The effect of horticultural trade on establishment success in alien terrestrial true ferns (Polypodiophyta)

Wildlife trade imposes direct and indirect negative impacts on the environment but particularly through the continual introduction of species beyond their native range. The high demand for various ornamental plant species at global scale, combined with modern and more efficient modes of trade (i.e., e-commerce) highlight the horticultural trade industry as a point of major conservation concern. Historically, ferns are well documented as being a popular ornamental plant group and recent studies have highlighted their high propensity towards invasion. Despite this, no studies have considered the role of trade as a driver of invasiveness in this large plant group. In this study we developed an inventory of traded terrestrial true ferns (Polypodiophyta) using catalogues from on-ground (i.e., brick and mortar) and e-commerce (i.e., online) nurseries across 6 major trading countries: Australia, Canada, New Zealand, South Africa, the United Kingdom, and the United States of America. A total of 382 traded ferns including alien and native species were identified. Generalised linear models were used to determine which market and species traits influenced the probability of establishment success in the country of trade. The final model highlighted that various market traits positively affected the likelihood of establishment success in alien ferns, i.e., the number of varieties and cultivars available for a species (a proxy for the effects of horticultural manipulation and ecotypic variation), a high market presence, and trade via e-commerce. Species traits such as a broad native range and successful establishment elsewhere were also important predictors. Three families (Dryopteridaceae, Pteridaceae, and Polypodiaceae) were identified as the most popular fern families in trade. These families have also contributed a significant portion of alien and invasive ferns in this and previous studies and are commonly promoted by horticulturalists. Twenty-nine out of 261 traded alien fern species were identified as having successfully invaded regions other than the country of trade as well as having a high market presence, with Dryopteris erythrosora, Dryopteris cycadina, Polystichum polyblepharum, and Cyrtomium falcatum being of particular concern. This study emphasised the role of horticultural trade in invasion success in alien ferns and highlighted the value of early detection through screening horticultural catalogues to identify introduced and potentially invasive species that should be monitored in trade.