Modeling the Riverscape Ecology of Fish Invasions Across an Artificial Irrigation Network

The invasion of river networks by alien fish species is an ongoing biodiversity crisis in many parts of the world. The threat posed by alien invasive fish to native fish conservation within riverscapes highlights the need to understand the spatial-temporal dynamics of these invasions, so that mitigating solutions can be found. Much research into the establishment and spread of alien fish has focused on the role of habitat adaptations of the aliens and biotic resistance by recipient communities. However, another critical driver of invasion that has not been adequately studied is propagule pressure: the quantity, quality, composition and rate of supply of alien organisms across the riverscape. Propagule pressure has been identified as a fundamental determinant of invasion success especially when multiple human-mediated introductions occur over time and has been suggested to serve as the basis of a null model for biological invasions. To understand the role of propagule pressure in alien fish invasions, we simulate the spread of alien fish through the Sundays River irrigation network in South Africa, which consists of multiple storage impoundments connected by water-transfer furrows. Because the connectivity and filling history is known for each impoundment, incidence of invasive fish can be linked to both inoculation rate and location within the network, allowing spatial-temporal models of propagule pressure to be built. In this research we test the null hypothesis that propagule pressure is the central driver of alien fish establishment. We begin by modeling potential rates of spread using a dynamic network model on a GIS representation of the irrigation network. The results from this model can then be compared with the field data of fish distributions across the impoundments, assessing the relative importance of network location, inoculation rate, local habitat suitability and community composition in determining the establishment success of invasive fish species. Lessons learned from this model system can be further applied for assessing the role of propagule pressure in alien fish invasions across natural riverscapes in South Africa.