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

Abstract

The freshwater zooplankton of New Zealand-Aotearoa are species-poor with only two indigenous Daphnia species: D. thomsoni, and D. tewaipounamu which was discovered in a few subalpine lakes in 2016. Over the last two decades two species of invasive non-indigenous Daphnia, D. pulicaria/pulex and D. galeata, have become well established in the plankton of many lakes in New Zealand-Aotearoa and their distribution appears to be increasing rapidly. 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. Although canonical variates analysis and boosted regression trees did not identify human access as an important predictor, both invasive Daphnia occurred significantly more often in lakes within 200 m of a road, consistent with our hypothesis that humans and their recreational activities are the most likely vectors and facilitators of the distribution of these Daphnia in New Zealand-Aotearoa. 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.