A biological invasion occurs when a species introduced deliberately or inadvertently by humans establishes a population far from its native home, maintains itself without human assistance, and spreads beyond the point of introduction (Richardson et al. 2000). Some definitions (e.g., President Clinton’s Executive Order 13112 of 1999) require that the spreading species have a harmful impact, but this is not a part of biologists’ definition. The rare occasions on which a species arrives on its own and spreads in a distant location—such as the African cattle egret reaching the New World—do not qualify as invasions. Although some invasions (e.g., ship rats on Mediterranean islands) occurred thousands of years ago (Ruffino and Vidal 2010), the major surge began with the European discovery and colonization of the New World, which initiated the widespread intercontinental movement of animals, plants, and humans known as the Columbian Exchange. Early explorers and colonists observed European plants in North America by the 17th century, and by the 19th century biogeographers routinely classified species as native, introduced, or of unknown origin (Chew and Hamilton 2010), but few concerned themselves with impacts of introduced species. A remarkable 1958 book for a lay audience by English ecologist Charles Elton, The Ecology of Invasions by Animals and Plants, described many invasion impacts. It is often cited as having founded the modern field of invasion biology (see Elton 2000). In fact, it was ahead of its time and had little effect. Rather, a project in the mid-1980s of the international Scientific Committee on Problems of the Environment engaged hundreds of scientists in an attempt to understand why only certain invasions led to impacts and how to minimize these (Simberloff 2010a). These efforts led to the rapid growth of a distinct science, invasion biology, and today thousands of researchers annually publish hundreds of papers on invasions. Invasions are idiosyncratic, and the routes to some impacts are so tortuous that one would never have predicted them.
Africa and the Demographic Consequences of the Columbian Exchange