Summary1. Body size differences among coexisting related species are common, but the actual effect of these differences in mitigating interspecific interactions, such as resource competition and reproductive interference, is poorly understood. 2. Local assemblages of the ground beetle genus Carabus (subgenus Ohomopterus) typically consist of two or more species of varying sizes. Through foraging and mating experiments using four Ohomopterus species in parapatry and sympatry, we examined whether interspecific body size differences are effective in partitioning food resources or reducing reproductive interference. 3. Because larval Ohomopterus feed exclusively on earthworms, body size differences may be related to partitioning earthworms of different sizes. However, larvae did not exhibit differences in selectivity or attack success on earthworms of different sizes based on larval body size, indicating little possibility of partitioning food by body size. 4. In contrast, interspecific mating behaviours, such as mate recognition, mounting, and copulation, were hindered when body size differences were large; copulation was frequently accomplished between parapatric species with smaller body size differences. 5. These results suggest that body size differences between species effectively reduce reproductive interference, rather than resource competition. Although body size differences in coexisting closely related species have been considered to function in resource partitioning, they may function primarily in reproductive isolation and thereby facilitate coexistence of species.

20Aim We tested the hypothesis of character release in body size among allopatric 21 populations of the carabid beetle Carabus japonicus by analysing geographical variation 22 in body size in relation to habitat temperature and sympatry/allopatry with the larger 23 congeneric species Carabus dehaanii. 24Location The main and satellite islands of Kyushu in the south-western part of the 25 Japanese archipelago. 26Methods We studied geographical variation in body length and genital size of C. 32Results While body size was positively correlated with annual mean temperature, it was 33 consistently small in sympatry but larger in some allopatric populations in warmer regions. 34The body size differences among populations were heritable. Allopatric C. japonicus 35 populations on satellite islands were derived from the sympatric mainland populations. 36In a few sympatric areas, mitochondrial haplotypes were shared between the species 37 because of introgressive hybridization, suggesting the occurrence of reproductive 38 interference between the species. We also found that genital size was not affected by 39 sympatry/allopatry but was positively correlated with body size. 40Main conclusions We demonstrated that the increased body size of allopatric C.

Diverse populations of invertebrates constitute the food web in detritus layers of a forest floor. Heterogeneity in trophic interactions within such a speciesrich community food web may affect the dynamic properties of biological communities such as stability.To examine the vertical heterogeneity in trophic interactions among invertebrates in litter and humus layers, we studied differences in species composition and variations in carbon and nitrogen stable-isotope ratios (d 13 C and d 15 N) using community-wide metrics of the forest floors of temperate broadleaf forests in Japan. The species composition differed between the two layers, and the invertebrates in the litter layer were generally larger than those in the humus layer, suggesting that these layers harbored separate food webs based on different basal resources. However, the d 13 C of invertebrates, an indicator of differences in the basal resources of community food webs, did not provide evidence for separate food webs between layers even though plant-derived organic matter showed differences in stable-isotope ratios according to decomposition state. The minimum d 15 N of invertebrates also did not differ between layers, suggesting sharing of food by detritivores from the two layers at lower trophic levels. The maximum and range of d 15 N were greater in the humus layer, suggesting more trophic transfers (probably involving microorganisms) than in the litter layer and providing circumstantial evidence for weak trophic interactions between layers at higher trophic levels. Thus, the invertebrate community food web was not clearly compartmentalized between the detrital layers but still showed a conspicuous spatial (vertical) heterogeneity in trophic interactions.Keywords Compartment AE Food web structure AE Microhabitat Electronic supplementary material The online version of this article (

The body size of a univoltine carabid beetle Carabus tosanus on Shikoku Island, Japan, was clearly smaller in higher-altitude populations (subspecies), which possibly represents incipient speciation. To explore the determinants of altitudinal differences in body size in this species, we studied the degree of phenotypic plasticity by conducting rearing experiments at two constant temperatures and examined genetic differences through interpopulation crosses. At 15°C, C. tosanus had a longer developmental period and a shorter adult body than at 20°C. Nevertheless, variation in body size due to temperature effects (phenotypic plasticity) was small compared to the interpopulation differences, which suggests substantial genetic differences between populations (subspecies) at different altitudes. In F 1 offspring from crosses between a low-altitude (subspecies tosanus) and a high-altitude population (subspecies ishizuchianus), adult body length was affected by the genotypes of both parents, with an interaction effect of parental genotype and offspring sex. Further analyses revealed that adult body length was affected by sex-linked factors in addition to autosomal factors. These genetic differences in body size may have resulted from adaptations to different altitudes and may be important for the process of incipient speciation because body size differences could contribute to premating reproductive isolation.

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