Ecosystem Effects Associated with Interactions between Northern Kōura (Paranephrops planifrons) and Invasive Mosquitofish (Gambusia affinis)

Abstract

In New Zealand there are two species of freshwater crayfish commonly referred to as the northern kōura (Paranephrops planifrons) and the southern kōura (Paranephrops zealandicus). Kōura are keystone species within New Zealand freshwater ecosystems influencing community and trophic structure through their various ecological roles as ecosystem engineers, detritivores, shredders and predators. However, kōura populations are currently in decline due to human mediated impacts including introductions of invasive fish. Understandings of the effects invasive fish have on kōura has been limited to research conducted on large speices that pose direct predatory impacts. Despite their co-occurrence in New Zealand freshwater ecosystems it is not known whether the invasive western mosquitofish interacts with the northern kōura or what ecological consequences are associated with their interactions. This is concerning given the ecological importance of kōura in New Zealand freshwater ecosystems and the impacts the western mosquitofish has within these systems. The overall aim of this study was to determine if the western mosquitofish impacts the ecology of the northern kōura and how their interactions effect ecosystem properties. A four week mesocosm experiment was conducted where the individual and interactive effects of these species on various ecosystem responses were determined. Ecosystem responses measured included physicochemical parameters and measures of ecosystem structure and function. As a pelagic-oriented fish, mosquitofish had a strong impact on the pelagic food web reducing mosquito larvae, macroinvertebrate and copepod abundance. However, this did not drive a trophic cascade (phytoplankton increase) over the four week experimental period. As kōura had a diet dominated by plant material they had no effect on zooplankton, mosquito larvae and macroinvertebrate abundance. However, a strong suppression effect on GPP, NPP and ER was induced in the presence of kōura. Indirect interactions between mosquitofish and kōura led to changes in the pelagic environment including an increase in periphyton concentrations. Kōura also dampened the effect of mosquitofish in reducing copepod abundance at week four of the experiment. Overall, this research has highlighted that indirect interactions between freshwater invaders and native species can alter ecosystem properties. This has been highlighted by the ecological outcomes associated with interactions between mosquitofish and northern kōura. Future research should further investigate the drivers of these interactive effects.