Geomorphology, biodiversity and ecosystem function, and food web dynamics in large riverscapes

Abstract

Humans have transformed the Earth's surface so extensively that we now play a dominant role in regulating geomorphological processes around the world. These effects are particularly prevalent in large rivers, which despite their high biophysical complexity, face widespread habitat alteration and simplification. The way species respond to such changes will unquestionably impact the structure and function of ecosystems, with cascading effects on numerous goods and services they provide. Consequently, efforts to understand how the physical habitat template shapes the structure and function of larger river ecosystems are critically needed to predict how future perturbations will impact these imperiled and iconic environments. My dissertation blends approaches from the physical and ecological sciences to uncover connections between the geophysical habitat template, biodiversity and ecosystem function, and the dynamics of food webs in large riverscapes. My questions were: 1) what are the spatial patterns and potential drivers of riverbed habitat, and how do they relate to process domain structures in large rivers? 2) how does the geophysical habitat template influence connections between invertebrate diversity and secondary production? and 3) how are trophic interactions supporting the federally endangered Pallid Sturgeon (Scaphirhynchus albus) shaped by the geophysical habitat template? I found consistent patterns of nested patchiness in riverbed substrate, indicating that multiple factors operating at different scales influence spatial patterns of substrate in the Missouri and Yellowstone Rivers. Invertebrate assemblages were tied to attributes of the geophysical habitat template, with strong relationships between large sediments and diversity. Invertebrate diversity, in turn, was positively related to secondary production, with the most diverse assemblages found in secondary production 'hotspots'. Finally, I found a general relationship between habitat diversity and trophic interaction strengths supporting Pallid Sturgeon, with geophysically diverse areas containing much weaker, and thus potentially stabilizing, interactions than homogenous areas. Additionally, habitat characteristics, such as sediment size, intensified these effects. Together, the chapters of my dissertation broadly highlight the role of nature's stage in governing patterns in biodiversity, secondary production, and trophic interactions across large and dynamic riverscapes, and provide empirical insights for guiding future management and conservation of large rivers in an increasingly simplified world.