Effects of consumer species richness and functional diversity on stream ecosystem processes

Background/Question/Methods

Biodiversity has generally been shown to have a positive effect on ecosystem function. While it has traditionally been measured as species richness, evidence suggests that functional diversity (FD) may be more important than species richness per se.  Additionally, many biodiversity studies have examined a single ecosystem process, but evidence suggests that biodiversity effects may vary depending on the process studied. This may be particularly true for secondary consumers, whose effects on ecosystem processes will generally be mitigated through food web interactions (e.g., trophic cascades). The objective of this research was to examine the relative importance of consumer FD and species richness in determining the effects of consumers on stream ecosystem function. We manipulated the diversity of 4 common consumers in stream mesocosoms: the predominately herbivorous central stoneroller (Campostoma anomalum), the omnivorous northern clearwater crayfish (Orconectes propinquus), mottled sculpin (Cottus bairdii), which are generalist benthic predators, and rainbow darters (Etheostoma caeruleum), which are largely benthic insectivores. We utilized a replacement design with mesocosms communities of no consumers, all combinations of two species, or all species present. We measured detrital breakdown rates, primary production, aquatic macroinvertebrate biomass and community composition, and consumer growth rates to determine the effects of consumer communities on stream ecosystem processes. A model selection approach will be used to compare the effect of species richness and FD on ecosystem processes.  

Results/Conclusions

Our preliminary results showed that E. caeruleum and C. bairdii were the most similar species in terms of FD, with O. propinquus falling in between these species and C. anomalum. Additionally, it appears that FD was a better predictor than species richness for growth rates of consumers (FD AICc  = -181.34 and species richness AICc = -113.63) but not for detrital breakdown (FD AICc = -25.87 and species richness = -27.83).  We found that growth rates were positively associated with FD, suggesting that less competition among consumers allowed for higher growth rates.  In contrast, species richness had a larger effect on detrital breakdown rates, suggesting that FD calculated solely on diet preferences may be less able to predict the effects of a consumer community on a single ecosystem process. This suggests that multiple ecosystem processes should be studied to accurately assess the effects of biodiversity on ecosystem function. However, further analyses will be done, including an examination of more ecosystem processes and a more detailed diet analysis to verify these preliminary results.