Trophic diversity increases ecosystem functioning in a co-evolved food web

Background/Question/Methods

Anthropogenically influenced invasions and extinctions have altered biological communities and their associated ecosystem services. Concern over the potential loss of ecosystem services has focused research on the relationship between biodiversity and ecosystem functioning (BEF). Although most studies document positive relationships between species richness and measures of ecosystem functioning, they also generally lack realism:  the entire range of species richness is rarely represented, complete food webs are not studied, and realistic assembly dynamics are not incorporated. Furthermore, functional diversity and ecological interactions can change even while species richness remains unaltered. We assessed how species richness and functional trophic diversity (TD) affect decomposition rates in a complete aquatic food web:  the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea.We conducted a greenhouse experiment in which we assembled 70 food webs ranging from 0 consumer species (i.e. containing bacteria only) to the full complement of 9 consumer species. For each species richness treatment, we divided the range of TD into seven equally-spaced levels spanning the lowest to the highest possible TD values. Each food web was assembled in an individual pitcher of an individual Sarracenia plant. To explore the influence of species richness and TD on decomposition, we measured the mass-loss of one carpenter ant (Camponotus pennsylvanicus) in each pitcher over a two-week period. 

Results/Conclusions

We identified a trophic cascade in which top-predators — larvae of the pitcher-plant mosquito — indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occurs in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of species interactions in determining how ecosystems respond to global change implies that changes in the strength of ecological interactions can cause ecosystem services to decline even when species richness and functional diversity are conserved.