Insect-flower network
In the order-level network structure between flower-visiting insects and 73 flowering species, almost all plant species had linkages with dipteran and/or hymenopteran insects (Fig. 5a). Dipteran insects visited most diverse flowers (H’ = 4.81) and showed the largest linkages with plant species among four orders. Hymenopteran insects also visited many flowers (H’ = 4.45) but the linkages with several plant species, that have strong linkages with dipteran insects, were limited. The diversity of foraging flowers of lepidopteran and coleopteran insects was moderate (H’ = 4.12 and 3.20, respectively). Species compositions of foraging flowers of lepidopteran and coleopteran insects were highly overlapping with those of hymenopteran and dipteran insects (0.88–0.95 in similarity), but the similarity of foraging flowers between lepidopteran and coleopteran insects was moderate (0.69). When the foraging patterns of dipteran insects and hymenopteran insects were compared, the niche overlap was relatively low (0.34). Thus, the flowers targeted by dipteran and hymenopteran insects were considerably different.
In the network structure between three major insect groups and 70 flowering species (Fig. 5b), syrphid flies, non-syrphid flies, and bumble bees linked to 62 (89%), 65 (93%), and 50 plant species (71%), respectively. The values of diversity index for visiting flowers were similarly high (H’ = 4.31–4.54). Similarities of foraging flowers were moderate between bumble bees and non-syrphid flies (0.72) and between bumble bees and syrphid flies (0.65), while the similarity between non-syrphid and syrphid flies was relatively low (0.57). Niche overlap among major insect groups was 0.303, suggesting that different insect groups tend to forage on different plant species irrespective of their wide foraging behavior.
GLM results conducted for the relationship between visitor frequency and the number of flowering species in each term revealed that the number of bee-visited species at the flowering stage was positively related to the abundance of hymenopteran insects (z = 2.39, p = 0.017) (Fig. 6a), whereas the number of fly-visited species at the flowering stage was independent of the abundance of dipteran insects (z = –0.53, p = 0.60) (Fig. 6b).