General population growth models with Allee effects in a random environment

Allee effects on population growth are quite common in nature, usually studied through deterministic models with a specific growth rate function.In order to seek the qualitative behaviour of populations induced by such effects, one should avoid model-specific behaviours. So, we use as a basis a general deterministic model, i.e. a model with a general growth rate function, to which we add the effect on the growth rate of the random fluctuations in environmental conditions. The resulting model is the general stochastic differential equation (SDE) model that we propose here.We consider two possible cases, weak Allee effects and strong Allee effects, which lead to different qualitative behaviours of the model.We will study the model properties for both cases in terms of existence and uniqueness of the solution, extinction and stationary behaviour of the population. The two cases will be compared with each other and with the general density-dependent SDE model without Allee effects.We then consider as an example the particular case of the classic logistic model and an Allee effect version of it.     

Dispersal effects on a discrete two-patch model for plant-insect interactions

A two-patch discrete time plant-insect model coupled through insect dispersal is studied. The model is based on three different phases: Plant growth is followed by the dispersal of insects followed by insect attacks. Our objective is to understand how different intensities of dispersal impact both local and global population dynamics of the two-patch model. Special attention is paid to two situations: When the single-patch model (i.e., in the absence of dispersal) is permanent and when the single-patch model exhibits Allee-like effects. The existence and stability of synchronous and asynchronous dynamics between two patches is explored. If the single-patch system is permanent, the permanence of the system in two patches is destroyed by extremely large dispersals and large attacking rates of insects, thus creating multiple attractors. If the single-patch model exhibits Allee-like effects, analytical and numerical results indicate that small intensity of dispersals can generate source-sink dynamics between two patches, while intermediate intensity of dispersals promote the extinction of insects in both patches for certain parameter ranges. Our study suggests a possible biology control strategy to stop the invasion of a pest by controlling its migration between patches.     

Vegetation in clear‐cuts depends on previous land use: a century‐old grassland legacy

Plant species richness in central and northern European seminatural grasslands is often more closely linked to past than present habitat configuration, which is indicative of an extinction debt. In this study, we investigate whether signs of historical grassland management can be found in clear‐cuts after at least 80 years as coniferous production forest by comparing floras between clear‐cuts with a history as meadow and as forest in the 1870s in Sweden. Study sites were selected using old land‐use maps and data on present‐day clear‐cuts. Species traits reflecting high capacities for dispersal and persistence were used to explain any possible links between the plants and the historical land use. Clear‐cuts that were formerly meadow had, on average, 36% higher species richness and 35% higher richness of grassland indicator species, as well as a larger overall seed mass and lower anemochory, compared to clear‐cuts with history as forest. We suggest that the plants in former meadows never disappeared after afforestation but survived as remnant populations. Many contemporary forests in Sweden were managed as grasslands in the 1800s. As conservation of remaining grassland fragments will not be enough to reduce the existing extinction debts of the flora, these young forests offer opportunities for grassland restoration at large scales. Our study supports the concept of remnant populations and highlights the importance of considering historical land use for understanding the distribution of grassland plant species in fragmented landscapes, as well as for policy‐making and conservation.     

环境污染中三维时变Volterra捕食-被捕食系统的持续生存

对环境容量很大且被污染的三种群时变系统进行了研究,给出了三维时变Ｖｏｌｔｅｒｒａ捕食－被捕食系统弱平均持续生存与绝灭的充分条件。     

Temporal patterns in Saturnidae (silk moth) and Sphingidae (hawk moth) assemblages in protected forests of central Uganda

Forest‐dependent biodiversity is threatened throughout the tropics by habitat loss and land‐use intensification of the matrix habitats. We resampled historic data on two moth families, known to play central roles in many ecosystem processes, to evaluate temporal changes in species richness and community structure in three protected forests in central Uganda in a rapidly changing matrix. Our results show some significant declines in the moth species richness and the relative abundance and richness of forest‐dependent species over the last 20–40 years. The observed changes in species richness and composition among different forests, ecological types, and moth groups highlight the need to repeatedly monitor biodiversity even within protected and relatively intact forests.     

盐城獐种群生存力的初步分析

本文采用Ｖｏｒｔｅｘ６．０计算机程序对盐城獐（Ｈｙｄｒｏｐｔｅｓｉｎｅｒｍｉｓ）种群的生存力进行了分析。结果表明：若无狩猎和海潮侵袭的影响,在能容纳５００头獐的栖息地中,盐城獐可长期生存。如狩猎死亡率达５％和１０％,拥有１００～２００头獐的种群分别在３２～５０年和１５～２７年内灭绝率超过５％;拥有３００～５００头的现存种群分别在９０～９５年和２９～４２年内灭绝率超过５％。在有海潮侵袭的栖息地中,不管现存种群多大,在未来２５年左右的时间内种群灭绝率大于５％,在５０年内种群绝灭的可能性超过５０％。根据该地的实际情况,建议在无海潮侵袭的堤内选择至少能容纳２００头獐的栖息地和在相临的堤外选择能容纳５００头獐的栖息地建立保护区,才能保证盐城獐种群长期生存。     

SIR传染病模型的参数反演

本文将Adams微分方程数值求解方法和神经网络技术相结合,提出了一种适合于SIR传染病模型参数反演的新方法,该方法可以根据不同的传染病类型和传染病发展的不同阶段反演得到SIR模型参数,从而为分析疫情的发展、制定和评判控制措施提供依据.最后本文以北京2003年公布的SARS疫情数据,采用上述方法进行参数反演,计算结果表明反演参数得到的SIR模型计算结果与实测数据相吻合.     

Extinction dynamics,population growth and seed banks

The goal of this paper is to test theoretical predictions about the effects of seed banks on population dynamics and extinction rates in variable environments using simulations based on data from a natural population of the winter annual Collinsia verna. In the simulations, we varied the frequency of demographically good and bad years and the autocorrelation between conditions in consecutive years to examine the impact of seed dormancy on population growth rate, extinction rate and time to extinction. The existence of a seed bank enhanced population growth rates under all environmental regimes except when good years were very frequent, but this enhancement was minimal. In addition, the presence of the seed bank decreased the likelihood of extinctions and increased the time to extinction. The time to extinction was longest when the environmental conditions were most unpredictable.     

冬小麦农田中净辐射的研究

本文对陕西泾阳地区冬小麦田净辐射进行了分段和分层模拟,取得了较好的拟合效果,相对误差在１０％以内。确定了麦田相对叶面积函数最大值的出现高度为４．５／７Ｈ,介于高粱（４／７Ｈ）和谷子（５／７Ｈ）之间。计算了不同发育期、不同层次至冠层顶部累积叶片的消光系数,结果表明,消光系数因发育期和叶片层次而异,拔节期,由旗叶向下,消光系数增大,乳熟期旗叶的消光系数最大,比拔节期大一倍,倒二叶以上两个层次叶片消光系数较小。就平均而言,冬小麦消光系数拔节期为０．５１,乳熟期为０．５６。本项成果为进一步开展麦田小气候研究提供了有意义的数据和方法。