Understanding and predicting the leading edge dynamics of invasive rusty crayfish (Orconectes rusticus) in the John Day River

Abstract

Continued ecological impacts of invasive species on freshwater ecosystems is one of the main challenges confronting ecologists and decision makers in conserving biodiversity and ecosystem function today. Efforts to prohibit the initial introduction of nonnative species are widely recognized to be the most cost-effective management and policy strategy. However, when aquatic invasive species become established and start spreading through the landscape, efforts to slow their proliferation remain severely limited by a lack of adequate forecasting tools and understanding of their secondary spread. My thesis aims to address these challenges by improving our understanding of and predicting the leading edge dynamics of the invasive rusty crayfish Orconectes rusticus (now Faxonius rusticus) in the John Day River (JDR) basin, a major tributary of the Columbia River in northeastern Oregon. In Chapter 1, I demonstrate the use of a spatially explicit individual-based model to recreate the invasion history of rusty crayfish in the JDR and forecast its future distribution. This study shows that controlling the spread of invasive species is possible even after their establishment, when control efforts can be effectively allocated, and that spatially explicit individual-based models can provide unique insight into the secondary spread of aquatic invasive species and concretely support decision makers in choosing optimal control strategies. Chapter 2 investigates whether phenotypic differences exist between rusty crayfish individuals at the boundary of their invasion range compared to their conspecifics closer to their initial location of introduction. I show that rusty crayfish in the JDR have developed less competitive morphology and better physiological condition as they spread towards the edge of their current invasion range and feed lower in the food web in invasion front populations than in core areas. By accounting for variations in temperature, primary productivity, and macroinvertebrate biomass throughout the invasion gradient of rusty crayfish, my research suggests that low conspecific densities and natural selection by spatial sorting are the primary drivers of these phenotypic changes, which suggests that these trends are likely to grow stronger over time as rusty crayfish keep spreading. Together, these chapters not only improve our understanding of the leading edge dynamics of aquatic invasive species such as rusty crayfish but also improve our ability to control their spread and reduce their impact on invaded ecosystems.