Increased pollinator activity in urban gardens with more native flora

Abstract

Declining pollinator abundance has become a major global environmental concern. Almost 90% of flowering plants rely on animal pollinators for reproduction, and negative effects of pollinator declines on crop production have been shown. Urbanization is at least partially responsible for pollinator declines, and public programs have been developed to encourage pollinator-friendly gardens. Here, in an observational study, we investigate the relationship between pollinator activity and the proportion of native species in unmanipulated private gardens in an urban area. Pollinator activity in each of ten gardens was recorded at nine times throughout the growing season. Pollinator frequency differed among gardens, and visitation was positively associated with percent area planted with native species, after correcting for effects of time of year, plant density and total garden area. The effect of proportion native plant area on pollinator activity differed among pollinator guilds, and was particularly strong for bumble bees and large bees. The observation of heightened pollinator activity with increasing native area in this correlational study suggests that cultivating native plant species should be encouraged in urban gardens. We discuss that, although such observational studies have the advantage of realism, they cannot determine underlying causal factors driving the observed correlation.