The role of climate-related disturbances on complex host-affiliate relationships remains largely understudied.  This lack of understanding is confounded by the fact that affiliate species vary in host specificity and often possess a suite of traits that make them differentially susceptible to disturbance compared to their respective hosts.  Here we investigated the host-affiliate, fish-mussel, relationships with current and future water availability modeled under the IPCC SRES A2 emissions scenario.  We used species-discharge relationships (SDR) for freshwater mussels and fish across 11 distinct hydrologic regions in the continental United States that we coupled to future water availability to model mussel and fish co-extirpations (n =343 rivers).  In addition, we used known host-affiliate matrices (presence-absence) to calculate host-overlap (specificity) and perform extirpation simulations to determine the relative importance of the loss of host from that of habitat loss (water availability or discharge).

Results/Conclusions

Our results indicate that the strength and predictability of SDR models vary geographically with strong richness-discharge patterns found in southern species-rich regions.  In addition, mussel SDR’s (mean slope = 0.14) were greater than those of fish (mean slope = 0.08) and mussel species losses associated with reduction in water availability (range = 0 - 40 species) were greater than those associated with loss of fish hosts (range = 0 - 18 species).  Mussel communities with greater host-redundancy had lower extirpation coefficients supporting the redundancy hypothesis.  In practice however, species-rich communities had lower host overlap, presumably due to increased niche differentiation for hosts. Thus, secondary extirpations associated with host losses are expected to be greater in species-rich communities.  Furthermore, our results indicate that over 25% of our study rivers will experience within group (fish or mussel) extirpations of over 10 species.  However, we have found no studies that address the functional consequences of aquatic taxa beyond 8 species.  Here, we report the first set of host-affiliate SDR relationships in freshwater but more studies are needed to improve these models and assess the functional consequences of species losses to the functioning of ecosystems.