A study recently published in the Enviroweeds List Server found that 94% of plant species invading North America had previously been cultivated.
Rod Randall, a weed scientist from Western Australia, stated that human dispersal, through love of plants, was more effective at dispersing plants around the globe than those same species that are accidentally dispersed as contaminants. Rod stated that not all the 27,000 species introduced into Australia became weeds but with such numbers involved, we certainly helped maximise the opportunities for a great many species. We also helped disperse them over huge distances.
The evidence that the cultivation of plants by humans plays a major role in weed invasion, is further illustrated by the fact that only 6% of the plants analysed in the American study had never been cultivated during the process of their naturalisation. This confirms that the vast majority of invasive plant species had previously been associated with humans in one way or other. The main point in the paper is that humans and their desires are one of the most significant drivers of environmental weed introductions.