Stable Population Structure Research Articles

Population structure and ecological performance of the mangrove Avicennia marina (Forssk.) Vierh. on the Arabian Gulf coast of Qatar

Abundance and size distribution of mangal vegetation was studied in four transects in relation to edaphic and microtopographic variability. Distribution of juveniles is related to prevailing biotic and abiotic factors. Abundance is lower at higher elevations (lower salinity) with a higher ratio in the large size class compared with trees at lower elevations. Juvenile numbers are not related to the size of the canopy. Instead numbers increase at lower elevations, accumulating in the basin at low tide mark. Tidal cycles interact with microtopography and exert a major effect on the population structure and the fate of juveniles. A stable population structure is deduced at intermediate elevations with equal numbers of large and small size classes amounting to 37% of the population. High nutritional conditions and ecophysiological adaptations permit the establishment of seedlings carried by tidal water. The mangrove population is expanding on mud deposition in the basin behind the tidal delta that is building up in the area. Although the physical environment has a marked effect on the dispersal and establishment of seedlings, masking growth and reproductive rates of parent trees, it is the biotic attributes of the juveniles that are much more effective in determining the ecosystem's boundary for the expanding population.

The pasture ryegrass plant, what is it?

Detailed studies of plant growth processes are important in understanding the performance and persistence of species in pastures, particularly in response to uncharacteristic environmental stress. The morphology of perennial ryegrass plants in mixed sheep grazed pastures was determined in self contained farmlets undercontrasting managements of rotational grazing, set stocking or a combination of both. Average size was 90 mg total DW, with 4- 5 tillers and 12-13 leaves, little different to white clover. Ryegrass exhibits strong clonal growth, with extension at the apex and death of the basal stem releasing branches to form new plants at regular intervals, maintaining a stable population structure of small plants all year. The normally short internodes on ryegrass stems can elongate to form stolon and elevate the apex to a more favourable position if survival is threatened. On average only 2530% of plants contained stolon at any one time. Because of high plant density the quantities of ryegrass stem present was often in excess of that produced by white clover in the same swards. Grazing management did not affect plant structure (numbers of tillers, leaves etc) only their size (dry weight), but had marked effects on pasture structure and subsequent survival of plants under stress. Keywords perennial ryegrass, plant morphology, grazing management, seasonal variation, plant survival, stolon formation

Ecology ofNothofagus menziesiiin the Catlins Ecological Region South-east Otago, New Zealand (III) growth and population structure

Abstract Nothofagus menziesii growth rate and population structure were studied in the Catlins Ecological Region. Most N. menziesii seedlings either exceeded 35 cm in height by 13 years or had died. A small proportion of suppressed seedlings persisted at less than 35 cm tall for up to 25 years. Diameter and age of stems larger than 10 cm d.b.h. were poorly correlated. Diameter growth rate variation, from 0.30 to 8.52 mm yr-1, was similar over the range of N. menziesii in the study area. Population structures showed continuous regeneration, except at low altitudes and on relatively fertile soils. At some sites, the other main tree species showed less stable population structures, a higher proportion of dead stems, or smaller basal area, in Nothofagus forest than in adjacent forest. It was concluded that N. menziesii forest has a continuously regenerating population, and is expanding its range in the Catlins at the expense of adjacent forest lacking N. menziesii, except on relatively warm and fertile sites.

Amyotrophic lateral sclerosis in Finland.

The epidemiology of amyotrophic lateral sclerosis (ALS) has been studied in Finland since 1963. The country is suitable for epidemiological investigations because of its stable population structure and accurate population registers.

Two-Sex Models: Chaos, Extinction, and Other Dynamic Consequences of Sex

Most demographic models consider only one sex, usually the female. The widespread occurrence of sexual dimorphism in life history traits and the occurrence of skewed and fluctuating sex ratios suggest that one-sex models or those dominated by one sex may often be less appropriate than two-sex models. Reproduction in two-sex models is a frequency-dependent nonlinear function (the birth or marriage function) of the relative abundance of males and females. In this paper, we examine the population dynamics resulting from three different two-sex, discrete-time, population-projection models. For a large class of birth functions, models without inter-stage mate competition are shown to converge to a locally stable adult sex ratio. Formulas for the stable population structure, stable sex ratio, and reproductive value at equilibrium are derived. When individuals of different stages compete for mates, the equilibrium population structure may become unstable. A sequence of bifurcations then occurs, leading to periodic oscillations, quasi-periodic fluctuations, and chaos as the intensity of competition increases. Finally, when per capita fecundity is a sigmoid function of the relative abundance of the other sex, perturbations of the sex ratio may lead to extinction.

Tree population structure of certain Himalayan forest associations and implications concerning their future composition

This study deals with the regeneration status of certain tree components of forests in the northwestern catchment of the river Gola in Kumaun Himalaya and explores possible compositional changes in time.Pinus roxburghii, Quercus floribunda andQuercus lanuginosa in their respective forest types had a stable population structure with a large percentage of young individuals (seedlings) and relatively few old ones, indicating that these species are frequent reproducers. The girth class distribution of the individuals ofQuercus leucotrichophora in theQuercus leucotrichophora forest suggests that this species is likely to disappear gradually. In this forest, although forest composition will remain unchanged for some time, a complete absence of seedlings of any species shows the possibility of its complete replacement by a scrub or a grassland vegetation. In the mixed forest, the dominant species, viz.,Quercus leucotrichophora, Persea odoratissima andBauhinia retusa did not have saplings/seedlings, indicating a possible change in forest composition in the near future whenSapium insigne, Cocculus laurifolius, Celtis eriocarpa andAcer oblongum are likely to gain dominance. Finally, suggestions are made to explain the poor regeneration ofQuercus leucotrichophora.

The Population Biology of Chamaelirium lutem, a Dioecious Member of the lily Family: Two-Sex Population Projections and Stable Population Structure

The Population Biology of Chamaelirium lutem, a Dioecious Member of the lily Family: Two-Sex Population Projections and Stable Population Structure

Permanent Settlement of Young People in the Region of the Baikal-Amur Trunk Railway

The construction of the Baikal-Amur Trunk Railway (BAM) and the economic development of the large territory surrounding it is an endeavor of national dimensions. And much depends in this regard on providing the region with labor, on creating a stable population structure. It is practically impossible today to solve the problem through natural population growth. The main solution is to attract labor from other areas and keep people here for a lengthy period.

The Population Biology of Chamaelirium Luteum, A Dioecious Member of the Lily Family: Two-Sex Population Projections and Stable Population Structure

Matrix projection models as typically applied to data from natural populations of an- imals and plants fail to distinguish between male and female contributions to population growth. The well-known Leslie model, for example, considers only females, and thereby ignores the interaction of females with males in the production of offspring. In the present paper, a two-sex size-specific population projection model is developed for Chamaelirium luteum, a long-lived dioecious plant. Application of this model to C. luteum is based on a 5-yr study of three large (N > 1000) natural populations of this forest-floor herb. Differences between sex classes (male, female, juvenile) in size- specific survivorship and size transitions are taken into account, as well as particular differences between males and females in age at first reproduction, flowering schedules, and reproductive output per individual. Projections of the three populations studied using this model generally showed a slight male bias in the adult sex ratio. Furthermore, comparison of observed size distribution and sex composition with projected stable populations indicated that the populations studied are near a stable structure and should show only modest changes in population size from year to year. These results are in accord with one's expectations for a long-lived species growing in an ecologically stable habitat.

Stable Population Structure and Reproductive Value for Populations with Complex Life Cycles

Classical demographic theory was developed to describe age—classified populations, in which knowledge of an individual's age accurately specifies its demographic state. In many species (e.g., plants, insects, fish) individuals of the same age may be in very different states; classical theory is inappropriate for such populations. In this paper, I present general formulae for the stable stage distribution (the analogue of the stable age distribution of classical demography) and the stage—specific reproductive value distribution for populations with arbitrarily complex life histories. The results are obtained by use of z—transforms of a simple graphical representation of the life cycle. They yield literal formulae, which can be used to draw qualitative conclusions about the general properties of the life history. They will also provide quantitative results if numerical values for the parameters are available. Since the selective pressures on life history traits can be expressed simply in terms of the stable stage distribution and reproductive value, these results render complex life cycles amenable to evolutionary analysis. The reproductive value formulae can sometimes be used to calculate optimal schedules of stage—specific reproductive effort.

POPULATION DYNAMICS AND DEMOGRAPHY OF AN ESTUARINE COPEPOD (PSEUDODIAPTOMUS EESSEI) IN LAKE SIBAYA, A SUBTROPICAL FRESHWATER COASTAL LAKE

SUMMARY Population size and stage structure of Pseudodiaptomus hessei (Mrazek) was studied over 27 months at a single site in subtropical Lake Sibaya. Population density varied seasonally from about 1 to 4 individuals per litre, but a fairly stable population structure was maintained by continuous breeding of this strongly multivoltine copepod. Embryonic durations were determined at several temperatures and used to estimate instantaneous birth rates, death rates and rates of population change. Overall mean values obtained for these population statistics were 0,345, 0,350 and, ignoring signs, 0,018 respectively, the former two values being particularly high. Some evidence linking death rates, and more particularly naupliar survival of P. hessei to cyclopoid copepods is presented; although it was not possible to discriminate between cyclopoid predation and cyclopoid competition, the former is favoured as the factor mostly influencing P. hessei mortality. Statistical analysis of interrelationships between va...

A refined estimator of measures of location of the age at first marriage

Because of bias of unknown sign and extent introduced by age misreporting when calculating the singulate mean age of marriage in the usual manner, Van de Walle has suggested a fairly robust estimator based on stable population structure. Unfortunately not much is known about the properties of this estimator. Various demographers have argued informally that it indeed estimates the SMAM; others feel that it instead estimates the mean age of marriage in a cohort, the mean age of marriage in the stable population, or the singulate median age of marriage. In this paper the properties of this estimator are examined. Further, extensions of the Van de Walle estimator based on regression are shown to be significantly superior to the estimator alone.

Distribution, burrowing, and growth rates of the clam Tridacna crocea on interior reef flats : Formation of structures resembling micro atolls.

Larvae of the burrowing clam Tridacna crocea (Tridacnidae) settle preferentially on top of detached coral heads lying on the surface of the interior reef flat in the Great Barrier Reef province. This species burrows as it grows, eroding the central top surfaces of coral boulders, producing structures that superficially resemble micro-atolls. Storm surges roll these coral heads onto the now flattened surface, killing the live population of clams, and exposing the fresh underside for unimpeded larval settlement. As these clams grow and burrow into the substratum, the coral head becomes progressively flattened and finally breaks apart. Field observations and growthring data documented growth rate; growth rates plus burrow volumes were converted to annual sediment production. At average population densities approximately 140 gm/m2/yr of coral are eroded. Concomitant with erosion is a calcium carbonate increase in the shell of these clams amounting to 60gm/m2/yr. Assuming a stable population structure, with annual mortality equal to annual estimated growth, total sediment production is 200 gm/m2/yr. Clams are usually aggregated at higher densities, however, with numbers regularly exceeding 100 clams/m2. Consequently maximum sediment production rate locally is often 4,500 gm/m2/yr.