Proximate structural mechanisms for variation in food‐chain length

Food‐chain length is a central characteristic of ecological communities because of its strong influence on community structure and ecosystem function. While recent studies have started to better clarify the relationship between food‐chain length and environmental gradients such as resource availability and ecosystem size, much less progress has been made in isolating the ultimate and proximate mechanisms that determine food‐chain length. Progress has been slow, in part, because research has paid little attention to the proximate changes in food web structure that must link variation in food‐chain length to the ultimate dynamic mechanism. Here we outline the structural mechanisms that determine variation in food‐chain length. We explore the implications of these mechanisms for understanding how changes in food‐web structure influence food‐chain length using both an intraguild predation community model and data from natural ecosystems. The resulting framework provides the mechanisms for linking ultimate dynamic mechanisms to variation in food‐chain length. It also suggests that simple linear food‐chain models may make misleading predictions about patterns of variation in food‐chain length because they are unable to incorporate important structural mechanisms that alter food‐web dynamics and cause non‐linear shifts in food‐web structure. Intraguild predation models provide a more appropriate theoretical framework for understanding food‐chain length in most natural ecosystems because they accommodate all of the proximate structural mechanisms identified here.