Abstract
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 the local flora in international trading ports, we conducted a floristic survey at each of the 10 grain landing ports and non-grain landing ports throughout Japan to compare the flora 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 flora 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. There was a tendency for the more abundant species at the grain landing ports to show higher contamination levels in grain commodities. These results indicate that contaminant seeds spill from imported grain in grain landing ports and the most common contaminant 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.
Highlights
An expansion of international trade has promoted the introduction and spread of alien species beyond their native range
A total of 238 alien species and 141 native species were recorded in spring, and 220 alien species and 223 native species were recorded in fall
Most of the highly abundant species were first recorded before 1950. These results indicate that weed seed contamination plays a role as a propagule pressure only at the grain landing ports, while a longer residence time promotes the establishment of alien species at both ports
Summary
An expansion of international trade has promoted the introduction and spread of alien species beyond their native range. Biological invasion has received considerable attention because invasive alien species often cause biodiversity losses, ecosystem modifications, and deleterious impacts on the economy (Mack et al 2000; Pimentel et al 2001). Potential alien species must successfully go through several stages to become fully invasive (Williamson and Fitter 1996; Blackburn et al 2011). The first stage is introduction: species are brought into new locations beyond the limits of the native range. The second is establishment: species make selfsustaining populations in locations where they are introduced. The third is spread: species expand their distribution in new locations away from the point of introduction
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