Niche partitioning of Sericinus montela and Atrophaneura alcinous in small scale riverside habitats

Coexisting butterflies on the same host plant are potentially competitors and are excellent models for studying niche partitioning.To explore the niche partitioning mechanism of Sericinus montela and Atrophaneura alcinous sharing Aristolochia contorta in riverside habitats, we analyzed environmental factors associated with larval distribution through field surveys, and subsequently examined leaf quality, larval preference, and performance using A. contorta leaves grown under contrasting light conditions identified as the primary axis of partitioning in the field surveys.Field surveys revealed that relative light intensity (RLI) was the environmental variable significantly associated with the occurrence of both species in opposing directions: S. montela was found mainly in the lower riverbank with high RLI (odds ratio per %: 1.009, 95% confidence interval: 1.002-1.016),while A. alcinous was found in the upper riverbank with low RLI (odds ratio per %: 0.979, 95% confidence Interval: 0.973-0.986).Sunny leaves were nutritionally superior to shady leaves, and S. montela habitats also had greater leaf number per ramet, indicating that both the quan- tity and quality of food resources were favorable.In the bioassays, A. alcinous tended to prefer shady leaves, and although neither species differed in performance between leaf types, A. alcinous showed higher assimilation rate but lower relative growth rate and total leaf consumption than S. montela.These findings suggest that two specialist herbivorous butterflies sharing a single host plant can partition microhabitat along a small-scale light gradient, and that light environment heterogeneity generated by microtopography and vegetation structure contributes to maintaining niche partitioning between the two species.From a conservation perspective, removal of trees on upper slopes for riverside maintenance may disproportionately affect A. alcinous, which depends on shaded habitats, and the maintenance of small-scale light heterogeneity may therefore be an important consideration for managing conditions conducive to the coexistence of the two species.