Pollination is essential for ecosystem functioning, yet our understanding of the empirical consequences of species loss for plant–pollinator interactions remains limited. It is hypothesized that the loss of abundant and generalized (well‐connected) species from a pollination network will have a large effect on the remaining species and their interactions. However, to date, relatively few studies have experimentally removed species from their natural setting to address this hypothesis. We investigated the consequences of losing an abundant, generalist native species from a series of plant–pollinator networks by experimentally removing the flowers of Helianthella quinquenervis (Asteraceae) from half of a series of 10 paired plots (15 m diameter) within a subalpine ecosystem. We then asked how the localized loss of this species influenced patterns of pollinator visitation, floral visitor composition, and interaction network structure. The experimental removal of Helianthella flowers led to an overall decline in plot‐level pollinator visitation rates and shifts in pollinator composition. Species‐level responses to floral removal differed between the two other abundant, co‐flowering plants in our experiment: Potentilla pulcherrima received higher visitation rates, whereas Erigeron speciosus visitation rates did not change. Experimental floral removal altered the structural properties of the localized plant–pollinator networks such that they were more specialized, less nested, and less robust to further species loss. Such changes to interaction network structure were consistently driven more by species turnover than by interaction rewiring. Our findings suggest that the local loss of an abundant, well‐linked, generalist plant can bring about diverse responses within intact pollination networks, including potential competitive and facilitative effects for individual species, changes to network structure that may render them more sensitive to future change, but also numerous changes to interactions that may also suggest flexibility in response to species loss.

Plants have evolved a variety of approaches to attract pollinators, including enriching their nectar with essential nutrients. Because sodium is an essential nutrient for pollinators, and sodium concentration in nectar can vary both within and among species, we explored whether experimentally enriching floral nectar with sodium in five plant species would influence pollinator visitation and diversity. We found that the number of visits by pollinators increased on plants with sodium-enriched nectar, regardless of plant species, relative to plants receiving control nectar. Similarly, the number of species visiting plants with sodium-enriched nectar was twice that of controls. Our findings suggest that sodium in floral nectar may play an important but unappreciated role in the ecology and evolution of plant–pollinator mutualisms.

Urbanization is rapidly growing worldwide, yet we still do not fully understand how it affects many organisms. This may be especially true for wild bees that require specific nesting and floral resources and have been threatened by habitat loss. Our study explores the response of wild bee communities to an urbanization gradient in the Chicagoland region of Illinois. Specifically, we explored how both landscape scale impervious surface and local floral diversity across an urbanization gradient influenced 1) the composition of local bee communities, 2) the richness of native and non-native bees, and 3) the composition of bee functional traits. Over the course of our study, we documented 2,331 bees belonging to 83 different species, 13 of which were not native to North America. We found that impervious surface influenced the overall composition of bee communities. In particular, highly urban areas were composed of more non-native bee species and fewer native bee species. Additionally, bee richness and native bee richness responded positively to floral resources. Bee functional trait responses were variable, with floral diverse sites supporting greater richness of ground nesting, eusocial, and generalist bees regardless of landscape-level impervious surface. Importantly, our study provides evidence that urban areas can support diverse bee communities, but urban and suburban bee communities do differ in composition. Thus, bee conservation efforts in urban areas should focus on creating floral diverse habitats to help support more bee species, specifically native bee species, while also considering which bees are best supported by these conservation efforts.