Multi-scale analysis of Unionid freshwater mussel community structure in the upper Tennessee River basin

Populations of freshwater mussels of the family Unionidae are experiencing a decline in the upper Tennessee River basin. An understanding of the processes by which mussel community structure is created and sustained may aid conservation efforts. Here, modelling was used to explore the patterns that developed in local, mesoscale, and regional communities when particular biotic or abiotic processes were specified, and to generate predictions to which upper Tennessee River mussel data were compared.

Locally competing mussel species can coexist at equilibrium only when each is sufficiently more successful on a different resource in terms of resource use, life history characteristics, or a combination of the two. Temporal variability in fecundity promotes coexistence of a poorer competitor, particularly when variability is high and uncorrelated between species. Neither resource partitioning nor the influence of temporal variability explained coexistence of two similar mussel species: coexistence may be influenced by past competition or processes at different scales.

At the mesoscale, competition can reduce species number and lead to nested species composition among sites, whereas long-distance dispersal consistent with metapopulation dynamics has the opposite effect. Patterns in three mussel communities were most consistent with structuring by competition, however, at the low species number per patch exhibited in real communities, differences between scenarios with and without competition were relatively minor.

Highest regional-scale species richness occurred at lowest critical fitness values for colonization and extinction, not at intermediate colonization levels as would be expected if environmental heterogeneity generated pattern. High-richness communities exhibited low variability among replicate scenarios and high resource specialization among species, consistent with classical competition theory. Field data were consistent with the range of model results where competition appeared to structure communities.

Different processes appear to structure freshwater mussel communities at different spatial scales: communities appear increasingly structured by biotic processes with increasing spatial scale. Biotic processes generate simple rules that guide a system’s development, while abiotic processes control expression of pattern at particular sites. Results of this study can be used to develop predictive models, and to interpret observed patterns of decline in the upper Tennessee River basin.