pH as a key factor defining the niche space of the alpine crustacean species Daphnia tanakai

Understanding the mechanisms of adaptation of alpine aquatic organisms to the alpine environment, one of the ecosystems most vulnerable to climate change, would help us predict how those organisms would respond to the upcoming climate change. Here, we examined how one of Japan's endemic crustaceans, Daphnia tanakai, which has been found only in alpine lakes and pools in high mountain moors, is adapted to the alpine environment and what restricts its distribution only to the alpine environment. Lab experiments on UVB tolerance and growth rate under different pH and temperature based on the field survey for pools in mountain moors revealed that D. tanakai was differentiated from two low-altitude species (Daphnia dentifera and Daphnia pulicaria) by its tolerance to low pH such as 4.5, which is equal to the mean pH of pools in the high mountain moors occupied by D. tanakai. The growth rate at pH 4.5 was, however, significantly lower than that at higher pH, suggesting that low pH was not necessarily an optimal condition, and that other factors restrict the distribution to alpine lakes. Vulnerability to invertebrate predators such as Chaoborus was suggested as such a factor by additional experiments on swimming activity and predation and the fact that D. tanakai has only been found in pools without Chaoborus. Because it is known that the pH of alpine lakes is tightly linked with temperature, global warming may diminish the advantage of D. tanakai and allow invasion of alpine lakes by unwelcome predators and competitors from lower altitudes.