Building a framework for predicting the effects of species addition and species loss on nutrient dynamics in freshwater ecosystems

Background/Question/Methods

Change in species composition due to losses in biodiversity or species invasion has a profound influence on ecosystem processes, such as nutrient cycling, in freshwater ecosystems. Human activities result in biodiversity losses and enhance the ability of invasive species to invade and persist in novel habitats. Species loss and invasions are expected to increase through time and advances in understanding the ecological consequences of these changes are needed to inform appropriate management and conservation decisions. The purpose of this study was to identify patterns across study sites and organisms to determine what specific species traits make organisms more apt to be drivers of ecosystem function. We also propose using volumetric excretion (Ev) as a unifying method to assess the impacts of species additions and losses in lotic ecosystems under different environmental conditions and spatial scales. We used three examples to highlight the roles of animals and their potential impact on nutrient cycling:  two native species that are threatened with extirpation (mussels in OK and tadpoles in Panama) and one species of highly invasive fish (in Mexico). We modeled Ev for ammonium (N) and soluble reactive phosphorus (SRP) for several organisms and we compared these to other published values of Ev.

Results/Conclusions

In the three studies we reviewed, aquatic organisms had profound effects on nutrient dynamics in freshwater ecosystems. We found that species functional traits, such as stoichiometry and feeding behavior, were important in determining the spatial extent of an organisms’ influence on nutrient dynamics in lotic ecosystems.  Additionally, the physiochemical characteristics of the environment, including nutrient limitation, discharge, temperature, and the biophysical traits of the organism, including trophic state, biomass and density, were all important factors in predicting the role of organisms in nutrient dynamics. For example, non-native loricariids are nocturnally-active, phosphorus-rich, detritivorous fishes that exerted a large influence on nutrient dynamics during the dry season in tropical rivers in Mexico. Native fish excretion produced approximately 10% of the NH4+ and 3% of the P demand in the Chacamax River. By contrast, loricariids produced approximately 255% of the NH4+ and 70% of the P demand of the river; thereby converting the river to a system where fishes played an important functional role in nutrient dynamics. The results from our study highlight the importance of generating uniform methods to express species-specific effects on nutrient dynamics in freshwater ecosystems.