Potential Impacts of Climate Change on Phenology and Fitness in the Endangered Diving Beetle <scp> <i>Laccophilus nakajimai</i> </scp>

ABSTRACT The effects of global warming on insects have been reported, including the expansion of their geographic range into cooler regions, as well as shifts in their range toward these regions. However, species that are unable to cope with climate change by shifting their geographic range, such as those inhabiting alpine zones or islands, may face habitat loss and serious consequences. Laccophilus nakajimai Kamite, Hikida and M. Satô (Coleoptera: Dytiscidae), a small diving beetle endemic to Yonaguni Island, Japan, is listed as “Vulnerable” on the national Red List. The insect occurs throughout the year and appears to be most numerous in early summer. However, little is known about its life history, including its voltinism and diapause characteristics. In this study, in order to clarify the effects of global warming on this species, we reared larvae and pupae of L. nakajimai under five constant temperature conditions—18°C, 22°C, 26°C, 30°C, and 34°C—and examined the relationships between temperature, developmental rate, body size, and survival rate. As a result, all three parameters—developmental rate, body size and survival rate—showed nonlinear relationships with temperature: developmental rate peaked at 30°C, body size was largest at 22°C, and survival rate was highest at 26°C. In addition, while the developmental rate increased above 22°C, newly emerged adults tended to be smaller, suggesting a trade‐off between developmental rate and adult body size. Considering the current breeding season in its natural habitat, ongoing global warming may shorten the development period, potentially altering its phenology. Moreover, a reduction in adult body size—critical for fecundity and predator avoidance—could negatively impact fitness. To develop effective conservation strategies, further investigation into the ecological consequences of these phenological shifts and body size is needed, both through field studies and laboratory experiments.