The diversification of phenotypes and species has been a major concern in evolutionary biology. Both resource competition and predator–prey interactions have been suggested as mechanisms that drive phenotypic divergence and speciation, but the latter has attracted much less attention than the former. The Karaftohelix land snails in Northeast Asia provide an excellent model to investigate the effects of competition and predation on the divergence process. This is because 1) all species are closely related and sometimes indistinguishable by genital morphologies and nDNA and mtDNA markers; 2) two different anti-predatory strategies are observed for each species in this group; and 3) multiple species have generally overlapping distributions. In this study, we conducted quantitative quadrat surveys and carbon/nitrogen stable isotope analyses to compare the microhabitat and trophic niches among Karaftohelix land snails and tested the effects of negative interspecific interactions as the cause of adaptive radiation in this taxon. All the results indicated that the differences in microhabitats and trophic niches among Karaftohelix land snails—especially between the most closely related sister species, Karaftohelix editha and Karaftohelix gainesi in northern Japan—were too small to promote the adaptive radiation in this group.