Historical contingency and productivity effects on food-chain length

Food-chain length (FCL) is a fundamental ecosystem attribute, integrating information on both food web composition and ecosystem processes. It remains untested whether FCL also reflects the history of community assembly known to affect community composition and ecosystem functioning. Here, we performed microcosm experiments with a copepod (top predator), two ciliate species (intermediate consumers), and bacteria (producers), and modified the sequence of species introduction into the microcosm at four productivity levels to jointly test the effects of historical contingency and productivity on FCL. FCL increased when the top predator was introduced last; thus, the trophic position of the copepod reflected assembly history. A shorter FCL occurred at the highest productivity level, probably because the predator switched to feeding at the lower trophic levels because of the abundant basal resource. Thus, we present empirical evidence that FCL was determined by historical contingency, likely caused by priority effects, and by productivity.

食物链长度（Food-chain length, FCL）是生态系统的核心属性，整合了食物网组成与生态系统过程的相关信息。目前尚不明确，食物链长度是否同样能够反映已知会影响群落组成与生态系统功能的群落组装历史。本研究针对桡足类（copepod，顶级捕食者）、两种纤毛虫（ciliate，中级消费者）以及细菌（生产者）开展微宇宙实验（microcosm experiment），并在四个生产力水平下调整微宇宙中物种的引入顺序，以联合检验历史偶然性与生产力对食物链长度的影响。当顶级捕食者最后被引入时，食物链长度会增加；由此可见，桡足类的营养级位置反映了群落组装历史。在最高生产力水平下，食物链长度更短，这可能是因为基础资源充足，捕食者转而取食更低营养级的生物。综上，本研究提供了实证证据，表明食物链长度由历史偶然性（大概率由优先效应驱动）与生产力共同决定。