Effects of urbanization on specialist insect communities of milkweed are mediated by spatial and temporal variation

Abstract

Abstract Urbanization drastically alters landscapes in ways that can threaten local biodiversity. Although species loss has been well documented in urban habitats, why some species persist and not others, and how species interactions change remain poorly understood, especially for species with specialized niche requirements. Here, we test the hypothesis that urbanization disrupts specialized plant–herbivore interactions, and these effects vary according to the characteristics of both cities and the features of the organisms themselves. To test this hypothesis, we surveyed milkweed specialist herbivore communities in the early and late growing season of the common milkweed (Asclepias syriaca) across six cities that varied in size by two orders of magnitude. Four results are most important to answering our research questions: (1) the abundance of herbivore species was higher in urban than in rural habitats, whereas leaf herbivory was higher in rural than in urban habitats, and these effects varied between seasons and among cities; (2) the higher species diversity and abundance are likely due to a few species being more associated with rural habitats and other species more common in urban habitats, and thus, urbanization alters insect community composition; (3) the species with the lowest dispersal abilities, Rhyssomatus lineaticollis and Liriomyza asclepiadis, were those that were most strongly affected by urbanization, with R. lineaticollis less abundant and L. asclepiadis more abundant in urban areas; and (4) urbanization influenced plant–herbivore interactions by reducing herbivory, but it did not alter species interactions among herbivorous insects. Biodiversity loss, particularly declines of habitat and host specialists, in urban areas remains a critical concern for conservation. Our results indicate that at least some specialist herbivores are more abundant in urban areas than in rural areas and that species interactions remain intact.

Keywords

• milkweed herbivore community
• plant–herbivore interactions
• species interactions
• urbanization