污染环境下单种群模型生存阈值

本论文研究了污染环境下毒素对单种群生存的影响。在环境容纳量较小的假设下建立了生物种群模型,在该模型中不但考虑了环境毒素浓度对生物个体生存的影响,还考虑了生物个体从食物链中吸收的毒素对其影响。通过研究得到种群一致持续生存和若平均持续生存的充分条件,同时得到种群持续生存依赖于模型参数和生物个体体内毒素净化率的某些充分条件．     

具污染与捕获的Logistic单种群的持续生存及绝灭

当今社会工农业的高速发展在给人们带来经济利益的同时,也造成了严重的环境污染．一些生物种群一方面受到污染的威胁,另一方面还要满足人们捕获的需求．这就使得环境污染中的种群捕获问题成为人们关心的问题．本文指出以往环境污染单种群模型的不足之处,基于文献[3]的基础上给出新模型;并得到了一致持续生存及灭绝的充分条件;而且通过具体例子给出两个控制变量：环境毒素输入率和捕获率之间的关系图．     

动物的易绝灭特征与保护优先性

各种人为干扰和自然因素促使大量物种走向濒危和绝灭。物种濒危和绝灭不是随机的。具有某些特征的物种容易濒危和绝灭 ,即易绝灭特征。易绝灭特征包括个体大 ,繁殖力低 ,扩散能力弱 ,营养级高 ,家域大 ,种群小 ,种群波动大 ,分布范围窄 ,种群密度低 ,栖息地特化程度高和特殊栖息地类型等。研究物种的易绝灭特征可以为生物多样性提供预防性 (proac tive)的优先保护措施。尽管物种的易绝灭特征已经用于实际的物种保护中 ,然而由于物种的各种特征对物种濒危和绝灭的影响十分复杂 ,各个易绝灭特征还有待于进一步深入的、准确的研究。探讨适合不同类群和不同地区物种的易绝灭特征是十分必要的。由于特殊地史发育、中医药传统和边境频繁的非法野生动物贸易 ,我国动物的濒危模式可能与国外有所不同。     

污染环境中单种群生存分析

本文研究了污染环境中种群的动力学模型,基于已有结果,考虑了内禀增长率为非线性函数的情形.利用微分方程定性理论中的比较定理,对于种群的持续生存与绝灭给出了判别条件.     

环境因子对荒漠沙蜥种群密度影响的研究

本文研究人类改造荒漠的活动,植被,潜在的可利用的食物资源,竞争种的密度,土壤理化性质等坏境因子对荒漠沙蜥种群密度的影响。结果说明：人类的活动对沙蜥种群密度没有显著影响;决定沙蜥种群密度的主导因子是潜在的可利用的食物资源,植被,土壤含水量,竞争种的密度。这些因子的任何改变都能改变沙蜥的种群密度,均具有调节种群的作用。     

播种时间对棉田害虫和天敌种群的影响

通过对3种不同时间播种的棉田内害虫、天敌系统调查,分析和比较了播种时间对棉田害虫、天敌种群和群落的影响。结果表明,播种期推后,可减轻或避免第二代棉铃虫的为害,加重第三代和第四代棉铃虫的为害;但不同播种日期对不同时期的棉蚜影响不同,苗蚜以迟播棉田内种群数量最高,伏蚜则以夏播棉田内种群数量最高。播种期的推后,不利于棉田捕食性瓢虫、蝽类、蜘蛛和寄生性天敌种群增长。棉田害虫和天敌群落多样性指数也随播种期的推后而下降．因此．应针对不同时间播种的棉田开展相应的害虫生态管理。     

生长季节和非生长季节交替出现的种群动态模型及环境变化的影响

有些生物的生长季节和非生长季节交替出现,本文建立了描述这种生物种群动态的方程,并研究了在环境稳定、随机波动、定向变化的情况下种群的变化方式,还讨论了种群的危害及濒危情况．生长季节延长时种群增大,有害生物的危害加重,濒危生物濒危程度减轻;生长季节缩短时种群减小,有害生物的危害减轻,濒危生物更加濒危或灭绝．     

刺五加种群生态学的研究 Ⅱ．刺五加的种群统计

在３种天然次生林类型中,以刺五加的无性系小株为统计单位,编制该种群的生命表,分析其种群动态．结果表明,刺五加种群有２个死亡率高峰,在不同的森林群落中其死亡强度有较明显差异．种群的总亏损度不同,这些都与群落生境有关．该种群的存活曲线趋近于ＤｅｅｖｅｙⅢ型．     

环境条件对伸展摇蚊实验种群繁育的影响

以伸展摇蚊为研究对象,重点考察了温度、光强对各生命阶段存活率和发育速率、成虫性别比和繁殖力以及种群动态的综合影响。通过F检验不同温度相同光照条件下摇蚊卵、幼虫和蛹的存活率的回归方程和和回归系数,发现了在相同的光强(800 lx或2000 lx)条件下,15—35℃温度范围内,摇蚊幼虫和蛹两种形态存活率与温度之间均呈显著相关,而幼虫期摇蚊的存活率比蛹期更易受温度影响,但是,温度变化并没有显著影响卵的存活率;且15℃和35℃两个极端温度不适合摇蚊的生存和繁育。其次通过正交试验,对在2个温度(15℃和30℃)和2种光强(800 lx和2000 lx)两种因素组合作用下的摇蚊存活率和发育速率进行极差分析和双因素方差分析,得到伸展摇蚊种群繁育的最佳条件为25℃,800 lx的结论,且两种因素对于摇蚊成虫的雌雄比影响并不显著,但是温度对三者(存活率、发育速率和雌雄比)的影响远大于光强。最后通过单因素方差分析关于不同光照和温度下对成虫繁殖力的结果,以及相同光照和温度下不同湿度(45%、65%、85%和95%)对成虫繁殖力的结果,总结出在25—30℃,800 lx光强下伸展摇蚊维持良好生命活力、顺利完成繁殖发育过程,85%—95%的相对湿度可以使羽化成虫保持较高的产卵水平。并根据所观察的种群生殖力资料计算得到相应温度(25℃和30℃)和光照条件(800 lx和2000 lx)组合下的实验种群繁殖特征生命表。结果表明在800 lx光强、25—30℃条件下,摇蚊能维持较高的种群净增殖率(R_0)与内禀增长率(r_m)。综上所述,25℃温度,800 lx光强和85%的湿度的条件更适合伸展摇蚊种群繁育。此结果为建立伸展摇蚊室内繁育的标准化条件及相应的种群发展规律和构建本土摇蚊种的毒性测试方法奠定了基础。     

Evolutionary rescue under demographic and environmental stochasticity

Populations suffer two types of stochasticity: demographic stochasticity, from sampling error in offspring number, and environmental stochasticity, from temporal variation in the growth rate. By modelling evolution through phenotypic selection following an abrupt environmental change, we investigate how genetic and demographic dynamics, as well as effects on population survival of the genetic variance and of the strength of stabilizing selection, differ under the two types of stochasticity. We show that population survival probability declines sharply with stronger stabilizing selection under demographic stochasticity, but declines more continuously when environmental stochasticity is strengthened. However, the genetic variance that confers the highest population survival probability differs little under demographic and environmental stochasticity. Since the influence of demographic stochasticity is stronger when population size is smaller, a slow initial decline of genetic variance, which allows quicker evolution, is important for population persistence. In contrast, the influence of environmental stochasticity is population-size-independent, so higher initial fitness becomes important for survival under strong environmental stochasticity. The two types of stochasticity interact in a more than multiplicative way in reducing the population survival probability. Our work suggests the importance of explicitly distinguishing and measuring the forms of stochasticity during evolutionary rescue.     

Evolution of dispersal in metapopulations with local density dependence and demographic stochasticity

In this paper, we predict the outcome of dispersal evolution in metapopulations based on the following assumptions: (i) population dynamics within patches are density-regulated by realistic growth functions; (ii) demographic stochasticity resulting from finite population sizes within patches is accounted for; and (iii) the transition of individuals between patches is explicitly modelled by a disperser pool. We show, first, that evolutionarily stable dispersal rates do not necessarily increase with rates for the local extinction of populations due to external disturbances in habitable patches. Second, we describe how demographic stochasticity affects the evolution of dispersal rates: evolutionarily stable dispersal rates remain high even when disturbance-related rates of local extinction are low, and a variety of qualitatively different responses of adapted dispersal rates to varied levels of disturbance become possible. This paper shows, for the first time, that evolution of dispersal rates may give rise to monotonically increasing or decreasing responses, as well as to intermediate maxima or minima.     

Linear analysis solves two puzzles in population dynamics: the route to extinction and extinction in coloured environments

In this paper, we give simple explanations to two unsolved puzzles that have emerged in recent theoretical studies in population dynamics. First, the tendency of some model populations to go extinct from high population densities, and second, the positive effect of autocorrelated environments on extinction risks for some model populations. Both phenomena are given general explanations by simple, linear, sto-chastic models. We emphasize the predictive and explanatory power of such models.     

Effects of environmental variation on extinction and establishment

Theoretical models predict that increasing environmental variation increases the probability of extinction, decreases the probability of establishment, and influences the distribution of times to extinction or establishment. We conducted an experiment with 281 independent populations of Daphnia magna under controlled laboratory conditions to test these predictions. Consistent with the theory, the fraction of populations going extinct increased and the fraction of populations establishing self‐sustaining populations decreased under higher levels of environmental variation compared with controls. Time to extinction decreased under higher levels of environmental variation, but we found no effect on time to establishment. These results are consistent with theoretical predictions from models of extinction. They therefore support the use of stochastic population models to predict the fates of introductions of non‐indigenous species or native endangered species based on historic fluctuations and/or expected future conditions.     

Pushed beyond the brink: Allee effects,environmental stochasticity,and extinction

To understand the interplay between environmental stochasticity and Allee effects, we analyse persistence, asymptotic extinction, and conditional persistence for stochastic difference equations. Our analysis reveals that persistence requires that the geometric mean of fitness at low densities is greater than one. When this geometric mean is less than one, asymptotic extinction occurs with high probability for low initial population densities. Additionally, if the population only experiences positive density-dependent feedbacks, conditional persistence occurs provided the geometric mean of fitness at high population densities is greater than one. However, if the population experiences both positive and negative density-dependent feedbacks, conditional persistence only occurs if environmental fluctuations are sufficiently small. We illustrate counter-intuitively that environmental fluctuations can increase the probability of persistence when populations are initially at low densities, and can cause asymptotic extinction of populations experiencing intermediate predation rates despite conditional persistence occurring at higher predation rates.     

Extinction in relation to demographic and environmental stochasticity in age-structured models

The demographic variance of an age-structured population is defined. This parameter is further split into components generated by demographic stochasticity in each vital rate. The applicability of these parameters are investigated by checking how an age-structured population process can be approximated by a diffusion with only three parameters. These are the deterministic growth rate computed from the expected projection matrix and the environmental and demographic variances. We also consider age-structured populations where the fecundity at any stage is either zero or one, and there is neither environmental stochasticity nor dependence between individual fecundity and survival. In this case the demographic variance is uniquely determined by the vital rates defining the projection matrix. The demographic variance for a long-lived bird species, the wandering albatross in the southwestern part of the Indian Ocean, is estimated. We also compute estimates of the age-specific contributions to the total demographic variance from survival, fecundity and the covariance between survival and fecundity.     

Modelling time to population extinction when individual reproduction is autocorrelated

In nature, individual reproductive success is seldom independent from year to year, due to factors such as reproductive costs and individual heterogeneity. However, population projection models that incorporate temporal autocorrelations in individual reproduction can be difficult to parameterise, particularly when data are sparse. We therefore examine whether such models are necessary to avoid biased estimates of stochastic population growth and extinction risk, by comparing output from a matrix population model that incorporates reproductive autocorrelations to output from a standard age‐structured matrix model that does not. We use a range of parameterisations, including a case study using moose data, treating probabilities of switching reproductive class as either fixed or fluctuating. Expected time to extinction from the two models is found to differ by only small amounts (under 10%) for most parameterisations, indicating that explicitly accounting for individual reproductive autocorrelations is in most cases not necessary to avoid bias in extinction estimates.     

Experimental support of the scaling rule for demographic stochasticity

A scaling rule of ecological theory, accepted but lacking experimental confirmation, is that the magnitude of fluctuations in population densities due to demographic stochasticity scales inversely with the square root of population numbers. This supposition is based on analyses of models exhibiting exponential growth or stable equilibria. Using two quantitative measures, we extend the scaling rule to situations in which population densities fluctuate due to nonlinear deterministic dynamics. These measures are applied to populations of the flour beetle Tribolium castaneum that display chaotic dynamics in both 20-g and 60-g habitats. Populations cultured in the larger habitat exhibit a clarification of the deterministic dynamics, which follows the inverse square root rule. Lattice effects, a deterministic phenomenon caused by the discrete nature of individuals, can cause deviations from the scaling rule when population numbers are small. The scaling rule is robust to the probability distribution used to model demographic variation among individuals.     

Effects of habitat size and quality on equilibrium density and extinction time of Sorex araneus populations

1. The effects of changes in habitat size and quality on the expected population density and the expected time to extinction of Sorex araneus are studied by means of mathematical models that incorporate demographic stochasticity.
2. Habitat size is characterized by the number of territories, while habitat quality is represented by the expected number of offspring produced during the lifetime of an individual.
3. The expected population density of S. araneus is shown to be mainly influenced by the habitat size. The expected time to extinction of S. araneus populations due to demographic stochasticity, on the other hand, is much more affected by the habitat quality.
4. In a more general setting we demonstrate that, irrespective of the actual species under consideration, the likelihood of extinction as a consequence of demographic stochasticity is more effectively countered by increasing the reproductive success and survival of individuals then by increasing total population size.