Adaptive significance of arboreality: field evidence from a tree-climbing land snail

Arboreality has evolved in a wide range of taxa, but its adaptive significance has rarely been examined in natural ecosystems. Euhadra brandtii sapporo is an arboreal land snail distributed in a restricted area of Hokkaido, Japan. We hypothesized that arboreality provides the species with significant survival advantages, which we tested via field observations and experiments. A monitoring census showed that E. b. sapporo hibernates in winter in the ground litter, climbs into the canopy in early spring and returns to the ground in late autumn. This seasonal movement appears to be effective for escaping from predation by ground-dwelling carabine beetles, whose activity was high during the summer based on a pitfall-trap census. Manipulative field experiments were conducted to compare survival rates in arboreal and ground-dwelling environments. We collected 120 E. b. sapporo individuals in summer and tethered 40 in tree canopies and 80 on the ground; half those on the ground were covered by baskets to prevent predation by large animals. The survival rate after 11 days was highest in the canopy, followed by that on the ground with a basket and was lowest on the ground without a basket. Predation was the main cause of death, but some died from other causes. Similar results were obtained in autumn, except for higher survival rates of the ground treatments. Analyses of carbon and nitrogen isotope ratios suggest that the land snail uses epiphytic lichens and mosses as food resources. In conclusion, arboreality has a marked advantage in reducing mortality in E. b. sapporo and is probably supported by food availability as well.