Establishment success of alien Daphnia in the ancient Lake Biwa: insights from sedimentary archives

Invasive species pose serious threats to global biodiversity, prompting studies to identify factors underlying invasion and establishment success. However, it's difficult to discern these factors due to the absence of survey data since the initial invasion, because introduced species often remain unnoticed until their population becomes large enough. Here, we investigated the establishment process of the alien zooplankton, Daphnia pulicaria, in the past 30 years using sedimentary archives at Lake Biwa. We performed genetic analysis on mitochondrial DNA sequences of the control region and ND5 gene of ephippia in sediments, as well as present females from water samples. Furthermore, we investigated the relationships between the abundance of D. pulicaria and its major predator fish, Ayu, based on the catch data, as well as its competitor, Daphnia galeata whose abundance was inferred from claw remains. Our analyses showed the studied mitochondrial sequences were identical between all samples. The abundance of D. pulicaria was not correlated with that of a competitor but was negatively with that of a predator. These findings suggest the successful establishment of D. pulicaria was primarily influenced by reduced predation pressure, with a limited impact from competitive interactions and potential hindrance for adaptation caused by loss of genetic variation.