Mustard plants distant from forest fragments receive a lower diversity of flower-visiting insects

Abstract

Mustard fields (Brassica campestris: Brassicaceae) are mass flowering crops attracting a wide diversity of flower-visiting insects. Many studies have shown that the diversity of insects is higher near forest fragments than farther away from the forest edge and that Apis-bees numerically dominate such ecosystems. In this study, we investigated how insect diversity changes with distance from the forest edge (100 m, 1100 m, 2100 m) in mustard crop fields in Nepal. The effects of distance on both the abundance and richness of insects were evaluated using generalized linear mixed models, while Hill numbers were used to describe species diversity. We performed ordination analysis and PERMANOVA to examine the dissimilarity between insect communities at different distances. Finally, percent similarity was used to describe which insect species contributed most to the dissimilarity among distances. Our findings suggest that richness and abundance of the flower-visiting insects differed between distances of 100 m and 2.1 km from forest fragments. We found that values of all diversity measures were higher nearer to the forest fragments and moderately distant from the fragments. Accordingly, the ordination analysis corroborated the GLMMs, showing that insect community composition near to and moderately far from the forest fragments differed from that farther in the field. Additionally, we detected that solitary bee species (Andrena spp., Halictus spp. and Megachile lanata) outnumbered Apis-bees, greatly contributing to community discrimination among distances. Overall, we demonstrated that insect diversity within mustard crops is not homogeneous in Nepal. Cultivated plants farther from forest fragments are facing a deficit of diversity of flower-visiting insects. Furthermore, the role and the nesting habitat of solitary bees sampled here should be investigated, as they may contribute to mustard pollination in Nepal.