一个竞争模型的一致持续生存

本文研究一个带有时滞的竞争模型的一致持续生存。首先证明了离散时滞不影响种群的一致持续生存,接着在种群增长率是周期的假设下讨论了正周期解的存在性,最后给出了连续时滞模型一致持续生存的充分条件。     

太白红杉种内和种间竞争研究

采用逐步扩大范围的方法确定影响对象木 (Objectivetree) 的最佳竞争范围, 利用单木竞争指数的改进模型对太白红杉 (Larixchinensis) 种内和种间竞争强度进行了定量分析, 并讨论了不同竞争强度下太白红杉的形态变化。结果表明 :随对象木胸径的增大, 由于太白红杉种群自然稀疏过程中密度调节作用, 植株距离增加, 种内竞争强度降低 ;太白红杉主要分布于亚高山地段, 群落内其它物种较少, 个体普遍较小, 结果种间竞争相对较弱, 种内与种间竞争关系顺序为 :太白红杉 太白红杉 >巴山冷杉 (Abiesfargesii) 太白红杉 >牛皮桦 (Betulaplatyphylla) 太白红杉 >其它树种 太白红杉 ;竞争强度和对象木胸径的关系服从幂函数关系 (CI =AD-B), 当太白红杉胸径达到 35cm以上时, 竞争强度几乎没有变化, 所得的预测模型能很好地预测太白红杉种内和种间的竞争强度 ;不同竞争强度下, 太白红杉主茎各层的分枝角度、总分枝数、当年生枝条长、平均枝长和活枝数均表现出显著的差异。表明采用逐步扩大范围的方法能有效地确定竞争木范围, 较好地反应太白红杉种内和种间的竞争关系。同时, 太白红杉通过自身形态变化, 提高了对光的截获能力和对不同竞争强度的适应能力。     

POLLEN-TUBE COMPETITION AND MALE FITNESS IN HIBISCUS MOSCHEUTOS

The stigmas of animal-pollinated flowers often capture more pollen than is needed to fertilize all available ovules, and mixed-donor pollen loads are probably common. When this is the case, variation in average pollen-tube growth rates can potentially affect the number of seeds sired by a given plant. Despite considerable interest in effects of postpollination processes on male fitness, little is known about the extent of variation in pollen performance among plants from natural populations. To examine this question in Hibiscus moscheutos (rose mallow), we conducted mixed-donor hand-pollination experiments with 39 pollen donors bearing distinctive isozyme markers. Pairs of competing donors were compared on sets of 11 to 15 recipient plants per pair. These donors often differed in the proportions of seeds they sired, with the maximum deviation from an expected ratio of 50:50 being 68:32. Furthermore, three intensively studied plants exhibited consistent trends in relative pollen performance when each was tested against (1) the same three competitors, and (2) groups of 14 competitors chosen at random from the study population. In a separate experiment, we investigated the effects of salinity stress and high soil nutrients on pollen performance. These environmental factors had anticipated effects on leaf production, flower production, and petal length, but style length and (most importantly) the number of seeds sired relative to a standard pollen donor were not affected. In summary, this study provides the strongest evidence to date that pollen-tube competitive ability varies among coexisting plants and may be an important component of male fitness in plants.     

ALGAL COMPETITION IN AN EPILITHIC RIVER FLORA1

Field and laboratory procedures demonstrate that the effluent from the Willimantic, Connecticut, sewage treatment plant changes the numerical composition of the dominant epilithic algae in the Shetucket River. This effect is due to the effluent altering the competitive balance between chlorella sp. and Achnanthes deflexa Reim. The chlorella completely dominates highly polluted stretches of the river, and shares dominance with A. deflexa in other areas. The degree of river pollution can be estimated by enumerating the natural standing crops of these 2 algae. A 2-membered assay utilizing these organisms is a sensitive laboratory pollution monitor and a useful predictive tool. Dominant stands of these epilithic species develop in polluted and unpolluted zones in this river due to competition for the limited area on the cobbles.     

GLOBAL ATTRACTIVITY IN A N-SPECIES COMPETITION DIFFERENCE SYSTEM

1．Introduction．TherehasbeenrecelltlyasteadyIncreaseIntheInterestondifferencesystems．Onecommonlyusedtechn［queIstoconsideracont［nuoustimemodelgovernedbyordinarydifferentialequationsandobtainadlscretlzatlonof＊thetlzatlonsamebyreplacingthetimeder孔vatlvebyanappropriatedifferencequotientasInthecaseofEuler’smethodforthenumericalsolutionoffirstorderordinarydifferentialequations．SuchaprocedurehasbeproposedfortheLotka－VolterraN－spedescompetitionsystemoftheformItisknownthat［Chen,1988,G…     

COMPETITION AS A CONTROL MECHANISM IN SUBALPINE MEADOWS

Interspecific competition in a stable subalpine meadow was investigated experimentally. Vegetation responds to gradients of moisture and disturbance which control species composition. Moisture and snow melt patterns produce a productivity gradient that affects the importance of competition in this vegetation mosaic. Reciprocal transplant and selective removal experiments demonstrated that the intensity of competition is directly related to productivity and that seedlings are more susceptible to competitive effects than are established plants. Competition may reduce diversity in stable, productive habitats, while disturbance and stress limit diversity directly. Habitats of moderate productivity and minor disturbance have maximum diversity. The experiments and observations combine to suggest that habitat heterogeneity and local disturbance interact to control biotic interference and that the relative importance of each varies with productivity.