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.

近日有研究报道，全球范围内局地人为灭绝事件已引发营养级联效应（trophic cascade）——该效应被定义为顶级消费者的扰动沿食物链传递至初级生产者的过程。当前针对顶级消费者灭绝（即物种存留状态）的效应研究，忽略了灭绝前可能发生的快速形态变化所带来的潜在级联效应。本研究依托包含青鳉鱼、植食性浮游动物与单细胞藻类的三级实验食物链开展研究，结果显示：仅通过调整单条青鳉的体长（从36.3 mm的大型个体至11.5 mm的小型个体）所引发的营养级联效应强度，高于仅通过改变平均体长（23.8 mm）个体的存留与否（存在/消失）所产生的效应。青鳉对浮游动物的捕食强度与鱼体长度及对应的口裂宽度呈准线性相关（而非幂指数相关），这表明由此产生的营养级联效应依赖于形态特征而非代谢水平。鱼体长度对浮游植物的影响强度显著高于其对浮游动物的影响，这是因为大型青鳉具备独特的能力以抑制大型植食性浮游动物，而这类大型植食者具备极强的牧食能力。综上，本研究结果表明，消费者体型通过决定其食性宽度，既可以驱动营养级联效应的发生，也能放大其强度。与之相反，当去除异速生长分量（即体型对形状的影响）后，鱼体形状对青鳉的捕食表现无显著影响。在自然生态系统中，人类活动导致的顶级消费者体型小型化现象普遍存在；若要缓解由此对生态系统功能与服务造成的扰动，亟需转变现有保护范式——从仅关注物种存留，转向同时保护物种的体型结构。