Process catalyzers in Amazonian rivers: large woody debris modifies ecosystem processes across freshwater habitats

Understanding the role of interactions in influencing community structure and ecosystem function is a goal in ecology, and identifying biotic entities that are strong interactors is imperative for setting targeted conservation strategies. Several different mechanisms have been linked with strongly interacting species (e.g., predation, competition, abiotic habitat modification), but the most important organisms often influence ecosystems in multiple ways. We propose that these strong interactors share a broad common feature: They catalyze ecosystem processes, such as rates of primary productivity, species interactions, and/or physical disturbances. We provide a case study of Spanish cedar (Cedrela odorata), focusing on its influence as a provider of large woody debris (LWD) on food web dynamics in tropical floodplain rivers and associated oxbow lakes. Large woody debris has been subject to considerable attention because of its perceived importance in creating geomorphologically favorable conditions for target commercial species (e.g., distribution of pools and riffle for salmonids). However, in this study we suggest that LWD catalyzes a suite of ecological processes in addition to geomorphology that determines its important role within aquatic communities. Through a factorial experiment manipulating large and small fish access to treatments with and without LWD piles, we tested the role of Spanish cedar in modifying interactions between different-sized fishes, invertebrates, and primary producers in a tropical floodplain river and associated oxbow lake. Path analysis revealed that fishes influence particulate matter accumulation and invertebrate abundances more so in wood piles than outside of wood piles in both river and lake ecosystem contexts. In addition to providing the first experimental test of factors controlling trophic dynamics in an Amazonian river, we suggest that understanding the role of organisms through the ecological processes they catalyze provides an overarching conceptual framework to link single species and ecosystem-based management strategies.