COS 307-4 - Density-driven interactions between two invaders with implications for endangered salmon

Abstract: Invasive species are a well-known threat to aquatic ecosystems, causing harm through predation on native species, competition for food and space, and overall disruption of food webs. Despite a well-established understanding of the individual effects of invasive species, the interactions between multiple invasive species and potential implications for native species are less understood. A rich body of ecological literature suggests that these interactions may be the sum of (additive), less than (antagonistic), or greater than (synergistic) the effects of individual invaders. By not accounting for the diverse ways that invaders might interact, standard single-species management strategies may be ineffective and unproductive. The John Day River Basin, Oregon hosts continuously expanding invasive populations of smallmouth bass (Micropterus dolomieu) and rusty crayfish (Faxonius rusticus), whose ranges are encroaching into critical spawning and rearing habitat of ESA-threatened Steelhead (Oncorhynchus mykiss). The goals of this study were to understand the interactions between bass and crayfish across a natural density gradient related to both species’ leading edges and to anticipate the net ecological outcomes. Using the HexSim framework, we created a spatially explicit, individual-based population model that includes an event sequence, parameterized values of each species’ life histories, and interspecific interactions based on a systematic literature review and field-based studies. Next, we simulated the population dynamics of each species under scenarios of projected climate change to examine the modulating role of conspecific and heterospecific interactions in shaping future distributions. Based on our understanding of species dynamics, the presence of both invasive species was predicted to result in a net negative impact on Steelhead recruitment and fry survival compared to areas in which only one invader is present. Population dynamics of both invaders were highly sensitive to the inclusion of biotic interactions in the models, suggesting its importance in understanding patterns of spread over time. The results of this research enabled a joint mechanistic and probabilistic prediction of multi-invader spread, which can be used to bolster current research and conservation efforts of Steelhead conservation. Additionally, model simulations revealed optimal locations and timing of control actions for one or both invaders that resulted in the greatest overall reductions in the invader's population size and slowing of range expansion in the river basin. The study elucidated and established a method for investigating potential ecological outcomes as a product of multiple interacting invasive species, which can be applied to other systems facing collective threats.