Assessing top-down and bottom-up control in a litter-based soil macroinvertebrate food chain

The relative importance of top-down and bottom-up control in setting the equilibrium abundances within trophic levels is examined in a comparative study on the litter-based food chain of a temperate deciduous forest. During two consecutive years, we estimated the abundances of macroinvertebrate detritivores and their predators on a natural gradient of annual litterfall. Detritus-based food chains are thought to be classical examples of donor-controlled systems. Indeed, both trophic levels showed higher abundances on sites with higher annual litterfall. Therefore; they appear to be bottom-up controlled. Using the Errors-in-Variables regression technique, we quantitatively compared our data with the equilibrium predictions of a set of simple trophic chain models including bottom-up effects with different types of functional responses (Beddington-DeAngelis, Hassell-Varley, and ratio-dependent). The model with a Hassell-Varley type functional response yielded the best adjustment to the data, although with a very high value of the mutual interference parameter suggesting the existence of overcompensating density dependence. Several changes to the structure of this model were considered. Their adjustment to the data consistently yielded such high values of the interference parameter.