Schwerdtfeger F
Niedersächsische Forstliche Versuchsanstalt Göttingen, Gottingen, Deutschland.
Oecologia. 1968 Nov;1(4):265-295. doi: 10.1007/BF00386685.
Since the twenties of our century, at least 15 theories worth discussing have been developped which intend to explain the causes of natural control of animal populations (for details see SCHWERDTFEGER, 1968). An attempt is made to integrate the different-partly contrary-ideas and new results into a general theory. The basis to start from is the cybernetic principle of feed-back mechanism introduced into population dynamics by WILBERT (1962): an actual value (e.g. the inside temperature of a refrigerator) is permanently changed by perturbances (the always higher outside temperature); through a regulator (a thermostat), each change puts in action a regulating variable (a cooling device) which alters the actual towards the index value (required inside temperature).The often complicated processes that take part in the natural control of populations are summarized in Fig. 6. The actual value is the existing population density (Abundanz). The perturbances primarily causing its fluctuations (Fluktuation) are fertility and immigration which raise the abundance, mortality and emigration which lower it.The amplitude of the fluctuation must be limited, if the population is not to die out or to destroy its habitat by continuous increase. It is determined (Determination) as a sort of index value, the lower limit of which corresponds in the extreme to the minimal density guaranteeing the existence of the population, while the upper limit is formed by the environmental capacity. The latter is determined either by the total supply of requisites and the ability of the animals to use it or by the local minimum of adverse effects. The capacity of the environment and therewith the amplitude of fluctuations can be fixed or variable. It is fixed in a population of Great Tits with territorial behaviour: in an oak stand, the number of breeding pairs cannot be higher than the number of territories fitting in. It is variable in the case of bark beetles living in wind thrown spruce trees: they may find 2 suitable trees this year, 100 the next after heavy winter storms and 10 the year after next. In this case, the change of determination was a change of environment, specifically of the supply of a requisite; it can as well be a change in the constitution of the population characterized by its demands and efficiencies: in the final effect it makes no difference wether more breeding space is offered or less is demanded. The variability of the amplitude in the case of the bark beetles is caused by chance; it can also be governed or self-induced. It is governed e.g. by the seasonal rhythm of climate: the average level of density in tsetse flies is higher during the rain season than during the dry season. It is self-induced in the case of an entomophagous parasite changing the density of its host insect and by this varying the supply of a requisite.Fertility permanently tends to raise the density of populations. Processes of limitation (Limitation) work against this tendency. With increasing abundance, density dependent factors become more effective as general regulators. In simple feed-back control systems (einfacher Regelkreis), regulation is solely performed by perfectly density dependent factors. An example is the cyclically fluctuating Field Vole: increasing mutual interference causes a crash of the abundance which thereafter rises again. In complex feed-back control systems (komplexer Regelkreis) density is kept on a low level for a shorter or longer period by random influences and delayed density dependent factors; now and then a real regulating factor has to interfere as an emergency brake to prevent the transgression of the upper density limit. That applies to many insect populations. The effect of the limiting factors as regulating variables consists in lowering fertility and immigration and raising mortality and emigration.The processes causing fluctuations result in an equilibrium density (Gleichgewichtsdichte): increase and decrease are counterbalanced. The level of the equilibrium density is differently situated within the amplitude and approximately in accordance with the long-term average of the actual densities.
自二十世纪二十年代以来,至少已发展出15种值得探讨的理论,旨在解释动物种群自然控制的原因(详情见施韦尔特费格,1968年)。人们试图将不同的——部分相互矛盾的——观点和新成果整合为一个通用理论。起始的基础是威尔伯特(1962年)引入种群动态学的反馈机制的控制论原理:一个实际值(例如冰箱内部温度)会因干扰(始终较高的外部温度)而不断变化;通过一个调节器(恒温器),每次变化都会启动一个调节变量(制冷装置),使实际值朝着指标值(所需的内部温度)变化。图6总结了参与种群自然控制的通常复杂的过程。实际值是现有的种群密度(丰度)。主要导致其波动的干扰因素是生育力和迁入,它们会增加丰度,而死亡率和迁出则会降低丰度。如果种群不想灭绝或因持续增长而破坏其栖息地,波动的幅度就必须受到限制。它被确定为一种指标值,其下限在极端情况下对应于保证种群生存的最小密度,而上限则由环境容纳量构成。后者要么由资源的总供应量以及动物利用它的能力决定,要么由不利影响的局部最小值决定。环境容纳量以及波动幅度可以是固定的或可变的。在具有领地行为的大山雀种群中,它是固定的:在一片橡树林中,繁殖对的数量不能高于可容纳的领地数量。对于生活在被风吹倒的云杉树上的小蠹虫来说,它是可变的:它们今年可能找到2棵合适的树,在严重的冬季风暴过后次年找到100棵,再下一年找到10棵。在这种情况下,确定值的变化是环境的变化,具体来说是一种资源供应的变化;它也可以是种群构成的变化,其特征在于需求和效率:最终结果是提供更多繁殖空间还是需求更少繁殖空间并无差别。小蠹虫情况下波动幅度的变化是由偶然因素引起的;它也可以是受控制的或自我诱导的。例如,它受气候的季节节律控制:采采蝇的平均密度水平在雨季高于旱季。在食虫寄生虫改变其寄主昆虫密度并由此改变一种资源供应的情况下,它是自我诱导的。生育力一直倾向于增加种群密度。限制过程则与这种趋势相反。随着丰度增加,密度依赖因素作为一般调节器变得更加有效。在简单的反馈控制系统中,调节仅由完全依赖密度的因素来执行。一个例子是周期性波动的田鼠:相互干扰的增加导致丰度崩溃,此后又再次上升。在复杂的反馈控制系统中,密度会因随机影响和延迟的密度依赖因素在较短或较长时间内保持在较低水平;时不时地,一个真正的调节因素必须作为紧急制动器进行干预,以防止超过密度上限。这适用于许多昆虫种群。作为调节变量的限制因素的作用在于降低生育力和迁入,并提高死亡率和迁出。导致波动的过程会产生一个平衡密度:增加和减少相互抵消。平衡密度的水平在波动幅度内处于不同位置,并且大致与实际密度的长期平均值一致。