NSF DEB-0211010: Effects of species composition and environmental context on redundancy
within a functional group: a test with freshwater mussels.

Species that are believed to play the same functional role in ecosystems are often placed in functional groups, types or guilds, and redundancy of ecological function is predicted to be highest within such groups. The concept of functional groups composed of interchangeable, redundant species 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 redundancy among species assigned to many functional groups or guilds is unknown and we do not understand the degree to which the functional roles of species change with environmental context. The vast majority of our knowledge of species roles and how they are influenced by the environment comes from studies of short-lived, terrestrial plants. We now need to broaden our understanding of species roles in other equally important systems.

Freshwater systems, and streams in particular, are losing biodiversity at a higher rate than terrestrial or marine systems, but few studies have examined functional redundancy in these systems. Freshwater mussels are a functional group of long-lived, benthic, burrowing filter-feeders that are thought to play an important role in stream ecosystem function. Within this functional group, the potential for redundancy in ecological roles should be high because species have similar life histories, typically occur as speciose assemblages, and there is little evidence for differences in microhabitat or resource preferences between species. However, we have almost no quantitative information on either the overall importance of mussel assemblages to stream function or the roles of 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 stream ecosystems, we must quantify the effects of both overall mussel abundance and individual species on ecosystem processes, and we must understand how these relationships may change with environmental conditions.

Our research is adressing the following questions:

(1) What is the overall importance of the mussel functional group to stream ecosystems? What ecological processes do mussels perform 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 vary in their performance of ecological processes? Do ecological effects of mussel species within an assemblage vary based on species composition and/or abundance?

(3) How are the overall importance of the mussel functional group and effects of species roles within the group influenced by environmental context? Are the ecological effects of the overall mussel assemblage and of individual species constant or do they change with environmental conditions?

 

 

We are addressing these questions with a series of complementary laboratory stream and field enclosure experiments that are measuring the ecological function of single and multi-species assemblages of mussels under varying abundance and environmental conditions. Laboratory mesocosm experiments are focusing on ecological processes performed by mussels (eg. filtering rates, nutrient excretion). Mesocosms are housed in a new 3000 ft2 greenhouse on the OU campus

Field enclosure experiments are examing the effects of mussels on the rest of the benthic community. Small-scale laboratory experiments and analyses will quantify the potential contributions (eg. nutrient excretion and biodeposition rates) of different mussel species to streams under differing conditions, and mechanisms 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 and redundancy in different stream systems