Effects of Species Composition and Environmental Context on Redundancy Within a Functional Group: A Test With Freshwater Mussels

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Species that are believed to play the same functional role in ecosystems are often placed infunctional groups, types or guilds, and redundancy of ecological function is predicted to be highestwithin such groups. The concept of functional groups composed of interchangeable, redundantspecies is appealing because it simplifies the study and management of ecological systems.However, there are problems limiting the applicability of this concept. The degree of redundancyamong species assigned to many functional groups or guilds is unknown and we do not understandthe degree to which the functional roles of species change with environmental context. The vastmajority of our knowledge of species roles and how they are influenced by the environment comesfrom studies of short-lived, terrestrial plants. We now need to broaden our understanding ofspecies roles in other equally important systems.Freshwater systems, and streams in particular, are losing biodiversity at a higher rate thanterrestrial or marine systems, but few studies have examined functional redundancy in thesesystems. Freshwater mussels are a functional group of long-lived, benthic, burrowing filter-feedersthat are thought to play an important role in stream ecosystem function. Within this functionalgroup, the potential for redundancy in ecological roles should be high because species have similarlife histories, typically occur as speciose assemblages, and there is little evidence for differencesin microhabitat or resource preferences between species. However, we have almost no quantitativeinformation on either the overall importance of mussel assemblages to stream function or the rolesof individual species. In recent years, many mussel populations have undergone a drastic decline.In order to predict how this loss of both species and overall mussel biomass will impact streamecosystems, we must quantify the effects of both overall mussel abundance and individual specieson ecosystem processes, and we must understand how these relationships may change withenvironmental conditions.The proposed research will address the following questions: (1) What is the overall importance ofthe mussel functional group to stream ecosystems? What ecological processes do musselsperform in streams? How do these processes relate to overall mussel abundance/biomass? (2)Are species within the mussel functional group ecologically redundant? Do mussel species varyin their performance of ecological processes? Do ecological effects of mussel species within anassemblage vary based on species composition and/or abundance? (3) How are the overallimportance of the mussel functional group and effects of species roles within the group influencedby environmental context? Are the ecological effects of the overall mussel assemblage and ofindividual species constant or do they change with environmental conditions? A series ofcomplementary laboratory stream and field enclosure experiments will measure the ecologicalfunction of single and multi-species assemblages of mussels under varying abundance andenvironmental conditions. Laboratory stream experiments will focus on ecological processesperformed by mussels (eg. filtering rates, nutrient excretion) and field enclosure experiments willexamine the effects of mussels on the rest of the benthic community. Small-scale laboratoryexperiments and analyses will quantify the potential contributions (eg. nutrient excretion andbiodeposition rates) of different mussel species to streams under differing conditions, andmechanisms underlying differences in ecological function between species (eg. gill morphology,behavior). Finally, experimental results will be combined with quantitative information from an NSF-funded mussel biodiversity inventory to make predictions about mussel functional role andredundancy in different stream systems.