Plant community structure in relation to grazing and environmental changes along a north-south transect in the western Kalahari

Sixteen vegetation types were described from a north-south transect in the western Kalahari. Pronounced differences were found between communities on the nutrient poor red Kalahari sand, covering most of the area and those on fine soils and white calcareous sand. Syntaxa resulting from severe overgrazing by livestock were in most cases clearly distinguished from the less disturbed vegetation. The communities on red sand consisted mainly of shrub savanna dominated by perennial tufted grasses, whereas in the vegetation on calcareous material and on overgrazed land, forbs, dwarf shrubs and shrubs played a more important role. The major communities on red sand showed a clear geographical zonation roughly corresponding to the gradient in mean annual rainfall and its interannual variation. In the northern and central Kalahari these syntaxa were dominated by species of Sudano-Zambezian origin and in the southern Kalahari by species showing Karoo-Namib affinities.     

Porphyra nereocystis: A dual-daylength seaweed

Conchospores from the perennial conchocelis phase of the annual, epiphytic, marine red alga Porphyra nereocystis Anderson, which in nature lives on the large annual kelp Nereocystis luetkeana (Mertens) Postels et Ruprecht, are released in culture only in response to a short-day photoperiod treatment followed by a long-day treatment. Each treatment requires a minimum of three to four weeks and is enhanced by lower temperature during the second photoperiod treatment. To our knowledge P. nereocystis is the first known dualdaylength seaweed and requires a short-day-longday treatment for completion of its life cycle. This stringent environmental control of its reproduction appears to be an adaptation to coordinate conchospore production with the seasonal availability of its host kelp Nereocystis.Abbreviations LD long day - SD short day - SLD shortlong day     

Age and growth of a colonizing minnow,Barbus anoplus,in a man-made lake in South Africa

Synopsis Aspects of the life history of Barbus anoplus were studied in Lake le Roux, a turbid man-made lake on the Orange River, South Africa. This minnow underwent a population explosion and successfully colonized the shoreline of the newly-formed lake during the early phases of reservoir filling. Male and female B. anoplus reach sexual maturity in one year at about 40 mm fork length. They have a multiple spawning habit with the first spawning in November–January and the second in February–March. The growth of the two resulting cohorts is discussed. It is proposed that the offspring from the second spawning not only acts as a ‘back-up’ but is capable of prolonging the life of that year-class into an additional reproductive season. Most of the minnows die after their second summer, but more offspring from the second spawning, especially females, live into a third summer. Females attain a larger maximum size (73 mm FL) and age (3–4 years) than males (60 mm FL, 2–3 years). B. anoplus is small and short-lived with a high seasonal reproductive potential, which is in contrast to the larger Barbus species in the Orange River system. These life-history traits enable the species to colonize and successfully inhabit unstable environments and probably account for its widespread distribution.     

A preliminary study on quantitative relations among growth,reproduction and mortality in fishes

As a quantitative approach to the life histories of fishes, the present paper attempted to predict a relation among reproduction, growth and mortality numerically with a technique of control theory, the discrete maximum principle. A method for predicting the relation was derived on the postulate that natural selection maximized the net reproductive rate subject to a few constraints. The derived method was applied to Atlantic cod and Atlantic herring populations in the Southern Gulf of St. Lawrence as numerical examples. The examples demonstrated that the theoretical reproductive effort and body weight were well consistent with the observed ones every age but the theoretical survival rates were slightly different from the observed ones. For the reasons mentioned below, however, it should be interpreted that the examples rather support the adopted postulate to a certain degree. First, in general, it is very difficult to obtain good estimates of the rates with traditional methods. Second, intense fishing pressure possibly changes the life history parameters to some extent in fish populations. Moreover, the examples also suggested that, to examine the postulate in further detail, similar analyses had to be made with the data of many fish populations on which intense fishing pressure had not been exerted.     

树鼩(Tupaia belangeri chinensis)精子发生的细胞学观察

对8只成体树鼩的睾丸进行了精子发生的细胞学和动力学观察。精原细胞可区分为A型和已分化定向的B型。生精上皮周期可分为12个连续阶段,不同阶段典型的细胞组合可被辨认。各阶段的相对持续时间以阶段Ⅴ、Ⅵ、Ⅶ的比率最高,分别为11.43,18.88、15.44;阶段Ⅸ、Ⅹ最低,为3.78和3.87。A型精原细胞在各阶段的数量分布并不保持恒定,阶段Ⅵ—Ⅷ和阶段Ⅸ时,分别出现两次成倍增长。B型精原细胞和前细线期精母细胞与每100个足细胞之比的平均值,在阶段Ⅰ、Ⅴ、Ⅶ分别为28.89、76.98、196.91。精细胞经14步变态为成熟精子。精子形成过程中顶体发育的形态学变化及成熟精子的形态,与灵长类动物相比具有较多的类似之处;而同啮齿类动物相比,存在明显的形态差异。     

Natural mortality at different population densities of a pine shoot moth,Evetria cristata

A field population of Evetria cristata was studied in 10 plots in 1962 and in 6 plots in 1963. These plots were divided into 2 or 3 groups of different population levels of the shoot moth in respective years. The survival of the insect was then analysed in these different groups of plots. The survival rate of E. cristata from eggs to adults in the first generation was found always higher in the group with low population density, which indicates the existence of some factors that affect the population more severely when the insect is more abundant. Lissonota evetriae and Pediobius sp. seemed to have killed more proportion of the hosts where the shoot moth density was high. However, the total effect of the all natural enemies was not always great in the plots with high density of the moth. The survival of the second generation of the moth in 1963 was observed to be much higher at any population level than in the other generations.     

A method for studying vegetation dynamics when there are no obvious individuals: Virtual-population analysis applied to the tundra shrub Betula nana L.

A method is presented to study dynamics of plants that cannot be separated into individuals such as many grassland, salt marsh and tundra species. A virtual population is created by using a permanent transect line through the vegetation and individuals are defined as the branch segments distal to the intercept with the transect line. Addition and loss of individuals together with growth or shrinkage form the basis for constructing a size-structured transition matrix. A discrete-event simulation demonstrates that: 1) a virtual population of individuals grows at the same rate as the parent population; and, 2) size-structured transition matrices for a virtual population and parent vegetation have similar dominant and subdominant eigenvalues so a virtual population can be used to describe the dynamics of a parent vegetation.Dwarf birch, Betula nana L., was studied in the northern foothills of the Brooks Range, Alaska, by using photography to record branch intercepts along permanent transect lines. The distal branch segments constitute a virtual population of the parent vegetation. Transects were photographed in 1985 and again in 1986 and changes of branch segments were used to construct two transition matrices for shrubs with and without elevated fertilizer treatment. Analysis of the virtual populations suggests that although Betula nana may show increased branch growth with increased fertilizer, in the long run this shrub may decline in the tundra in response to such treatment.     

关于茭白黑粉菌的正名问题

茭白黑粉菌在文献中出现Yenia esculenta(P.Henn.)Liou及Ustilago esculenta P.Henn.两个名字。现经形态发生学的研究以明确新名字是否有效。此菌冬孢子没有休眠期,孢子成熟后在适宜的环境中即可萌发。萌发的最适pH值为6;最适温度为25％;不需要外界光的诱导,在黑暗条件下也能正常萌发;培养基中的营养成份对孢子萌发的速率有一定影响。冬孢子在不同的环境条件下,其萌发形态及方式都是稳定的:孢子萌发时产生有隔的担子或先菌丝(promycelium)。该先菌丝起初可以是无隔的,但生长到一定程度,便产生分隔,分隔多为2—4个。初生的先菌丝和小孢子是单核的,但成熟的先菌丝、小孢子及短菌丝细胞内均是双核的。可以认为,该菌的双核期在整个生活史中占有相当长的时间,它只有短暂的单核期。没有发现先菌丝、小孢子彼此之间或相互间融合的现象;未见到双核的融合及减数分裂过程。茭白黑粉菌在人工组合培养基上生成厚壁的孢子,较自然界中的冬孢子为大,但萌发方式则相同。最后作者等认为茭白黑粉菌仍应保留在Ustilago属中,而Yenia esculenta(P.Henn.)Liou的名字是无效的。     

猕猴(Macaca mulatta)生命表研究

本文研究了猕猴雌性生命表。南湾猕猴各年度全群平均存活率高于93％,特定年龄组逐年存活率直到13岁仍达0.81以上,种群内禀增长率为0.1327,年周增长率为1.1419。用莱斯利矩阵对种群发展作预测,将预测值与1985-1987年的实测值作卡方适应性检验,两者无显著性差异。进一步分析发现该方法预测南湾猕猴种群动态,在7年内是有效的。