Proximate and ultimate determinants of food chain length

Food chain length has been recognized as an important ecosystem characteristic and a fundamental ecosystem attribute: it influences community structure,ecosystem functions and bioaccumulation of contaminants in top predators.Previous studies have shown that food chain length varies greatly among systems. Defining the determinants of food chain length has been a primary research focus for food web ecologists,and has contributed to ecological theories. Three main hypotheses,i.e. resource availability,ecosystem size,and the disturbance hypothesis,which argues that the stability of local dynamics and the level of disturbance contribute to food chain length have been extensively examined. However,much less attention has been paid to proximate changes in food web structure that link variation in food-chain length to the ultimate dynamic mechanism. Proximate mechanisms could cause variation in food-chain length by either: additive mechanism,through addition or removal of top predators; insertion mechanism,through addition or removal of intermediate predators; omnivory mechanisms,changes in the degree of trophic omnivory by the top predator. Recent advances in stable isotope analysis have rekindled interest in examining factors that determine food chain length in ecological communities. In this paper,we reviewed recent studies from close and open freshwater systems to explore the factors that determine the relative importance of resource availability,disturbance and ecosystem size on food chain length,and how they may be affected by food web architecture. We then provided a framework to investigate how these hypotheses operating at different spatial scale. We explore the implications of these mechanisms for understanding how proximate changes in food-web structure link variation in food-chain length to the ultimate mechanism. Finally,we gave suggestions in applying the framework to consolidate the hypotheses across ecosystems in future studies.