Data from: Morphological drivers of trophic cascades

Worldwide, local anthropogenic extinctions have recently been reported to induce trophic cascades, defined as perturbations of top consumers that propagate along food chains down to primary producers. This focus on the effects of top-consumer extinction (i.e. of species presence) ignores potential cascading effects of the rapid morphological changes that may precede extinction. Here, we show in an experimental, three-level food chain including medaka fish, herbivorous zooplankton and unicellular algae that varying body length of a single fish from large (36.3 mm) to small (11.5 mm) induced a stronger trophic cascade than varying an average-sized (23.8 mm) fish from being present to absent. The strength of fish predation on zooplankton scaled quasi linearly (not with a power exponent) with fish body length and associated gape width, suggesting that the resultant trophic cascade was morphology (not metabolism)-dependent. The effect of fish body length was stronger on phyto- than on zooplankton, because large-sized fish had the unique ability to suppress large-sized herbivores, which in turn had high grazing capacities. Hence, our results show that consumer body size, by setting diet breadth, can both drive and magnify the strength of trophic cascades. In contrast, fish body shape had no significant effect on fish predatory performances when its allometric component (the effect of size on shape) was removed. In the wild, human-induced body downsizing of top consumers is widespread, and mitigating the resultant perturbations to ecosystem function and services will require a paradigm shift from preserving species presence towards preserving species size structure.近期有研究报道，全球范围内局地人为导致的物种灭绝可引发营养级联（trophic cascade），该过程被定义为顶级消费者（top consumer）的扰动沿食物链传递至初级生产者（primary producer）的现象。现有研究多聚焦于顶级消费者灭绝（即物种存续与否）的影响，却忽略了灭绝前可能出现的快速形态变化（morphological change）所带来的潜在级联效应。本研究在包含青鳉鱼（medaka fish）、植食性浮游动物（herbivorous zooplankton）与单细胞藻类（unicellular algae）的三级实验食物链中发现：仅改变单条青鳉的体长（从36.3 mm至11.5 mm）所引发的营养级联效应，强于将平均体长（23.8 mm）的青鳉从存在变为缺失所产生的效应。鱼类对浮游动物的捕食强度与鱼体体长及对应口裂宽度（gape width）呈准线性比例关系（而非幂指数关系），表明由此产生的营养级联效应取决于形态而非代谢（metabolism）。鱼体体长对浮游植物的调控效应强于浮游动物，这是因为大型青鳉可特异性抑制大型植食性浮游动物，而这类大型植食者具备更强的摄食能力。因此，本研究结果表明：消费者的体型通过决定食性宽度（diet breadth），既可以驱动营养级联效应的产生，也能放大其强度。与之相反，当去除异速生长分量（allometric component，即体型对体型形状的影响）后，鱼体形状对鱼类的捕食表现无显著影响。在自然环境中，人类活动导致的顶级消费者体型小型化现象普遍存在。若要缓解由此引发的生态系统功能与服务（ecosystem function and service）扰动，需实现研究范式的转变：从仅保护物种存续，转向保护物种的体型结构。