Macroecology of a host-parasite relationship

The larvae of freshwater mussels are obligate ectoparasites on fishes while adults are sedentary and benthic. Dispersal of mussels is dependent on the movement of fish hosts, a regional process, but growth and reproduction sh ould be governed by local processes. Thus, mussel assemblage attributes sho uld be predictable from the regional distribution and abundance of fishes. At a broad spatial scale in the Red River drainage, USA, mussel species ric hness and fish species richness were positively associated; maximum mussel richness was limited by fish richness, but was variable beneath that constr aint. Measured environmental variables and the associated local fish assemb lages each significantly accounted for the regional variation in mussel ass emblages. Furthermore, mussel assemblages showed strong spatial autocorrela tion. Variation partitioning revealed that pure fish effects accounted for 15.4% of the variation in mussel assemblages: pure spatial and environmenta l effects accounted for 16.1% and 7.8%. respectively. Shared variation amon g fish, space and environmental variables totaled 40%. Of this shared varia tion, 36.8% was associated with the fish matrix. Thus. the variation in mus sel assemblages that was associated with the distribution and abundance of fishes was substantial ( > 50%), indicating that fish community structure i s an important determinant of mussel community structure. Although animals commonly disperse plants and, thus, influence the structure of plant commun ities, our results show a strong macroecological association between two di sparate animal groups with one strongly affecting the assemblage structure of the other.