Urbanization can cause major changes in ecosystems. Although many studies have shown that urbanization affects the abundance and species richness of plants and animals, few studies have focused on the effects of urbanization on species interactions between plants and animals. To clarify whether urbanization changes species interactions, we investigated species interactions between herbaceous plants and heteropteran insects (Hemiptera) at 20 grassland sites along an urban-rural gradient in the Kinki region of Japan. In July and August of 2020 and July of 2021, we recorded the species and abundance of heteropterans feeding on the reproductive organs of the herbaceous plants that grew at a transect (length, 250 m; width, 1 m) at each site. The number of plant species and reproductive organs was also recorded at each transect. Impervious surface areas surrounding the study sites were quantified by GIS and defined as urban areas. A total of 2045 individuals (nymphs and adults) of 45 heteropteran species were observed feeding on the reproductive organs of 56 plant species. Increasing the amount of urban area surrounding the study sites did not affect the species richness of plants and heteropteran insects. However, heteropteran abundance and the number of plant reproductive organs decreased with an increasing amount of urban area. Also, the number of interaction links between heteropteran and plant species and the network structure (nestedness) decreased with the increasing urban area. The decrease in plant and heteropteran abundance may change the structure of plant–heteropteran interactions in an urban environment.
