一类具有退偿增长曲线的生物种群的捕获优化问题

本文研究一类具有延偿增长曲线的生物种群,讨论了该种群的动力学性质及捕获问题,确定了最优捕获努力量、相应的种群密度和最大可持续捕获量、讨论了开放条件下的生物资源的管理。     

污染与捕获对Logistic种群的影响

本文研究污染与捕获并存时Logistic种群的β生存问题．证明了种群若不永远β生存，则必在有限时间内绝灭．给出了种群β生存、β绝灭条件，并对临界情况作了讨论．     

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．     

生物环及其测度

本文将一类生物种群抽象为“生物环”并定义了生物环上的测度,利用这一测度研究了两类生物种群的可测性．     

岛屿生物地理学与集合种群理论的本质与渊源

岛屿生物地理学和集合种群理论是目前生物多样性保育所依赖的主要生态学理论。人们通常强调这两种理论的区别,对它们之间的关联却很少注意到。事实上,这两种理论是同根同源的。以经典集合种群理论的创始者R．Levim对他与岛屿生物地理学的创始者R．H．MaeArthur的合作过程以及岛屿生物地理学对他提出集合种群理论的影响的回顾为基础,分析比较了岛屿生物地理学、经典集合种群理论、以Hanski为代表的现代集合种群理论的基本假设、研究范式和核心思想的异同,简要介绍了多物种集合种群与集合群落研究的差异,最后分析了岛屿生物地理学和集合种群理论在生物多样性保育实践中的应用和存在问题。     

一个生物种群非线性泛函微分方程模型整体解的聚集性

自然界的种群,如鱼类、鸟类和有蹄兽类有聚集成群的现象,研究种群的聚集性是生物数学中的一个有趣的课题．本文讨论了一个有关生物种群的非线性泛函微分方程模型整体解的聚集性和稳定性问题．     

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．     

具脉冲扩散效应的单种群动力学模型

讨论了两斑块间脉冲扩散的单种群动力学模型,利用离散动力系统频闪映射理论,得到了种群持续生存的充分条件．结论31,1~了现实的生物种群动力学性质,也丰富了脉冲微分方程理论．     

濒危植物海南龙血树的种群结构与动态

了解种群的生存现状与动态趋势对濒危植物的保护至关重要。为了更好地保护濒危海南龙血树种群,对海南岛不同生境的11个海南龙血树(Dracaena cambodiana)种群进行了调查;并利用个体基径数据建立各种群的年龄结构和静态生命表,得到种群年龄结构动态指数,绘制种群存活曲线和生存函数曲线,从而获得海南龙血树的种群结构与动态。结果表明:海南龙血树种群年龄结构与种群所处的生境关系密切,海边的NS和YL种群结构为衰退型,LN和CH居群为稳定型,而非海边种群为增长型,没有老龄个体;11个种群中,EXL种群显示出最高的增长趋势和最好的种群稳定性,NS和YL种群稳定性最差,种群抗干扰能力极低;11个种群的存活曲线均属于Deevey-Ⅰ型,但生存函数分析显示海南龙血树种群生活史早期和晚期脆弱,中期稳定,这表明海南龙血树种群所处的生境更适合成年个体生存,现有的生境对幼龄个体的环境筛作用较强。海南龙血树属于退偿物种,针对其种群的生存现状,应采取多种方法相结合的方式进行保护与复壮。     

种群空间格局研究的零频率方法

在建立S~2=a'(-lnP_0)~b'模型的基础上,结合Gerrard＆Chiang模型(m=a(-InP_0)~β)和扩散系数C,本文提出一个种群空间格局研究的零频率方法.根据种群聚集临界零频率,可对空间格局连续统作出定量描述,按参数(b'-β)可将生物种群划分为“聚集度非零频率制约型”、“聚集度逆零频率制约型”和“聚集度零频率制约型”3种类型.通过13种生物种群资料的实例验证,表明新模型能客现地反映出种群空间格局的本质特征,应用于低密度种群已显示出它的独到之处.此外,文中还推导出了确定理论抽样数的公式和简易序贯抽样决策模型.     

Interaction of growth with social behaviour in Tilapia zilli raised in three different temperatures

Three thermal levels were tested on 10-day old T. zilli fry obtained from one spawning to study the effects of temperature on the occurrence of growth depensation in the population reared under controlled environmental conditions. Results showed highest individual growth rate, mortality rates and growth depensation at 30 °C whilst the greatest biomass increase was obtained at an intermediate temperature (27 °C). The enhanced depensation and high mortality rates at the upper limit of the thermal gradient for growth of T. zilli were attributed to an alteration in the behaviour of fish which favoured a few, large, dominant and socially aggressive individuals.     

An equation of growth of a single species with realistic dependence on crowding and seasonal factors

Various biological phenomena lead to single species models where the relative rate of increase is a non-monotone function of the density, i.e. depensation models. A brief survey of the literature and some new models are given. A nonlinear nonautonomous O.D.E. is proposed as a general depensation model in a periodically fluctuating environment. Results on existence, multiplicity and global stability of periodic solutions are given.     

Growth depensation and group behaviour in juvenile hybrid striped bass Morone chrysops×Morone saxatilis: effects of group membership, feeding method, ration size and size disparity

The effects of group membership (three group sizes), feeding method (single site v. scattered), variation in ration size (low v. high) and starting size disparity (range of starting sizes) on growth, growth depensation and group behaviour of juvenile hybrid striped bass (HSB) Morone chrysops female × Morone saxatilis male were tested. Group membership, starting size disparity and ration size affected the growth, growth depensation and behaviour of juvenile HSB. Growth was higher in fish held within large group memberships and high ration treatments. Growth depensation (size variation among individuals of the same age) in group memberships of three and 12 fish per tank was minimal when compared to intermediate memberships (six fish per tank). In addition, small fish held in tanks with fish of intermediate sizes demonstrated higher growth when compared to small fish housed with large individuals. Some behaviours, specifically activity and scramble feeding, were more elevated in large group memberships when compared to groups with fewer members. Activity was also found to increase over time during size disparity testing. Results suggest that HSB are ideally suited for high-density, intensive aquaculture since they appear to thrive in high group memberships of similar starting sizes and when a sufficient amount of feed in administered.     

Possible interpretation problems for the current CITES listing criteria in the context of marine fish species under commercial harvest

The current criteria for listing species on CITES Appendices are summarized briefly. A précis is given of problems with these criteria in a marine species context, as identified by an FAO ad hoc Expert Group meeting in November 1998. Three aspects of the criteria that are of concern to the author are discussed briefly. These are the definition of a "decline," specifications for taking account of uncertainty, and the desirability of using existing stock-recruitment data for marine fish species to estimate the spawning biomass level at which critical depensation might occur. This last would provide guidance in specifying resource levels at which there is a threat of extinction when reformulating the CITES criteria. Received: May 6, 1999 / Accepted: December 11, 1999     

Can intense predation by bears exert a depensatory effect on recruitment in a Pacific salmon population?

It has long been recognized that, as populations increase in density, ecological processes affecting growth and survival reduce per capita recruitment in the next generation. In contrast to the evidence for such “compensatory” density dependence, the alternative “depensatory” process (reduced per capita recruitment at low density) has proven more difficult to demonstrate in the field. To test for such depensation, we measured the spawner–recruit relationship over five decades for a sockeye salmon (Oncorhynchus nerka) population in Alaska breeding in high-quality, unaltered habitat. Twenty-five years of detailed estimates of predation by brown bears, Ursus arctos, revealed strong density dependence in predation rate; the bears killed ca. 80 % of the salmon in years of low salmon spawning abundance. Nevertheless, the reconstructed spawner–recruit relationship, adjusted to include salmon intercepted in the commercial fishery, provided no evidence of demographic depensation. That is, in years when few salmon returned and the great majority were killed by bears, the few that spawned were successful enough that the population remained highly productive, even when those killed by bears were included as potential spawners. We conclude that the high quality of breeding habitat at this site and the productive nature of semelparous Pacific salmon allowed this population to avoid the hypothesized depressed recruitment from depensatory processes expected at low density. The observed lack of demographic depensation is encouraging from a conservation standpoint because it implies that depleted populations may have the potential to rebound successfully given suitable spawning and rearing habitat, even in the presence of strong predation pressure.     

Effects of Group Membership and Size Distribution within a Group on Growth Rates of Juvenile Sablefish Anoplopoma fimbria

Group membership can confer both advantages and disadvantages to growth in juvenile fishes. The balance between costs and benefits of social interactions can shift depending on such factors as the composition of the group (density and size disparity) and the availability of food. We examined the effect of these factors on absolute growth and growth depensation in juvenile sablefish, Anoplopoma fimbria. Increasing density and increasing size disparity had little influence on absolute growth rates of juvenile sablefish and the effects of these social factors were not modified by ration level. In experiments testing density effects, absolute growth did not differ among groups of 1, 3, or 10 fish held at high rations, but at low rations single fish exhibited a different pattern of size-dependent growth compared to fish in groups. In experiments testing disparity effects, absolute growth did not differ between groups with an even size distribution and groups with a mixed size distribution. The relative size of an individual within a group, i.e., small, medium, or large, also did not modify growth, despite evidence of higher chasing behavior in mixed size distributions. Although the growth of small fish was not diminished in the presence of large fish, negative impacts of size disparity were expressed in high levels of cannibalism, which occurred in 42% of groups with a mixed size distribution. Significant growth depensation over time occurred in the density experiment, but not in the size disparity experiment, possibly due to the shorter duration of the latter experiment. We suggest that growth depensation was generated by individual variability in growth capacity rather than social effects on growth rates. Schooling behavior, measured by group cohesion indices, increased with fish size and was higher in groups with an even vs. a mixed size distribution. These results for sablefish are consistent with other schooling species in which growth variability is determined by exploitative competition and/or genetic variability in growth capacity rather than interference competition.     

Pollen limitation or mate search need not induce an Allee effect

When a process modelling the availability of gametes is included explicitly in population models a critical depensation or Allee effect usually results. Non-spatial models cannot describe clumping and so small populations must be assumed very diffuse. Consequently individuals in small populations experience low contact rates and so reproduction is limited. In Nature invasions into new territory are unlikely to be as diffuse as those described by non-spatial models. We develop pair approximations to a probabilistic cellular automata model with independent pollination and seed setting processes (equivalently mate search and reproduction processes). Each process can be either global (population-wide) or local (within a small neighbourhood) or a mixture of the two. When either process is global the resulting model recaptures the Allee effect found in non-spatial models. However, if both processes are at least partially local we obtain a model in which Allee effects can be avoided altogether if individuals are suitably strong pollinators and colonisers. The Allee effect disappears because small populations are dramatically more clumped when colonisation is local and less wasteful of pollen when pollination is local.     

Transition from a random to an ideal free to an ideal despotic distribution: the effect of individual difference in growth

Individual differences in growth can lead to a monopolistic form of food competition. We studied the long-term transition in the mode of competition and the distribution of individuals between food patches of the cloned salmonid fish, Oncorhynchus masou ishikawae, in the laboratory. This transition was accompanied by growth depensation, i.e., the increase over time in the variance of size between individuals resulting from the differences in individual growth rates. The 120-cm experimental tanks were divided into two compartments (patches) between which an opaque partition was placed. Fish were able to move freely between the patches and therefore were able to assess the patch quality using long-term memory, but they were not able to see the food input in the other patch directly. The distribution between the two food patches, the amount of food gained, and the growth and the agonistic behavior of four groups of six individuals were observed over 4 weeks. We found that (1) within-group variation in body weight increased with time; (2) on average, the better patch was used by more individuals than predicted by a random distribution but fewer individuals than predicted by an ideal free distribution, and (3) the distribution and pattern of resource use by the fish changed over the 4-week experimental period from a random distribution to an ideal free distribution and finally to an ideal despotic distribution. We suggest that growth depensation causes the long-term change in the spatial distribution and pattern of resource use by competitors. Received: December 19, 2000 / Accepted: March 19, 2001     

Forest-level analysis of stability under exploitation: depensation responses and catastrophe theory

Stability analysis of whole forests is proposed as a qualitative tool for the study of forest responses to partial or patchy harvests or mortality. Instead of modeling every tree or stand, aggregate tree biomass is modeled. In order to aggregate stands, spatial effects must be incorporated. It is shown that depensation growth responses (reduced growth at low biomass) are common because forests often modify harch environments to be more suitable for their growth. Depensation can result from thinning, partial mortality, patchy cutting, or clearcutting, depending on forest type and abiotic factors. Examples of these types of behaviors are given. Stability analysis of different growth regimes under exploitation are related to catastrophe theory and to optimal harvesting policies. Such qualitative analysis is shown to be applicable to data-poor regions such as the tropics where there is great concern over responses of forests to exploitation.     

Individual growth variation of red-spotted masu salmon,Oncorhynchus masou rhodurus,in a mountain stream

Individual growth patterns in red-spotted masu salmon,Oncorhynchus masou rhodurus, were examined by mark and recapture in a mountain stream, central Japan. The growth pattern varied substantially among individuals of the same age cohort in the stream. Mean absolute growth rates of the individuals were neither significantly different between years, study sections along the stream course, or sexes, but showed significant differences between seasons. No correlation was found between the individual growth rate of fish and the area of pools they inhabited. However, there was considerable growth variation among inhabitants of the same pool. Within a pool, larger individuals grew more rapidly than smaller ones, despite there being no significant relationship between individual growth rates and initial body weights in the stream overall. Individual growth differences probably resulted from growth depensation caused by intraspecific competition within individual pools.