THE COMPLEX RESPONSE OF HIGHLY MODIFIED RIVERS TO ADDITIONAL DRIVERS OF CHANGE

Large rivers and their floodplains are among the most productive ecosystems, globally, yet are the ecosystem most affected by humans. Few studies explicitly explore multiple anthropogenic drivers in large river systems. To improve our knowledge of the influence of anthropogenic drivers in large rivers, we need to investigate ecosystem response at broad spatial and temporal scales, whereas most studies focus on single drivers and small scales. Our study uses a nearly 60-year, river-wide dataset to determine if fish diversity in the Illinois River (ILR; Illinois, USA) changed in response to two major, system-wide anthropogenic drivers: policy-based water quality improvements (principally the Federal Water Pollution Control Act [FWPCA, ca. 1948] and the Clean Water Act [CWA, ca. 1972]) and invasion by bigheaded carps (BHC, ca. 2000; i.e., silver carp Hypophthalmichthys molitrix and bighead carp H. nobilis). We analyzed diversity changes for the entire river system, between distinct geomorphic zones, and among functional feeding guilds. Although the response was complex, overall, fish diversity increased substantially in the whole river, and the upper and lower river, and several FFGs displayed diversity increases more impressive than the combined group.