Download Species Richness and Trophic Diversity Increase Decomposition in a Co-Evolved Food Web

Abstract

Ecological communities show great variation in species richness, composition and food web structure across similar and
diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the
maintenance of diversity and the potential effects of species losses and gains on ecosystems. While research often focuses
on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can
provide further insight into the relationship between diversity and ecosystem functioning and elucidate potential
mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity 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 identified a trophic cascade in which top-predators — larvae of the pitcherplant
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 occur 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 trophic diversity and species
interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species
richness does not give a complete picture as to how ecosystems may change with the loss or gain of species.