A top predator forages low on species-rich tropical food chains.

Emergent properties of food webs, including food-chain length (FCL), may differ across latitudinal gradients because of strong differences in biodiversity and productivity between warm and cold regions. Theory predicts long food chains in the tropics because of high species richness and productivity, but empirical data suggest otherwise. Here I show that an opportunistic top predator common to coastal rivers and streams, the Australian Longfinned Eel (Anguilla reinhardtii), feeds ∼1 trophic position higher in temperate systems (4.7 ± 0.3 [SD]) than in tropical systems (3.8 ± 0.5). This result suggests shorter food chains that contain a diverse array of large-bodied herbivores and omnivores that act as prey for generalist predators, such as eels, in tropical systems. The resulting altered size spectrum limits the size and trophic position of top predators that can be supported in the face of constraints from known limits to FCL, including productivity, ecosystem size, and disturbance. This framework has general application for food webs and suggests unique structural and functional attributes of highly threatened tropical freshwater ecosystems. [ABSTRACT FROM AUTHOR]