Impacts of the invasive New Zealand mud snail in aquatic food webs: Community patterns, competitive effects and trophic interactions

Biological invasions have become increasingly common in our globalized society. An important goal of invasion research is to understand how the successful establishment of invasive species can both affect and be affected by the diversity of recipient communities. In chapter one, I introduce my study system and several important concepts in invasion ecology that are addressed by this dissertation. In chapter two, I use the framework of the "invasion paradox" to examine the relationship between the establishment of high densities of the introduced New Zealand mud snail (Potamopyrgus antipodarum) and native community diversity and structure across a range of aquatic systems. Within a highly invaded estuarine system, I examine interactions between this snail and native consumers to better understand the competitive impacts of this species (chapter three). Finally in chapter four, I address the direct and indirect interactions between this snail, predators, and their native prey. Across multiple regions, the highest densities of P. antipodarum were found in systems with lower diversity and density of native invertebrates, however within river systems, there was no correlation between the density of mud snails and native diversity or density. There was no evidence for a negative impact of P. antipodarum on a native estuarine species that overlaps with mud snails in resource use. However, there was evidence for both direct trophic interactions between P. antipodarum and native predators, and indirect effects, in the form of asymmetrical apparent competition with native prey. Indirect effects such as these are less frequently studied but can be important in structuring communities, and demonstrate the complexity of determining the impact of a given invader. Like many successful invasive species, Potamopyrgus antipodarum are tolerant of a wide-range of environmental conditions, allowing them to invade many different habitat types. It is likely that the factors influencing both the success and impact of P. antipodarum differ between these types of communities, and further research into the long-term, multitrophic effects of this introduced species is important.