Neither source nor trap: Urban gardens as habitat for nonmigratory monarch butterflies in Northern California

Abstract

Urban ecosystems can act as hotspots for diverse taxa, especially pollinators. However, it is not clear whether urban environments function as high-quality habitat as opposed to demographic sinks or ecological traps. In recent years, a nonmigratory, winter-breeding population of monarch butterflies has established in urban gardens in Northern California, and there are conflicting hypotheses about whether these urban populations are beneficial or detrimental to the larger migratory monarch population. We tested whether the winter-breeding monarch butterfly population was primarily supported by the larger migratory one using monthly surveys of monarchs and milkweeds throughout urban gardens in the California East Bay. If the winter-breeding population were a trap, we expected increases in abundance and decreases in parasite prevalence to be timed with monarch migration into our study area. Demographic patterns of winter-breeding monarchs were not consistent with an influx from the migratory population. Population size was highest during summer months, when milkweed density was most abundant, not during monarch migration. Parasite loads were consistently high but increased during fall migration, in direct opposition to our prediction. During summer, monarch butterfly larva:egg ratios were lower than in other months, possibly due to predation by synanthropic species such as wasps, but predation did not prevent population growth. These demographic patterns contrast with recent studies of monarch butterflies in eastern North America. They also illustrate the importance of understanding mechanistically how species persist in urban environments and the potential of urban communities to function in novel ways as opposed to replicating natural habitat.