Predicting distribution and establishment of two invasive alien Daphnia species in diverse lakes in New Zealand-Aotearoa

The freshwater zooplankton of New Zealand-Aotearoa are species-poor with only two indigenous Daphnia species: D. thomsoni, and D. tewaipounamu. Over the last two decades, two species of invasive non-indigenous Daphnia, D. pulicaria/pulex and D. galeata, have become well established in many lakes in New Zealand-Aotearoa and their distribution appears to be rapidly increasing. We report the current distribution of these two invasive species in approximately 300 lakes throughout New Zealand-Aotearoa and test the hypothesis that the distribution and establishment of these invaders is more closely related to human access to a lake than to any environmental attributes of a lake's water or biota and consider potential implications of our findings for lake management. Boosted regression tree analysis identified total nitrogen and distance from the coast (D. pulicaria) and annual lake temperature (D. galeata) as key variables explaining current distributions. Inclusion of spatial autocorrelation overwhelmed other explanatory variables, highlighting the greater likelihood of spread from known source locations. Proximity to a road was an important predictor for D. pulicaria, and when tested separately both invasive Daphnia occurred significantly more often in lakes within 200 m of a road. Critically, D. galeata's realised niche completely envelopes D. thomsoni's, which could force the native species to become extinct through competitive exclusion. These results suggest that the spread of D. pulicaria and D. galeata to lakes where they do not yet occur is unlikely to be prevented unless public access to these waterways is denied or strictly controlled.