Are food chains limited by foraging efficiency?

Posted on December 15, 2009

What determines the height of an ecological pyramid?  Although the number of energy transfers between the base and the top of a food chain is a fundamental feature that affects the structure and function of ecosystems, a general theory of food chain length is still elusive.  A study of ten species-rich food webs in the Parana River Basin, Brazil showed that the length of aquatic food chains varied significantly between ecosystems, being longest in the least productive habitats (reservoirs) and shortest in high-gradient rivers.  This finding contradicts the commonly cited “productivity hypothesis” that predicts that food chain length should be limited by energy availability.  A possible explanation is that different types of food chain vary in terms of the relative sizes of species at successive levels in the feeding pyramid.  For example, the most effective predators of zooplankton are small optimally-foraging fish, but detritus and filamentous algae tend to be eaten by larger fish species.  Therefore, more transfer stages are needed to pass energy to the top of plankton-based food chains than to the end of chains based on detritus or algae.  By affecting the relative importance of different energy sources, both natural hydrological factors (the local landscape) and artificial structures (e.g., dams) can influence the length of aquatic food chains.

Reference:  Hoeinghaus, D.J., Winemiller, K.O. & Agostinho, A.A.  2008.  Hydrogeomorphology and river impoundment affect food-chain length of diverse Neotropical food webs.  Oikos 117, 984-995.