Biological invasions in brackish waters

Abstract

Invasive non-native species (NNS) cause deleterious ecological, economic, social impacts worldwide. Brackish waters are transitional areas subject to invasion by taxa that may subsequently spread into adjacent freshwater and marine systems. The tendency for ports to be situated in brackish estuaries makes them particularly susceptible to ship-borne introductions of NNS. Despite being reported as heavily invaded ecosystems with a disproportionately high abundance of NNS, studies on biological invasion in brackish waters has received considerably less attention than truly freshwater and marine systems. In this thesis, I investigate the discovery, ecological impacts, and management of NNS in brackish waters. I begin by conducting a systematic review and meta-analysis comparing the ecological impacts of NNS in brackish, freshwater, and marine environments around the globe. I reveal that NNS have led to an overall decrease in the abundance and diversity of the resident communities in freshwater and marine environments, but not in brackish waters. NNS in brackish estuaries provide novel habitats, which can facilitate the establishment of other NNS, leading to an invasional meltdown. Next, I explore the first records of all freshwater, brackish and marine NNS in Great Britain and show that many NNS were first discovered in estuaries and often aggregated around the major shipping ports. This result provides support that many marine and freshwater NNS in Great Britain could have been introduced via international ships visiting brackish waters. I then focus on one NNS, the Gulf wedge clam, Rangia cuneata, which is a brackish water bivalve currently invading European waterways. I show that the growth and feeding rates of R. cuneata differed at the two extremes of salinity tolerated by this species, and consider how this intraspecific variation might drive context-specific ecological impacts. I go on to demonstrate the potential detrimental impacts of R. cuneata as a result of their much higher feeding rates and greater functional responses compared to a sympatric, native unionid mussels. I finish by investigating two methods to control invasive species in brackish waters. I show that using saline, but not freshwater, shock could effectively control R. cuneata. I then demonstrate the effectiveness of two new formulations of microencapsulated biocides in controlling R. cuneata. Taken together, these studies provide important insights into the discovery, impacts and management of NNS in brackish waters. My findings demonstrate the need for greater monitoring for NNS in brackish systems, and especially those subject to high ship traffic. The control methods proposed here have wide application in many brackish systems and for many species, thus providing much needed effective tools for the rapid response to the discovery of high risk invasive NNS in this important but overlooked transitional habitat.