The only way is down: placing amphibian ponds on plateaux protects against Gambusia colonization

Invasive species pose a significant threat to global biodiversity, prompting the need for novel management strategies. We investigated the effect of pond placement in preventing colonization by Gambusia holbrooki, an invasive fish impacting amphibians, including the threatened green and golden bell frog (Litoria aurea). Because Gambusia moves between aquatic systems during surface flow events, we aimed to determine whether constructing ponds on elevated plots devoid of fish was less likely to be colonized due to the absence of two-way water connectivity with sources from below. Over eight consecutive years, we monitored Gambusia presence at 50 ponds within an industrialized area of Kooragang Island, New South Wales, Australia, home to one of the largest extant subpopulations of L. aurea. This included 24 constructed ponds, 19 of which were on elevated plots, along with 26 historic ponds already present prior to the study. None of the ponds constructed on elevated plots were ever colonized by Gambusia during the study period, while most (three out of five) constructed at lower elevation and historical ponds (25 out of 26) were. Our findings indicate that the elevated plots acted as plateau safe havens, with elevation difference rather than proximity to Gambusia sources the key driver preventing colonization. These plateaus are now functioning as some of the only Gambusia-free breeding habitat for L. aurea in this area. This research highlights the role of landscape topography and intentional pond placement in safeguarding newly constructed wetlands from invasive fish, providing insights for wetland conservation and biodiversity protection.