Life histories and soil water content preferences of sympatric exotic and native terrestrial isopods

Exotic species have negative effects on the ecosystems they enter, but often coexist with native species. Clarifying mechanisms behind the coexistence of native and exotic species is important for evaluating effects of such exotic species on native ecosystems. In Japan, at least seven terrestrial isopod species are considered exotic and some have been found with native species. The mechanisms of their coexistence have never been researched. This study examines the hypothesis that an exotic isopod species, Armadillidium vulgare, and native Mongoloniscus koreanus differ in their life histories and soil water content preferences, and that these differences facilitate their coexistence. Field observation revealed that populations of the two species were stably maintained despite large overlap in their life histories and breeding periods, although A. vulgare showed more variability in some aspects of life history than M. koreanus. Two types of choice experiments were carried out to identify their preferences for soil water content levels. The “alone” experiment, where one individual was used per experimental box, showed that the two species were different in preference for soil water content. However, the “group” experiment, where ten individuals of the two species (five of each species) were used per experimental box, suggested that A. vulgare changed its preference from when it was alone, showing preference similar to that of the M. koreanus individuals in the same box. Thus, we concluded that the two species can coexist at the study site, but that the hypothesized mechanisms for their coexistence were not proven in this study.