Transportation of nonindigenous species via soil on international aircraft passengers' footwear

The potential for transported soil to harbour and spread nonindigenous species (NIS) is widely recognised and many National Plant Protection Organisations (NPPOs) restrict or prohibit its movement. However, surprisingly few studies have surveyed soil while it is in transit to provide direct support for its role in accidental introductions of NIS. Moreover, there are few border interception records for soil organisms because they are neither easily detected nor routinely isolated and identified. Better data would improve evaluations of risks from soil transported via different pathways, enable targeting of management resources at the riskiest pathways, and support development of new risk management methods. We surveyed organisms present in soil that had been removed from footwear being carried in the baggage of international aircraft passengers arriving in New Zealand and recorded high incidences, counts and diversities of viable bacteria, fungi, nematodes and seeds, as well as several live arthropods. These included taxa that have not been recorded in New Zealand and were therefore almost certainly nonindigenous to this country. In each gram of soil, there was an estimated 52-84% incidence of genera that contain species regulated by New Zealand's NPPO, which suggests many were potentially harmful. Variation in the incidences and counts of soil organisms with sample weight, footwear type and season at the port of departure indicated it may be possible to develop methods for targeting management resources at the riskiest footwear. Comparisons with previously published data supported the hypothesis that survival of soil organisms is greater when they are transported in protected (e.g. in luggage) rather than unprotected environments (e.g. external surfaces of sea containers); this offers opportunities to develop methods for targeting management resources at the most hazardous soil pathways.