大小、花量等开花性状,共14个量化指标,结合种群土壤养分含量及不同种群的生境类型和干扰情况,分析异质生境下大旗瓣凤仙花种群生长状况。【结果】 18个大旗瓣凤仙花种群规模表现出很大差异,种群数量为20~10000株以上、面积3~10000 m2以上,生境中郁闭度和土层深度也各不相同;不同种群植株高度、冠幅和分枝数差异明显,最大值分别为最小值的3.23、3.13和7.50倍;就开花特征而言,花量差异可达10倍以上,花朵长、花朵宽 （旗瓣宽）、花纵深和翼瓣宽等均在少数种群间表现出显著差异,最大值比最小值分别高出43.67%、 27.73%、 26.62%和25.96%;不同种群土壤养分含量差异较大,主要归因于人类活动。聚类分析显示18个种群中11个综合生长状况良好,以种群11最佳, 7个种群生长状况较差,以种群17最差,较好、较差种群在生境类型的优劣上体现出对应关系。人为干扰对于大旗瓣凤仙花种群生长具有影响,但未发现明显规律性。【结论】异质生境下大旗瓣凤仙花不同种群间株高、冠幅、分枝数和开花数量差异较大,上述4个指标是影响种群延续性的重要参数;大旗瓣凤仙花对土壤要求不高,土壤不是影响大旗瓣凤仙花种群生长差异的首要因素;种群11整体表现最好,可作为优异种质资源进一步开发利用。 相似文献
16.
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
17.
In animals, appendages develop in proportion to overall body size; when individual size varies, appendages covary proportionally. In insects with complete metamorphosis, adult appendages develop from precursor tissues called imaginal disks that grow after somatic growth has ceased. It is unclear, however, how the growth of these appendages is matched to the already established body size. We studied the pattern of cell division in the tobacco hornworm Manduca sexta and found that both the rate of cell division and the duration of growth of the wing imaginal disks depend on the size of the body in which they develop. Moreover, we found that both of these processes are controlled by the level and duration of secretion of the steroid hormone ecdysone. Thus, proportional growth is under hormonal control and indirectly regulated by the central nervous system. 相似文献
18.
西藏不同产区藏鸡体重和体尺性状的测定与分析 总被引：1，自引：0，他引：1
藏鸡是生活在青藏高原上的原始地方小型鸡种,为了了解西藏不同分布区藏鸡体型特征与体重、体尺性状间的关系,测定了西藏5个典型产区的成年藏鸡体重和体尺性状指标,并进行了相关分析。结果表明,嘉黎县藏鸡体重、多项体尺性状指标较高,尼木县藏鸡较低;体重与体尺性状间存在极显著的相关关系(P<0.01),其中体重与胫长的相关系数最高,达0.763 5;建立的最优回归方程表明,对体重影响的重要性依次为胫长、胸围、胸宽、胸骨长和髋宽;主成分分析显示,5个主成分含有体重和体尺8项指标90.27%的信息量,说明不同产区藏鸡体重和体尺性状存在差别,但都具有体重轻、体型小、身体较长、胫长长等特点。 相似文献
19.
Berry JA 《Science (New York, N.Y.)》1975,188(4188):644-650
I have focused on examples of plant adaptations to environmental conditions that range from adjustments in the allocation of metabolic resources and modification of structural components to entirely separate mechanisms. The result of these modifications is more efficient performance under the stresses typically encountered in the plants' native habitats. Such adaptations, for reasons which are not entirely clear, often lead to poorer performance in other environmental conditions. This situation may be a fundamental basis for the tendency toward specialization among plants native to specific niches or habitats. The evolutionary mechanisms that have resulted in these specializations are very large-scale processes. It seems reasonable to suppose that the plants native to particular habitats are relatively efficient in terms of the limitations imposed by those habitats, and that the adaptive mechanisms these plants possess are, compared to those which have evolved in competing organisms, the most succesful biological means of coping with the environmental stresses encountered. I believe that we can learn from nature and utilize the adaptive mechanisms of these plants in agriculture to replace in part our present reliance on resources and energy to modify the environment for plant growth. By analogy with natural systems, improved resource utilization will require specialization and greater knowledge of the limitations of a particular environment and plant genotype. For example, the cultural conditions, plant architecture, and physiological responses necessary to achieve high water use efficiency from our crop species with C(4) photosynthesis probably differ from those required to achieve maximum total growth. Also, efforts to control water application to eliminate waste carry with them the risk that the crop could be injured by inadequate water. Thus, greater demands would be placed on the crop physiologist, the plant breeder, and the farmer. Planting and appropriate management of adapted crop genotypes could enable cultivation of many areas presently considered unusable because of environmental extremes or shortage of resources, and may lead to more efficient resource utilization on land already under cultivation. The costs or benefits of this cannot yet be estimated. However, I suspect that the greatest potential for application of such techniques will be in the developing rather than the developed regions of the world. The genetic and functional diversity of plants is a tremendous biological resource. The capacity of plants to adjust in the future to changing environmental conditions depends on this diversity and on evolutionary processes of nature. Wild plants may provide a source of genetic material to improve crop plants. Also, as advocated by McKell (22), wild plants can be utilized to a greater extent directly by man. Long-term research efforts and commitment to preserve natural habitats and their populations of wild plants will be required to maintain and more effectively utilize this resource. 相似文献
20.
不同地理群体菲律宾蛤仔生长发育的比较 总被引：4，自引：0，他引：4
于2007年6月-11月,对菲律宾蛤仔Ruditapes philipinarum莆田群体（PT）、大连群体（DL）和东京群体（DJ）的生长发育情况进行了比较。结果表明：菲律宾蛤仔3个群体的受精率、孵化率无显著差异（P〉0．05）;卵径、D形幼虫大小、单水管稚贝大小差异显著（P〈0．05）;附着规格、亲贝形态,PT群体与DL、DJ群体间差异显著（P〈0．05）。浮游期间,幼虫生长速度依次为PT〉DJ〉DL,分别为（9．37±0．44）、（9．28±0．59）、（8．70±0．51）um／d,PT、DJ群体与DL群体差异显著（P〈0．05）;913龄时,PT、DL、DJ群体幼虫的存活率分别为（95．08±1．87）％、（85．22±3．34）％、（75．32±3．49）％,彼此间差异显著（P〈0．05）。变态期间,3个群体幼虫的生长速度依次为PT〉DJ〉DL,分别为（2．54±0．39）、（2．37±0．52）、（2．05±0．45）um／d,彼此间差异显著（P〈0．05）;变态率、变态时间、变态规格,PT群体与DL、DJ群体间差异显著（P〈0．05）。室内培育阶段（20—40日龄）,3个群体稚贝的生长速度依次为PT〉DL〉DJ,分别为（22．60±4．04）、（20．67±4．74）、（18．74±5．15）um／d,PT、DL群体与DJ群体间差异显著（P〈0．05）;40日龄时,PT、DL、DJ群体稚贝的存活率分别为（87．07±4．29）％、（66．73±6．43）％、（35．75±4．84）％,彼此间差异显著（P〈0．05）。室外养成阶段（80—160日龄）,3个群体稚贝的生长速度依次为PT〉DL〉DJ,分别为（86．94±21．72）、（75．23±16．91）、（67．25±18．26）um／d,彼此间差异显著（P〈0．05）;160日龄时,门、DL、DJ群体幼贝的存活率分别为（90．21±4．68）％、（66．73±4．94）％、（56．98±6．58）％,彼此是差异显著（P〈0．05）。 相似文献