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A winner in the Anthropocene: changing host plant distribution explains geographic range expansion in the gulf fritillary butterfly

The changing climate is altering the geographic distributions of species around the world with consequences for population dynamics, resulting in winners and losers in the Anthropocene. Agraulis vanillae, the gulf fritillary butterfly, has expanded its range in the past one hundred years in the western United States. We combine time series analysis with species distribution modeling to investigate factors limiting the distribution of A. vanillae and to predict future shifts under warming scenarios. In the western US, where we have time series and geographic data, urban development has a positive influence (the host plant is an ornamental in gardens), being associated with year of colonization. Colonization was also associated to a lesser extent with winter maximum temperatures, while a negative impact of minimum temperatures and precipitation was apparent on population growth rates. Across the country, the butterfly is primarily limited by host availability and positively affected by human presence. Perhaps counter-intuitively for a largely tropical ectotherm, current effects of a warming climate (in the years post-colonization) are either negative (on population growth rate) or indirect, likely mediated through availability of areas that can support the host. Under future climate scenarios, conditions are predicted to become more suitable for Passiflora in many urban areas, which would likely result in further expansion of A. vanillae during the dispersive season. These results illustrate the value of combining time series with spatial modeling to understand and predict shifting geographic ranges in the Anthropocene.