Experimental tests of trophic dynamics: towards a more penetrating approach

The complex interactions between primary producers, herbivores, carnivores, and detritivores have resulted in the burgeoning field of trophic dynamics. One important contribution is the Fretwell and Oksanen theory (FO theory). The FO theory proposes that the productivity of the environment determines the length of the trophic chain, which, they suggest, is directly related to whether the system is being controlled by top-down forces (odd numbered length of trophic chain) or bottom-up forces (even numbered length of trophic chain). Recent evidence from experiments by L.H. Fraser and J.P. Grime claims to support the FO theory but the methodology has been criticised by D.C. Moon, P. Stiling and M.V. Cattell for hidden treatments and pseudoreplication. We reject these criticisms and recommend an approach to the study of trophic dynamics involving the aggregation of organisms into functional groups, direct quantitative measurements of trophic processes using field manipulations, inferences based upon the use of field probes and synthesis of ecosystems in closed microcosms.