Relative resource abundance explains butterfly biodiversity in island communities Naoaki Yamamoto, Jun Yokoyama, and Masakado Kawata* Department of Ecology and Evolutionary Biology, Graduate School of Life Sciences, Tohoku University, Aoba-ku Sendai 980-8578, Japan Edited by James H. Brown, University of New Mexico, Albuquerque, NM, and approved May 1, 2007 (received for review February 20, 2007) Ecologists have long been intrigued by the factors that control the diversity of species is determined by interspecific competition pattern of biodiversity, i.e., the distribution and abundance of and the diversity of resources. Many studies have demonstrated species. Previous studies have demonstrated that coexisting spe- that coexisting species partition their resources (15–17), which cies partition their resources and/or that the compositional simi- lends support to the niche-assembly model. However, recent larity between communities is determined by environmental fac- studies have shown that the neutral theory can explain the tors, lending support to the niche-assembly model. However, no biodiversity of several plant communities (1, 18), although other attempt has been made to test whether the relative amount of studies have refuted the theory (17, 19–21). An appropriate test resources that reflects relative niche space controls relative species that differentiates between the niche and neutral models is to abundance in communities. Here, we demonstrate that the relative assess the different predictions from the competing theories (17, abundance of butterfly species in island communities is signifi- 19). The neutral model predicts that the compositional similarity cantly related to the relative biomasses of their host plants but not between communities will decrease as the distance between two to the geographic distance between communities. In the studied points increases (17). In contrast, the niche-apportionment communities, the biomass of particular host plant species posi- model predicts that the similarity of relative species abundance tively affected the abundance of the butterfly species that used between communities will increase with an increase in the them, and consequently, influenced the relative abundance of the similarity of relative resource abundance. The relative contri- butterfly communities. This indicated that the niche space of bution of distance and resource abundance to the similarity butterflies (i.e., the amount of resources) strongly influences but- should be evaluated (22). terfly biodiversity patterns. We present this field evidence of the Previous studies supporting the niche theory have demon- niche-apportionment model that propose that the relative amount strated that coexisting species partition their resources or that of niche space explains the pattern of the relative abundance of the community composition is related to environmental conditions. species in communities. However, these studies did not evaluate the relative abundance of available resources; consequently, the relationships between neutral theory ͉ niche theory ͉ relative species abundance niche space and species abundance were not directly tested. Thus, to date, there has been no attempt to test whether the iodiversity is often considered to be synonymous with spe- relative abundance of available resources, i.e., available niche Bcies richness and relative species abundance (1). For more space, affects the relative abundance of species. In this study, than half a century, ecologists have paid attention to the factors butterfly communities were examined because both the larval that determine the relative species abundance in ecological and adult stages of butterflies depend almost entirely on specific communities; i.e., the commonness and rarity of species (2). Host plants for their dietary requirements, and information as to plants and their herbivore communities are good systems for which host plant a butterfly uses is generally available. In examining how the diversity of herbivores is influenced by their addition, we could estimate the host plant abundance using a resources; indeed, insect ecologists have focused considerable high-resolution aerial photograph and field surveys. Therefore, attention on the question of how host plant communities affect using the host plant–butterfly system, we were able to directly the species richness and composition of insects. Although a test the relationship between fundamental niche space and recent review (3) pointed out that little progress on this subject relative species abundance. has been made since the 1980s, a number of important studies have demonstrated that a large number of plant species is Results frequently correlated with a large number of insect species We examined butterfly species diversity on five of the Urato (4–6). Experimental studies have also demonstrated a positive Islands, Japan (Fig. 1). Throughout the sampling period, a total relationship between the diversity of plant species and the of 39 species were identified and used in the study (see Materials diversity of consumers (7–9). The abundance of resources also and Methods: Mahanashi, 30; Hoh, 25; Katsura, 31; Nono, 34; represents an important factor in the structuring of insect Sabusawa, 35); these butterflies potentially use 58 host plant communities (10). A number of studies have demonstrated that species [supporting information (SI) Fig. 3]. Of the butterfly resource abundance explains the variation in the abundance and species, nine are restricted to the use of a single host plant species, species richness of herbivorous insects (11–14). However, these and, for eight species, the host plants are used by no other studies on host plant and herbivore communities have treated species richness and species composition as components of the diversity, and no studies have examined the relative abundance Author contributions: N.Y. and M.K. designed research; N.Y., J.Y., and M.K. performed patterns of both host plants and herbivores. research; N.Y., J.Y., and M.K. analyzed data; and M.K. wrote the paper. On the other hand, recent theoretical arguments have focused The authors declare no conflict of interest. on whether the neutral or niche theory better explains the This article is a PNAS Direct Submission. patterns of relative species abundance. The neutral theory (1) of Freely available online through the PNAS open access option. biodiversity based on a dispersal-assembly perspective assumes Abbreviation: AIC, Akaike information criterion. that the relative abundance of species in communities is deter- *To whom correspondence should be addressed. E-mail: [email protected]. mined by random dispersal and stochastic local extinction. This article contains supporting information online at www.pnas.org/cgi/content/full/ Conversely, the niche-assembly view proposes that coexisting 0701583104/DC1. species should have different niches so that the abundance and © 2007 by The National Academy of Sciences of the USA 10524–10529 ͉ PNAS ͉ June 19, 2007 ͉ vol. 104 ͉ no. 25 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0701583104 Downloaded by guest on September 27, 2021 2 km 1 2 3 4 5 6 7 8 9 10 11 12 Matsushima Bay Hoh Urato Islands Pacific Ocean Nono Mahanashi Katsura 2 km Sabusawa Fig. 1. Maps of the Urato Islands and vegetation classification. Eight communities were considered when the Katsura and Sabusawa Islands were divided into two and three areas, respectively, as shown by the black lines. 1, residential areas, roads, and other man-made structures; 2, rice fields; 3, other crop fields; 4, grasslands with vegetation Ͻ50-cm high; 5, grasslands dominated by Miscanthus sinensis and other tall grasses; 6, wetland vegetation mainly dominated by Phragmites communis; 7, bamboo or dwarf bamboo thickets; 8, Cryptomeria japonica plantations; 9, Pinus densiflora (P. thunbergii in part) forests; 10, Machilus thunbergii forests; 11, deciduous forests mainly dominated by Quercus serrata; 12, mixed secondary forest dominated by Juglans mandshurica var. sachalinensis, Celtis sinensis var. japonica, and Neolitsea sericea. butterfly species. In this study, plant biomass was estimated as source, but there was no significant correlation between the the weight of the edible parts of the plants. An analysis of the matrices of Resource and Distance (r ϭ 0.3945, P ϭ 0.103). The diversity and abundance of host plants and butterfly species was multiple regression of the Resource and Distance matrices on conducted for eight communities, of which two communities Butterfly matrix indicated that the relative abundance pattern of were classified on Katsura Island and three were classified on butterflies was highly significantly influenced by the relative Sabusawa Island (the analysis of five communities, where each abundance pattern of their host plants, but not by geographic community corresponded to a single island, was also conducted, distance (Table 1 and Fig. 2). It is possible that communities and the results are provided in SI Table 3 and SI Fig. 5). This within islands were similar in the relative abundance of butter- community division was based on the observation that Katsura flies. When within- and between-islands factors were used as Island and Sabusawa Island support different vegetation types independent variables instead of the Distance matrix for the (Fig. 1; see also Materials and Methods). multiple regression, the Butterfly matrix was not significantly The number of butterfly species did not differ greatly among influenced by within-island effects