den Boer P J
Biologisch Station, Biological Station of the Agricultural University Wageningen, Kampsweg 27, 9418 PD, Wijster, The Netherlands.
Oecologia. 1981 Aug;50(1):39-53. doi: 10.1007/BF00378792.
The survival time of small and isolated populations will often be relatively low, by which the survival of species living in such a way will depend on powers of dispersal sufficiently high to result in a rate of population foundings that about compensates the rate of population extinctions. The survival time of composite populations uninterruptedly inhabiting large and heterogeneous areas, highly depends on the extent to which the numbers fluctuate unequally in the different subpopulations. The importance of this spreading of the risk of extinction over differently fluctuating subpopulations is demonstrated by comparing over 19 years the fluctuation patterns of the composite populations of two carabid species, Pterostichus versicolor with unequally fluctuating subpopulations, and Calathus melanocephalus with subpopulations fluctuating in parallel, both uninterruptedly occupying the same large heath area. The conclusions from the field data are checked by simulating the fluctuation patterns of these populations, and thus directly estimating survival times. It thus appeared that the former species can be expected to survive more than ten times better than the latter (other things staying the same). These simulations could also be used to study the possible influence of various density restricting processes in populations already fluctuating according to some pattern. As could be expected, the survival time of a population, which shows a tendency towards an upward trend in numbers, will be favoured by some kind of density restriction, but the degree to which these restrictions are density-dependent appeared to be immaterial. Density reductions that are about adequate on the average need even not occur at high densities only, if only the chance of occurrence at very low densities is low. The density-level at which a population is generally fluctuating appeared to be less important for survival than the fluctuation pattern itself, except for very low density levels, of course. The different ways in which deterministic and stochastic processes may interact and thus determine the fluctuations of population numbers are discussed. It is concluded that some stochastic processes will operate everywhere and will thus necessarily result in density fluctuations; such an omnipresence is much less imperative, however, for density-dependent processes, by which population models should primarily be stochastic models. However, if density-dependent processes are added to model populations, that are already fluctuating stochastically the effects are taken up into the general, stochastic fluctuation pattern, without altering it fundamentally.
小型孤立种群的生存时间往往相对较短，因此以这种方式生存的物种的存续将取决于其足够高的扩散能力，以产生一定的种群建立速率，该速率约能补偿种群灭绝速率。持续栖息在大型异质区域的复合种群的生存时间，在很大程度上取决于不同亚种群数量波动不均等的程度。通过比较19年里两种步甲复合种群的波动模式，证明了将灭绝风险分散到波动情况不同的亚种群中的重要性。这两种步甲分别是：云纹步甲，其亚种群数量波动不均等；黑头婪步甲，其亚种群数量平行波动。二者均持续占据着同一大片石南丛生的区域。通过模拟这些种群的波动模式，并由此直接估算生存时间，来检验实地数据得出的结论。结果表明，前一个物种的预期生存能力比后一个物种强十倍以上（其他条件保持不变）。这些模拟还可用于研究各种密度限制过程对已按某种模式波动的种群可能产生的影响。不出所料，数量呈上升趋势的种群的生存时间会因某种密度限制而增加，但这些限制与密度的依赖程度似乎并不重要。平均而言大致适度的密度降低甚至不必仅发生在高密度时，只要在极低密度时发生的几率较低即可。除了极低的密度水平外，种群通常波动的密度水平对生存的重要性似乎不如波动模式本身。文中讨论了确定性过程和随机过程相互作用从而决定种群数量波动的不同方式。得出的结论是，某些随机过程将在各处起作用，因此必然导致密度波动；然而，对于密度依赖过程而言，这种无处不在的情况就不那么必要了，因此种群模型应主要是随机模型。但是，如果将密度依赖过程添加到已经随机波动的模型种群中，其影响会被纳入一般的随机波动模式，而不会从根本上改变它。
