A simple energy flow hypothesis is inadequate to explain the patterns of food chain lengths observed in nature: although the species at the end of the food chain are limited by their food supply, a good correlaiton between overall level of energy flow and food chain length does not follow. Rather, food chain lengths are set by a combination of 1) how violently ecosystems are disturbed (variance of energy flow), and 2) how quickly their constituent species can recover from such disturbances (rate of recovery being determined by the flux of energy and nutrient flows and the openness of nutrient cycles). Ideas concerning disturbance, resilience and species persistence are mooted. Spatial and temporal variations of energy and nutrient cycles are argued to be the essential ingredients in any comprehensive theory of trophic structure. -P.J.Jarvis