Data from: Negative relationships between population density and metabolic rates are not general

Population density has recently been suggested to be an important factor influencing metabolic rates, and to represent an important ‘third axis’ explaining variation beyond that explained by body mass and temperature. In situations where population density influences food consumption, the immediate effect on metabolism acting through specific dynamic action (SDA), and downregulation due to fasting over longer periods, is well understood. However, according to a recent review, previous studies suggest a more general effect of population density per se, even in the absence of such effects. It has been hypothesised that this results from animals performing anticipatory responses (i.e. reduced activity) to expected declines in food availability. Here we test the generality of this finding by measuring density effects on metabolic rates in 10 clones from two different species of the zooplankton Daphnia (Daphnia pulex Leydig and D. magna Straus). Using fluorescence-based respirometry we obtain high-precision measures of metabolism. We also identify additional studies on this topic that were not included in the previous review, compare the results, and evaluate the potential for measurement bias in all previous studies. We demonstrate significant variation in mass-specific metabolism among clones within both species. However, we find no evidence for a negative relationship between population density and mass-specific metabolism. The previously reported pattern also disappeared when we extended the set of studies analysed. We discuss potential reasons for the discrepancy among studies, including two main sources of potential bias (microbial respiration and declining oxygen consumption due to reduced oxygen availability). Only one of the previous studies gives sufficient information to conclude absence of such biases, and consistent with our results no effect of density on metabolism was found. We conclude that population density per se does not have a general effect on mass-specific metabolic rate.种群密度（population density）近年来被认为是影响代谢速率（metabolic rates）的关键因素，同时也是解释体重与温度之外的代谢变异的重要“第三维度”。在种群密度影响食物摄取的情境中，其通过特异性动力作用（specific dynamic action, SDA）对代谢产生的直接效应，以及长期禁食引发的代谢下调机制，已得到充分阐释。然而，根据近期一篇综述，此前的研究表明种群密度本身存在更为普适性的效应，即便不存在上述食物摄取相关的影响。有假说认为，该现象源于动物对预期的食物可获得性下降做出了预期性响应（即降低活动水平）。本研究以两种水蚤（Daphnia，枝角类浮游动物）的10个克隆株为实验材料，测定种群密度对代谢速率的影响，以此检验前述发现的普适性；所选用的物种分别为蚤状溞（Daphnia pulex Leydig）与大型溞（D. magna Straus）。本研究采用基于荧光的呼吸代谢测定法（fluorescence-based respirometry），获取了高精度的代谢水平测量数据。此外，本研究还筛选出此前综述未纳入的相关主题研究，对其结果进行比对，并评估所有既往研究中存在测量偏倚的可能性。本研究证实，两个物种的克隆株之间均存在显著的单位体重代谢速率（mass-specific metabolism）差异。但本研究未发现种群密度与单位体重代谢速率之间存在负相关关系的证据。当拓展分析的研究样本范围后，此前报道的相关模式也不再显著。本研究探讨了不同研究间结果不一致的潜在原因，其中包括两类主要的偏倚来源：微生物呼吸作用，以及因氧气可获得性下降引发的耗氧量降低。仅有一项既往研究提供了足够信息，可证实不存在此类偏倚，且其结果与本研究一致，未发现种群密度对代谢速率存在影响。综上，本研究得出结论：种群密度本身并不会对单位体重代谢速率产生普适性影响。