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201.
Abstract. Arthrocnemum macrostachyum is a perennial species acting as a primary colonizer of salt pans in Mediterranean high salt marshes. Salicornia ramosissima, an annual, occurs in salt pans under Arthrocnemum canopies and in open areas. The aim of this study was to analyse, in wild populations and a transplant experiment, how S. ramosissima population dynamics and growth are affected by A. macrostachyum. The environmental conditions within the patches of Arthrocnemum were less stressful than in the open areas, with lower radiation levels and salinity concentrations. In the inner areas of A. macrostachyum patches, density‐dependent mortality processes of S. ramosissima seedlings led to low densities of adult individuals with greater morphological development and reproductive success than in open areas. However, at the edges of Arthrocnemum patches facilitation of seedling survival favoured high densities. Environmental stress hindered development, decreased reproduction and premature death. These results are in agreement with the general theory of factors controlling vegetation distribution that biotic interactions dominate in low stress environments, while abiotic interactions dominate under harsher environmental conditions. A. macrostachyum plays an essential role in the succession in these salt pans, facilitating seed production and stimulating nucleation processes in S. ramosissima. 相似文献
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Summary The expression of many genetic defects may be suppressed by proper medical care or even by changing the environmental conditions.
We have used the Penna model of ageing to show that such efFects may be responsible for increasing the human life expectancy
during the 20
th
century. This effect is equivalent to the shift of the threshold (T) in the Penna model, which determines how many deleterious, expressed mutations kill an organism. For long genomes, the shift
of T changes the age distribution significantly with negligible relative changes in the maximum life span, while for short genomes,
the shift of T changes both, the age distribution as well as the maximum age. Unfortunately the same simulations show that the strategy
of enhancing the medical care requires more and more effort to keep the mortality rate of our populations at the same lower
level and that some new defects could be exposed to selection. 相似文献
206.
A. Der Van Linde 《Biometrical journal. Biometrische Zeitschrift》1989,31(3):371-378
Papadakis' method is shown to be an example of an estimation procedure for a heterogeneity function localized in a reproducing kernel Hilbert space. Within this framework some current models for the evaluation of field trials are discussed. 相似文献
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C. D. Mathers 《Biometrical journal. Biometrische Zeitschrift》1984,26(1):33-38
The heterogeneous Poisson process with discretized exponential quadratic rate function is considered. Maximum likelihood estimates of the parameters of the rate function are derived for the case when the data consists of numbers of occurrences in consecutive equal time periods. A likelihood ratio test of the null hypothesis of exponential quadratic rate is presented. Its power against exponential linear rate functions is estimated using Monte Carlo simulation. The maximum likelihood method is compared with a log-linear least squares techniques. An application of the technique to the analysis of mortality rates due to congenital malformations is presented. 相似文献