The international grain trade is a major pathway for the introduction of alien plants because grain commodities can be contaminated with various weed seeds. To evaluate how alien weed seeds derived from imported grain commodities affect local vegetation in international trading ports, we conducted a vegetation survey on a set of grain landing ports and non-grain landing ports in nine regions throughout Japan to compare the vegetation between these two types of ports. We also surveyed weed seed contamination of wheat imported into Japan, and the contamination rate was calculated for each species based on our survey and previous studies on weed seed contamination. The vegetation clearly differed between the grain landing ports and the non-grain ports. In the grain landing ports, alien species were more abundant than in non-grain landing ports. All indicator species of the grain landing ports were reported to be contaminants in grain commodities. Furthermore, there was a tendency for the more abundant species at the grain landing ports to show higher contamination rates in grain commodities.These results indicate that contaminant seeds spill from imported grain in grain landing ports, and highly contaminated species are likely to become established. We clearly show that weed seed contamination in grain commodities plays an important role in propagule pressure. Gathering information about the prevalence of weeds in grain-exporting countries and monitoring the weed species composition in imported grain commodities is becoming increasingly important for predicting the unintentional introduction of troublesome weeds and identifying effective weed management options.

The international grain trade is a major pathway for the introduction of alien plants because grain commodities can be contaminated with various weed seeds. To evaluate how alien weed seeds derived from imported grain commodities affect local vegetation in international trading ports, we conducted a vegetation survey on a set of grain landing ports and non-grain landing ports in nine regions throughout Japan to compare the vegetation between these two types of ports. We also surveyed weed seed contamination of wheat imported into Japan, and the contamination rate was calculated for each species based on our survey and previous studies on weed seed contamination. The vegetation clearly differed between the grain landing ports and the non-grain ports. In the grain landing ports, alien species were more abundant than in non-grain landing ports. All indicator species of the grain landing ports were reported to be contaminants in grain commodities. Furthermore, there was a tendency for the more abundant species at the grain landing ports to show higher contamination rates in grain commodities. These results indicate that contaminant seeds spill from imported grain in grain landing ports, and highly contaminated species are likely to become established. We clearly show that weed seed contamination in grain commodities plays an important role in propagule pressure. Gathering information about the prevalence of weeds in grain-exporting countries and monitoring the weed species composition in imported grain commodities is becoming increasingly important for predicting the unintentional introduction of troublesome weeds and identifying effective weed management options.