Interspecific difference and frequency dependence in habitat use of coexisting Ohomopterus ground beetle species under reproductive interference

In reproductive interference (RI), the fitness of individuals or populations is decreased through reproductive interactions with other species. This process results in positive frequency dependence, hindering species coexistence. However, theory predicts that species can coexist under weak RI. Habitat segregation can decrease the opportunity for reproductive interactions between species. Thus, a difference in habitat preference between species may weaken RI and facilitate coexistence. We examined this hypothesis by investigating the habitat uses of closely related Ohomopterus ground beetle species under RI, Carabus insulicola and C. esakii, in limited zones of sympatry at their distributional boundary. The effect of RI may be stronger for females of C. esakii than those of C. insulicola due to asymmetry in the genital size mismatch between the species (C. insulicola males have larger genitalia). Field surveys of local abundances and associations with local environmental parameters revealed contrasting habitat uses between the species. C. insulicola preferred open environments, while C. esakii inhabited forest environments. Interestingly, the habitat use of C. esakii, not C. insulicola, changed depending on the frequency of the other species; the species utilized habitats with a low frequency of C. insulicola. The difference in habitat use and its dependence on the frequency of the other species may facilitate species coexistence by promoting habitat segregation within a continuous landscape. Our findings provide insights into the importance of (plastic) trait differences in species distributions and coexistence under RI.