Nonnative species engage in a suite of novel interactions in their introduced ranges, the collective outcome of which will determine their success as well as the fate of resident species. The major goal of my dissertation research is to examine how food web interactions influence long-term ecological and economic impacts of an invasive omnivore. My study system consists of the invasive rusty crayfish (Orconectes rusticus) and its interactions with fish, aquatic plants, and snails in north temperate lakes of northern Wisconsin and Michigan.
I studied aspects of intraguild predation between O. rusticus and smallmouth bass (Micropterus dolomieu) that were previously unknown using a combination of field surveys, video surveillance, diet reconstructions, and a controlled feeding experiment. I found that higher densities of O. rusticus increased rates of bass nest abandonment, but that consumption of crayfish by bass may overall lower abandonment. Additionally, consuming the resource pulse of bass eggs confers a growth benefit for O. rusticus. Together, these results support our field observations on why O. rusticus and M. dolomieu can coexist in abundance in north temperate lakes.
I used a long-term multi-lake survey of crayfish, macrophytes, and snails, as well as germination assays, to examine the potential for natural recovery following significant declines in O. rusticus abundance. I also examined how spatial heterogeneity influences both negative impacts and recovery over time. In general, crayfish impacts are realized quickly and recovery is gradual and incomplete. The magnitude of response scales with the magnitude in crayfish change, but this relationship is influence by substrate and is taxon-specific.
I developed a bioeconomic framework to evaluate which management strategies in response to O. rusticus are most likely to produce positive net economic benefits. The model simulations reveal that when prevention fails, immediate action to reduce O. rusticus will produce the greatest net economic benefit. I identified that the density-impact relationship between the invasive species and the ecosystem service has the greatest influence on model predictions.
My dissertation research contributes to our understanding of how complex food web interactions influence the net impact of an invasive species. This work provides new information about an intraguild predator and prey relationship, and describes limitations and conditions for natural recovery among multiple trophic levels in lakes impacted by an invasive omnivore. Further, I developed a management tool that can be adapted and applied to a variety of systems to help make time-sensitive decisions about invasive species control.