Data from: Systematic deviations from linear size spectra of lake fish communities are correlated with predator-prey interactions and lake-use intensity

Size structure of organisms at logarithmic scale (i.e. size spectrum) can often be described by a linear function with a negative slope; however, substantial deviations from linearity have often been found in natural systems. Theoretical studies suggest that greater nonlinearity in community size spectrum is associated with high predator-prey size ratios but low predator-prey abundance ratios; however, empirical evaluation of the effects of predator-prey interactions on nonlinear structures remains scarce. Here, we aim to empirically explore the pattern of the size-specific residuals (i.e. deviations from the linear regression between the logarithmic fish abundance and the logarithmic mean fish size) by using size spectra of fish communities in 74 German lakes. We found that nonlinearity was strong in lakes with high predator-prey abundance ratios but at low predator-prey size ratios. More specifically, our results suggest that only large predators, even if occurring in low abundances, can control the density of prey fishes in a broad range of size classes in a community and thus promote linearity in the size spectrum. In turn, the lack of large predator fishes may cause high abundances of fish in intermediate size classes, resulting in nonlinear size spectra in these lakes. Moreover, these lakes were characterized by a more intense human use including high fishing pressure and high total phosphorus concentrations, which have negative impacts on the abundance of large, predatory fish. Our findings indicate that nonlinear size spectra may reflect dynamical processes potentially caused by predator-prey interactions. This opens a new perspective in the research on size spectrum, and can be relevant to further quantify the efficiency of energy transfer in aquatic food webs.

对数尺度下的生物体型结构（即体型谱（size spectrum））通常可通过斜率为负的线性函数描述；然而，自然生态系统中常观测到显著的线性偏离。理论研究表明，群落体型谱的非线性程度越高，往往对应更高的捕食者-猎物体型比，但更低的捕食者-猎物丰度比；然而，针对捕食者-猎物相互作用对体型谱非线性结构影响的实证评估仍较为匮乏。本研究依托74个德国湖泊的鱼类群落体型谱数据，旨在实证探究体型特异性残差（即鱼类对数丰度与对数平均鱼类体型之间线性回归的偏差）的分布模式。研究结果显示，在捕食者-猎物丰度比高但体型比低的湖泊中，体型谱非线性程度显著。更具体而言，本研究结果表明，唯有大型捕食者即便种群丰度较低，也能够调控群落内广泛体型类群的猎物鱼类种群密度，进而维持体型谱的线性特征。反之，大型捕食性鱼类的缺失会导致中型体型鱼类种群丰度过高，进而使这些湖泊的体型谱呈现非线性特征。此外，这类湖泊往往伴随更为强烈的人类活动干扰，包括高强度捕捞压力与偏高的总磷浓度，这两者均会对大型捕食性鱼类的种群丰度产生负面影响。本研究结果表明，非线性体型谱或可反映由捕食者-猎物相互作用驱动的群落动态过程。这一发现为体型谱研究提供了全新视角，同时可为量化水生食物网的能量传递效率提供参考。