A horizon scanning approach to rapidly detect alien fern species through horticultural trade

Horticultural trade is a well-documented pathway of introduction for numerous invasive species globally, including ferns. In this study, we analysed trade in terrestrial true ferns (Polypodiophyta) in six anglophone countries: Canada (CA), the United States of America (US), Great Britain and Ireland (GB, for ease of reading), South Africa (ZA), Australia (AU), and New Zealand (NZ). The study provides an overview of fern trade and explored the relationship between trade and alien fern introductions with a view to better inform management interventions. Using a horizon scanning approach in consulting horticultural catalogues, we identified a total of 382 fern species currently traded by 148 traders in a period of just six months. International trade was observed in only three countries with most trade occurring at national scales and e-commerce was not the dominant mode of trade noted in this study, with a relatively higher proportion of species traded on-ground. Alien species accounted for more than 60 % of the total number of traded species in most countries except in AU and NZ, and a surprising number of species (11-14 species per country) known to be naturalised or invasive in their country of trade remain actively traded, with fewer species in CA (2) and AU (5). A total of 194 species noted in trade have not previously been recorded as alien in plant species inventories and did not have an invasion status assigned in their countries of trade. We identified 62 species of concern (i.e., potential future invaders) with Dryopteris erythrosora, Anisocampium niponicum, Polystichum polyblepharum, Austroblechnum penna-marina subsp. penna-marina, Asplenium nidus, Dicksonia antarctica, Polypodium vulgare, and Adiantum raddianum indicated as priority species for regulation in trade due to their high market presence. Citizen science records were noted for very few species of concern with only two records indicating the occurrence of two species in natural or semi-natural areas. This research constitutes one of few studies that have applied a horizon scanning approach using horticultural catalogues to identify alien species, and highlights the efficiency of this approach as a tool for the early detection of potentially invasive species.