Data_Sheet_2_Bird Census Data Do Not Indicate a Lack of Impact on Songbirds From the Growth of Avian Predator Populations in Britain in the Late 20th Century.PDF

The possible role of avian predators in limiting songbird populations has been largely discounted since the publication of findings showing a lack of statistical association in United Kingdom bird census data between changes in prey species populations and those of a range of predatory species, including raptors and corvids. I re-applied the methodology behind these findings, covering a wider range of prey species and using site-level modeling to estimate predator abundance instead of a mixture of spatial modeling and raw count data. A significant aggregate predator effect was found in 33 out of 40 prey species, compared to only 10 out of 27 in the original study, as well as a higher rate of significant individual predator effects, with 41 significantly negative and 84 significantly positive effects out of a total of 320. The greater explanatory power of predator variables estimated using site-level modeling suggests that this has significant advantages over the use of predator variables derived from spatial modeling, which may not capture variation in predator abundance at a local scale, or from raw count data, which may lead to attenuation of effect estimates. The prevalence of positive associations between predators and prey is consistent with a common response to local habitat variation, which may absorb negative covariance resulting from the impact of predators on prey populations. Both positive and negative predator-prey associations may also occur as a result of independent demographic processes that manifest as sequential habitat occupation or withdrawal. Analyses of census data cannot discriminate among these possible scenarios and may therefore have limited value in determining whether predators have been limiting prey populations. Inference to a lack of impact of avian predators on prey populations from such analyses may therefore be unsafe, and a role for increased predator numbers remains a viable hypothesis with respect to bird population declines. The recent neglect of this possibility should therefore be urgently reversed, with a particular need for field experiments that can support strong inference regarding population limitation of songbirds by avian predators.

自英国鸟类调查数据中缺乏统计关联性的发现公布以来，关于鸟类捕食者在限制鸣禽种群中可能扮演的角色已被广泛忽视。这些发现显示，在猎物种群变化与包括猛禽和鸦科在内的多种捕食物种之间，并未发现统计上的关联。我重新应用了这些发现背后的方法论，覆盖了更广泛的猎物种群，并采用基于地点的模型来估计捕食者数量，而非混合使用空间模型和原始计数数据。在40种猎物种群中，有33种发现了显著的聚合捕食者效应，而原始研究中仅在27种中发现了10种，同时，显著的个体捕食者效应率也有所提高，其中320个效应中有41个显著负效应和84个显著正效应。使用基于地点的模型估计的捕食者变量具有更高的解释力，这表明其在使用源自空间模型的捕食者变量之上具有显著优势，后者可能无法捕捉到局部尺度上的捕食者数量变化，或者从原始计数数据中，可能会导致效应估计的衰减。捕食者与猎物之间正关联的普遍性符合对局部栖息地变化的共同响应，这可能吸收了捕食者对猎物种群影响产生的负协方差。捕食者与猎物之间的正负关联也可能源于独立的人口统计过程，这些过程表现为连续的栖息地占用或撤退。由于调查数据分析无法区分这些可能的情景，因此其在确定捕食者是否限制了猎物种群方面的价值可能有限。因此，从这种分析中推断鸟类捕食者对猎物种群没有影响可能是不安全的，捕食者数量增加在鸟类种群下降的背景下仍是一个可行的假设。因此，应当紧急逆转近年来对这一可能性的忽视，特别是需要能够支持对鸟类捕食者限制鸣禽种群进行强烈推断的现场实验。