Comparative food web impacts of a native and an invasive fish in dynamic floodplain wetlands

Invasive species cause major declines in biodiversity globally through a range of mechanisms including predation, displacement of endemic taxa, and complex food web impacts. While the dramatic effects of invaders have been well documented, less is known about cases where global invaders and native species coexist, the factors that facilitate this, and the comparative food web effects of invasive and native competitors. The invasive poeciliid fish Gambusia holbrooki (Eastern Mosquitofish) has been shown to have dramatic impacts on freshwater systems on four continents. These include trophic cascades resulting in algal blooms, changes in invertebrate community composition, and displacement of endemic fish and frogs. However in some situations they coexist with other fish species, with apparently only minor effects on native biodiversity and ecosystem processes. My thesis examines the impacts of Gambusia and a similar sized native fish (Hypseleotris spp.) in the hydrologically and seasonally dynamic floodplain wetlands along the Ovens River in south-eastern Australia, where these species commonly co-occur. These wetlands systems typically undergo seasonal flooding during spring, but contract during summer, giving rise to high fish population densities, and potentially intensified biotic interactions. Gambusia and Hypseleotris population dynamics were monitored bi-monthly in 15 wetland pools over 2.5 years (2007-09) to determine if local seasonal habitat conditions, including floods and drying, influenced coexistence (Chapter Two). In two of these pools, I tested the degree of dietary overlap between Gambusia and Hypseleotris by examining fish gut contents and carrying out seasonal stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of fish and invertebrate communities (2008-2009) (Chapter Three). I then experimentally compared the ability of the two fish to trigger trophic cascades in a year-long mesocosm experiment (2009-10). This monitored the top-down effects of Gambusia and Hypseleotris on invertebrates, algae and nutrients. Treatments included Gambusia only, Hypseleotris only, mixed-predator (Gambusia/Hypseleotris) and controls (no fish) (Chapter Four). A second experiment examined the effects of the same treatments under seasonal habitat contraction (April-July 2010) (Chapter Five). I found that hydrological disturbance, and Gambusia’s and Hypseleotris’ different breeding strategies, may facilitate coexistence. While these species coexisted year around, Gambusia’s ability to breed viviparously through an extended period of the year may have allowed it to increase numbers in the absence of potentially population-limiting floods. Gambusia and Hypseleotris had similar trophic positions but their dietary overlap was temporally dynamic, with Gambusia appearing to shift towards preying more broadly on consumers of a range of basal resources in winter. Despite Gambusia’s documented ability to facilitate trophic cascades, the indirect effect of the two species on algal biomass was weak and generally similar, but varied seasonally. Seasonal shifts in algal biomass and the presence of a predator appeared more important in facilitating trophic cascades than predator species. In autumn and winter, trophic cascades induced by both species occurred only when habitats were contracted, highlighting the important role that the spatial and seasonal dynamics of habitats can have in determining the trophic dynamics of both native and invasive species. In summary, despite previously reported strong top-down effects, Gambusia invasion may only alter prey communities at certain times of year and given particular ecological conditions in dynamic floodplain wetland pools where Hypseleotris is already present. Despite being taxonomically unrelated and differing in life history traits, Gambusia and Hypseleotris appear to have strong dietary overlap and largely equivalent weak and seasonally variable top-down effects, showing the potential for the impacts of invasive species to be moderated by local conditions and interactions with native species.