In colonizing the world, humans have moved many other organisms with them, intentionally or by accident. Centuries ago, alien species provided humans with food, fiber, shelter, and cultural ties with their homelands. Much effort went into introducing, cultivating, or other wise nurturing a wide range of alien species. The shift from viewing alien species as welcome components of the biota to recognizing many of them as worrying diseases, pests, and weeds has occurred relatively recently. Although by far the biggest impacts of biological invasions have been felt only in the last 50 years, there are some examples of widespread invasions that date back much further. For instance, Charles Darwin wrote of invasive populations of the alien thistle species Silybum marianum and Cynara cardunculus covering many square kilometers in Argentina in 1833. Regions colonized by Europeans received a deluge of alien species at the same time as major human driven transformation of these systems started. Recently, technological innovations have enabled humans to move almost any species around the world much quicker and in bigger numbers than ever before. Major biogeographical barriers that had separated biotas of different parts of the world for millennia are now easily breached, and biological invasions have quickly become widespread and pervasive.
Since ethical issues (limited opportunity for experi ments in the wild due to the danger of introducing damaging alien species) and the historical character of invasion ecology (most invasions are recognized too late to be studied from the start), most insights on invasion ecology have emerged from comparative studies of geographically distant regions and their introduced biotas. Natural experiments, created by the centuries of human driven translocation of species around the globe, are valuable sources of information in invasion ecology.
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