A trait-based approach to species' roles in stream ecosystems: climate change, community structure, and material cycling.

The sustained decline in habitat quality and community integrity highlights the importance of understanding how communities and environmental variation interactively contribute to ecosystem services. We performed a laboratory experiment manipulating effects of acclimation temperature (5, 15, 25, and 35 degrees C) on resource acquisition, assimilation and subsequent ecosystem services provided by eight freshwater mussel species. Our results suggest that although freshwater mussels are broadly categorized as filter feeders, there are distinct nested functional guilds (thermally tolerant and sensitive) associated with their thermal performance. At 35 degrees C, thermally tolerant species have increased resource assimilation and higher rates of contributed ecosystem services (nutrient excretion, benthic-pelagic coupling). Conversely, thermally sensitive species have decreased assimilation rates and display an array of functional responses including increased/decreased benthic-pelagic coupling and nutrient excretion. Although thermally sensitive species may be in poorer physiological condition at warmer temperatures, their physiological responses can have positive effects on ecosystem services. We extrapolated these results to real mussel beds varying in species composition to address how shifts in community composition coupled with climate change may shift their contributed ecological services. Comparative field data indicate that two co-existing, abundant species with opposing thermal performance (Actinonaias ligamentina, Amblema plicata) differentially dominate community biomass. Additionally, communities varying in the relative proportion of these species differentially influence the magnitude (benthic-pelagic coupling) and quality (N:P excretion) of ecosystem services. As species are increasingly threatened by climate change, greater emphasis should be placed on understanding the contribution of physiological stress to the integrity and functioning of ecosystems.