The present results demonstrate that the relative abundance of resource biomass has a highly significant effect on the relative abundance of consumer species. The results indicate that the niche space of butterflies (i.e., the amount of their resources) strongly influences the abundance of butterflies and, consequently, butterfly biodiversity patterns. This was also supported by the results that the total biomass densities significantly affect the total biomass densities of all of the butterfly species and that the biomass of certain particular host plant species explains the abundance of the butterfly species that use them. The positive relationships between the total biomass and total butterfly biomass suggest that resource availability actually regulates butterfly abundance. However, butterfly species richness did not increase with increasing butterfly biomass densities. Thus, the increasing total biomass density of butterflies was attributable not to an increase in species richness but to an increase in the densities of certain species that was caused by large increases in the biomass of their host plants. For instance, several species (e.g., Graphium sarpedon nipponum, Papilio machaon hippocrates, Artogeia melete, and Colias erate poliographys) could be found at high densities in one or more communities, and the densities of these species were significantly affected by the biomass densities of their host plants (Table 2). In the present study, the species richness of butterflies was not related to that of their host plants, and this might be because the number of butterfly species did not differ greatly among the communities. Therefore, in the present communities, species richness was not useful for comparing communities; however, the relative abundance of species was an important aspect characterizing the biodiversity of butterfly communities. The relative amount of resources was a major factor that explained the biodiversity pattern of the butterfly communities.
The neutral theory predicts that the compositional similarity between butterfly communities will decrease as the distance between two communities increases because dispersal is a predominant factor determining community composition. Butterflies may be able to disperse to distant locations; however, the distances between the studied communities were not very large. A recent study showed that the similarity between communities begins to decrease at a distance of several hundred kilometers (25). Hence, there may have been an adequate number of migrants between communities regardless of distance. However, at present, there is no available data that demonstrates the relationship between the number of migrants and the distance between the studied communities. The distance effects should be examined to confirm that there might be no distance effects in the present communities. As expected, our studies demonstrated that geographic distance did not have a significant effect on butterfly diversity. In addition to the distance effect, the similarity between communities within islands might be larger than that between communities on different islands if migrations occur more frequently within islands. However, there was no evidence to suggest that communities within islands were more similar than those on different islands. Therefore, the present results do not support the neutral model at least for the studied spatial scale (
In this study, one of the major factors that might have contributed to the significant effect of the relative abundance of host plant species on the relative abundance of butterfly species is the positive relationship between the abundance of each butterfly species and the biomass of its host plant species. For 60% of 39 butterfly species, the biomass densities of the host plants were observed to have positive effects on the densities of the butterfly species that used them, and for 30% of the species, the effects were significant. These positive effects of the host plant biomass found in many (but not all) of the butterfly species indicate a significant relationship between the relative biomass of host plants and relative abundance of butterflies. Two additional factors may have contributed to the results, particularly in the case of those species that did not exhibit high positive relations between butterfly densities and their host plant biomass. First, some species alter their host plant preference depending on the relative abundance of potential host plant species and the abundance of other (competitor) butterfly species that use the same host plants. For instance, weakly negative relations were observed between the densities of Papilio protenor and the host plant Zanthoxylum ailanthoides. This might be because the increasing biomass of Z. ailanthoides increased the densities of the competitors Papilio macilentus and Papilio bianor dehaanii, resulting in Papilio protenor changing its host plants. Second, unknown ecological or environmental factors associated with the relative abundance of host plant species might influence the relative abundance of butterflies. To examine the relative contribution of these factors, additional studies are required. For instance, to confirm the potential for host plant switches by butterflies, the actual resource selection by butterflies should be observed; this would be an interesting topic for future research.
Previous studies on butterflies and moths have demonstrated the relationship between lepidopteran species richness and plant species richness (26, 27). However, the local movements of adult butterflies might be influenced by nectar resources. Indeed, a study on butterfly communities in open successional fields demonstrated that butterfly species richness was explained by flower abundance (28). However, butterflies oviposit on specific host plants and their larvae depend on these plants. Hence, if adult butterflies do not frequently disperse from the natal island or community, the abundance of adult butterflies will be related to that of the juveniles that depend on the host plant abundance on that island. In this study, the locations of the sampling points were selected at random with regard to the locations of host plants and nectar plants. Thus, the estimated densities of butterflies are considered to reflect the butterflies indigenous to each island.
Most studies on plant–herbivore communities have focused on species richness and species composition as representative components of biodiversity; however, few studies have treated the effect and consequences of the relative abundance of species. Although species richness and species composition are significantly correlated to ecosystem functioning (29), differences in relative species abundance can affect ecosystem functioning. For instance, the portfolio effects of ecosystem stability could be considerably influenced by relative species abundance. Even if species richness does not change, large increase in particular species might decrease stability by the portfolio effects (29). In the present study, increasing the biomass of host plants promoted an increase in the densities of certain butterfly species, thus causing a change in relative species abundance. Thus, the present studies indicated that vegetation changes caused by differences in land use would largely affect butterfly community stability by changing the abundance of particular species.
There are several hypotheses explaining the relative abundance of species and diversity [reviewed by Tokeshi (16)]. Previous studies designed to test the neutral and niche theories have examined the relationships between community compositions and environmental gradients, and have provided no direct evidence for the effects of relative resource abundance on relative species abundance patterns. These results are field evidence of the niche-apportionment model that proposes that the relative amount of niche space explains the pattern of relative species abundance at the consumer level. The results clearly demonstrate that butterfly communities depend on available resource abundance. This implies that the conservation of butterfly diversity and that of plant diversity should consider not only the maintenance of species richness but also prevent the prevalence of a few dominant species.