Species richness of mussel assemblages and trait guilds in relation to environment and fish diversity in streams of Illinois, the USA

Fish hosts are critical for freshwater mussels. However, correlation between mussel and fish species richness (SR) is variable. Here, we examine how the environment affects this variability. We calculated mussel–fish SR ratios for 459 stream sites to capture the different responses of mussel and fish SR to environmental variables. We used Random-Forests (RF) to model how the SR ratio varied with environmental gradients. We also modeled the percent SR of mussel guilds with different hosting and life-history strategies. The SR ratio was generally low, but highly variable across sites. The ratio variability strongly affected mussel-fish SR correlations and can be explained by environmental factors. Given that environmental gradients also differ among datasets or regions, mussel-fish SR correlation can be expected to vary. The responses of the SR of mussel guilds agree with the expectations. However, land-use and climate co-vary with other environmental variables, and their effects are difficult to assess. We conclude that mussel and fish respond differently to many environmental variables; the importance of fish SR to mussel SR may vary with the environmental settings of streams but often appears to be low. Thus, environmental data may be generally sufficient for mussel-SR modelling and mussel diversity conservation planning.