Trophic Cascades: Abiding Heterogeneity and the Trophic Level Concept at the End of the Road

Trophic cascades have been the subject for considerable research during the last decade. A review of the literature suggests that trophic cascades have been observed in many kinds of systems and organisms. However, the extent to which trophic cascades have community-wide effects (e.g. affecting total primary producer biomass density) is only indisputably demonstrated in aquatic systems. The lack of unequivocal evidence for community-wide trophic cascades from terrestrial systems may be due to (1) the absence of appropriately sealed experiments involving mammal herbivores or (2) that community-wide trophic cascades are less common in terrestrial systems. Experimental and theoretical studies have shown that adaptive foraging and consumer nutrient recycling are important elements of trophic cascades. In most studies, covariation in abundance has been found between adjacent levels that can in many cases be related to horizontal (several species at each trophic level) and vertical (species feeding from more than one trophic level) heterogeneity in food webs. A major cause to vertical heterogeneity is the presence of size structure in populations. The dynamic implications of horizontal and vertical heterogeneity cause the trophic level concept to be of limited value for the analysis of trophic dynamics. Theoretical and experimental studies should profit from focusing on how flexible behavior. nutrient reycling and horizontal and vertical heterogeneity affect indirect interactions in general. An approach based on community modules is suggested to be the most profitable route to take to make progress in this research area.