Freshwater ecosystems often face multiple human impacts, including climate change and invasive species. It is therefore important to understand how climate change might alter the impact of introduced species on stream ecosystems. Climate-driven changes in rainfall are predicted to decrease streamflow and increase flash flooding in tropical freshwater ecosystems. Increased flooding can give exotic species access to new habitats and increased drought events can enhance competitive effects on native species. Guppy fish have been introduced to many tropical freshwater ecosystems purposefully for mosquito population control or accidentally through aquarium trade. Our study investigates how climate-driven changes in flow and guppy fish introductions alter nutrient dynamics in Hawaiian streams. During the dry season of 2015 and 2016 we measured stream nutrient (nitrogen (N), phosphorus (P)) demand and nutrient supply via excretion of three native invertebrates as well as non-native guppy fish, which together comprised 80% of total animal biomass in these streams. We estimated the demand and supply rate in three guppy-free and three guppy-invaded streams across a rainfall gradient along the North Hilo region of Hawaii Island that mimicked predicted changes in stream flow.
Results/Conclusions
Predicted climate-driven changes in flow significantly decreased the total amount of nutrients exported by guppy-free streams. However, when streams are invaded, non-native guppies and climate change have synergistic effects on nutrient dynamics. Both nitrogen (0.68 – 3.95 mg N/m2/hr) and phosphorus demand (0.44 – 4.64 mg P/m2/hr) were lower in guppy-invaded streams, but did not vary consistently with stream flow. Guppy population N excretion was positively affected by climate-driven changes in flow. Native insect populations excreted up to 4x less nitrogen in guppy-invaded streams, but this difference decreased to 1.5x in drier streams. Total community excretion in guppy-free streams supplied 20% of the N demand, which decreased to 5% with climate-driven changes in flow. In guppy-invaded streams total community excretion supplied 60% of the N demand under current climate conditions, but did not consistently vary with predicted changes in flow. P showed similar patterns as N, but community excretion only supplied between 0.6% and 4% of the P demand. Guppy introductions appear to mediate effects of climate-driven changes in flow in nutrient poor tropical streams, but could exacerbate nutrient pollution in streams already dealing with eutrophication.