Resource productivity and availability impacts for food-chain length

Ecologists have focused on food-chain length (FCL) for the past eight decades as an index of food-web structure. Here, I review the hypotheses determining FCL with a focus on resource productivity and availability effects on FCL. First, I introduce the mainstream hypotheses to explain FCL variations: productivity, ecosystem size, and disturbance. For the existing productivity and productive space hypotheses, I stress the importance of using resource availability to estimate the productivity effect on FCL. Using a FCL dataset from 15 ponds, I tested the resource stoichiometry effect on FCL for ponds with between carbon:nitrogen ratio of primary producers and FCL. Moreover, I provide a perspective for studying resource availability and stoichiometry effects on FCL and of the alternative hypotheses of productive-space and ecosystem size. Finally, I suggest the future directions of the FCL study: a resource subsidy and climate change effects on FCL and food-web structure.