Exploring Individual and Synergistic Effects of Introduced Species on a Reservoir Food Web: an Ecosystem Modeling Approach

Though current literature is replete with studies on the effects of introduced species on ecosystem structure and function, only recently has research highlighting the importance of interactive effects of multiple introductions emerged.  Further, though often assumed to be undesirable, the effects of single introductions are frequently unclear, and the combined effects of multiple, sequential invasions are even less understood.  Reservoirs are particularly susceptible to invasion and often contain several introduced fish species, making them optimal systems in which to quantify the ways that invaders alter community structure and trophic dynamics.  Lake Norman, the largest reservoir in North Carolina,  has been subjected to multiple species introductions since its impoundment in 1963, and negative effects of four specific species (flathead catfish, alewife, spotted bass, and white perch) on established populations and trophic dynamics in the system have been implied (e.g., population declines in certain sportfish) but not directly quantified.  We used an Ecopath with Ecosim modeling approach to help understand the effects of these introductions and to provide insight into potential management strategies.  Specifically, we parameterized an Ecopath model with eighteen functional group using data collected from Lake Norman in 2007-2010.  Because synergies among multiple invaders may produce effects greater than the sum of their parts, we ran Ecosim to investigate the effects of each introduced species individually, and combinations of species simultaneously, on trophic interactions and community structure in the lake.   For individual species, we simulated the invasion process using the four introduced species in the order that they invaded Lake Norman: white perch, followed by spotted bass, alewife, and flathead catfish.  Ongoing modeling efforts will simulate and compare all individual, pair and three-way combinations of the four introduced species to determine the ways that single invaders might influence systems differently than multiple, sequential invasions.  Because of their ability to quantify complex species interactions, ecosystem models are an important, and often underutilized, component of food web studies.  This investigation provides a rare opportunity to quantify the ways in which multiple introduced species impact an established reservoir ecosystem, which provides insight into whether multiple invaders function independently or synergistically in food webs, and allows modeling of potential mitigation or management strategies for these frequently invaded systems.