Holyoak Marcel, Baillie Stephen R
NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot, UK.
British Trust for Ornithology, Thetford, Norfolk, UK.
Oecologia. 1996 Oct;108(1):54-63. doi: 10.1007/BF00333214.
If censuses are taken at less than generation intervals, the number of successive censuses in which a given individual is recorded will depend on longevity. Repeatedly recording the same individuals could produce under-estimates of population variability and influence detection of density dependence. We investigated this possibility in 60 time series of abundances of British birds compiled from the Common Birds Census data and then used simple population models to illustrate the proposed mechanism. Species had average lifespans of 2-10 years and were censused annually. Density dependence was detected (at P<0.05) much more frequently in bird species with long lifespans than in those with short lifespans; 75% of the 12 longest-lived species showed density dependence compared to 46% of all species. Population variability measured in annual censuses (termed "annual variability") was lower in bird species with longer lifespans. We used discrete time models based on difference equations to demonstrate how longevity influences population variability and detection of density dependence in series of annual censuses. A model in which only first-year birds experienced density dependence was rejected because annual variability was greater and detection of density dependence was less likely when longevity was greater, the opposite of the observed effects of longevity in birds. A model in which all age classes experienced density dependence gave time series with lower annual variability and in which density dependence was detected more frequently when longevity was greater, which is the pattern observed in British birds. Analysis of data from this model showed that the amount of density dependence actually present caused only small changes in annual variability, whereas detection of density dependence from simulated series was strongly influenced by annual variability. The high annual variability of series from short-lived bird species could mask any density dependence that was present. Correcting for trends lead us to detect density dependence in 75% of the 12 longest lived bird species. There is no reason to believe that this rate is not also representative of short-lived species.
如果普查间隔时间短于世代间隔，一个特定个体被记录的连续普查次数将取决于其寿命。重复记录相同个体可能会低估种群变异性，并影响对密度制约的检测。我们利用从普通鸟类普查数据汇编而成的60个英国鸟类丰度时间序列，对这种可能性进行了调查，然后使用简单的种群模型来说明所提出的机制。这些物种的平均寿命为2至10年，每年进行普查。在寿命长的鸟类物种中，检测到密度制约（P<0.05）的频率比寿命短的物种高得多；12种最长寿物种中有75%显示出密度制约，而所有物种的这一比例为46%。在年度普查中测量的种群变异性（称为“年度变异性”）在寿命较长的鸟类物种中较低。我们使用基于差分方程的离散时间模型来证明寿命如何影响年度普查序列中的种群变异性和密度制约的检测。一个仅第一年鸟类经历密度制约的模型被否决，因为当寿命更长时，年度变异性更大，检测到密度制约的可能性更小，这与在鸟类中观察到的寿命影响相反。一个所有年龄组都经历密度制约的模型给出了年度变异性较低的时间序列，并且当寿命更长时，检测到密度制约的频率更高，这是在英国鸟类中观察到的模式。对该模型数据的分析表明，实际存在的密度制约量仅导致年度变异性的微小变化，而从模拟序列中检测到密度制约则受年度变异性的强烈影响。短命鸟类物种序列的高年度变异性可能掩盖了任何存在的密度制约。校正趋势后，我们在12种最长寿鸟类物种中的75%检测到了密度制约。没有理由认为这个比率不代表短命物种。
