Are there direct and cascading effects of changes in grazer and predator species richness in a model system with heterogeneously distributed resources?

Abstract

The consequences of biodiversity loss on ecosystem processes and services have attracted substantial scientific effort over the last two decades. Most experiments have been small-scale and aimed to minimise environmental heterogeneity. Environmental heterogeneity is, however, known to be important in mediating species coexistence in both space and time. In an attempt to evaluate the potential importance of herbivore and predator richness in controlling the biomass and evenness of lower trophic levels, we manipulated the number of coexisting consumer species in a marine algae-grazer-predator microcosm system with a heterogeneous resource base. We performed two experiments where we manipulated the richness of four grazers and one where the richness of two predators, one crab and one shrimp, was manipulated. Even though the algal assemblage was diverse and potentially allowed for resource partitioning among grazers, plant consumption was mostly an effect of one effective grazer: the gastropod Littorina littorea. There were no significant richness effects on grazing rates of having four coexisting grazer species: the mixture with four grazers was approximately equal to the average of the four monocultures. Similarly, there was no emergent effect with two predator species present, as opposed to one species. The crab was most efficient in decreasing grazer biomass, whereas the effect of the shrimp was much weaker. Predators did not have any cascading effects on the algal community. In line with previous research, our results suggest that resource heterogeneity must be accompanied by broad enough trait diversity in order for resource partitioning to occur.

Appendix A

Table 2 The 18 algal species found in the intertidal zone of our study area, and the amount of biomass (g) grazed by the four grazers in the pilot experiment. The table also shows which class each species belongs to: Rhodophyceae (red algae), Phaeophyceae (brown algae) and Chlorophyceae (green algae)

Table 3 The algal species used in each of the three experiments

Table 4 The four grazers used in the experiment, their mean biomass (g) (based on data from Matthiessen et al. 2007) and number of specimens and expected biomass (g) stocked for each species and treatment