Climate suitability and human influences combined explain the range expansion of an invasive horticultural plant

Ecological niche models are commonly used to identify regions at risk of species invasions. Relying on climate alone may limit a model's success when additional variables contribute to invasion. While a climate-based model may predict the future spread of an invasive plant, we hypothesized that a model that combined climate with human influences would most successfully explain its present distribution. We used the ecological niche model MaxEnt to test our hypothesis with Japanese honeysuckle (Lonicera japonica), a common invasive horticultural plant in the United States. We first predicted the future range expansion of the species in the United States using a model that was trained on the climate conditions in its native range. We then tested the ability of a climate-based model, which was trained on climate conditions in the invaded range, to predict the current distribution in the United States. Finally, we tested whether including a measure of human influence would improve this model. Our results indicate that, despite L. japonica's 200-year invasion history, it is expected to spread beyond its current US range. Climate and human influence combined explain the current distribution. Modeling the spread of invasive horticultural plants using climate alone risks under-predicting areas with poor climates and high human influence. Therefore, planting invasive horticultural species should be discouraged as even suboptimal climates may result in further range expansion.