Urban gardens and life cycle plasticity of pollinators in the arid West

Changes in growing season length are one of the biggest anthropogenic environmental impacts. Although the effects of warming on spring emergence time are well documented, this is only one way in which anthropogenic changes in seasonality affect insect life cycles. In the arid West, cultivated plant choices and supplemental irrigation substantially increase the seasonal availability of resources for pollinators in urban and suburban gardens. The consequences of this change are well known for monarch butterflies (Danaus plexippus), which transition from seasonally migratory to nonmigratory year-round breeding when host plants are available throughout the year. However, monarch butterflies are not unique. We have documented that yellow-faced bumble bees (Bombus vosnesneskii) show extended periods of queen activity in urban landscapes, and are currently testing effects of extended host and nectar plant availability on voltinism and survival of pipevine swallowtails (Battus philenor). For all three taxa, the impacts of these changes on population viability are complex, involving a mixture of costs and benefits. Overall, these case studies highlight the remarkable plasticity of insect life cycles. They also highlight the ways in which urban landscapes represent novel ecosystems, that have the potential to support pollinator populations but are unlikely to replicate natural systems.