Most species of plants that we see on the North American continent today are not native. There are many causes for the migration of plants across the globe, a majority of which humans have a hand in. The basis of a plant being introduced from its native origin to a new location is that humans or other animals bring them, either intentionally or unknowingly. There are several ecological theories as to how this causes invasive species that overtaken nonative land and out compete the preexisting species. Firstly, when humans intentionally bring a plant across oceans and mountains to a new location, we are performing an informed selection of plants that are likely to become invasive. We pick plants that can survive drought to weather the traveling required to move to a new location, plants that grow faster and heardier, are less susecptible to pathogens, and are likely already well suited for the new environment. These traits all lead to a plant that will be highly competetive in the new community and likely will beat native species. Secondly, because an invasive species can travel from any part of the world to the new location and become rampant, the likelihood that another plant somewhere is more specialized and better at survival than the native plant is high. This can be compared to local marathons that are open to a global population, for example, the Boston Marathon. No one from Boston usually wins, because the likelihood that the BEST person is from the exact town that the marathon is heald in, when the entire world's population is allowed to compete, is low, while the opposite is high.
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