Humans as Global Plant Dispersers: Getting More Than We Bargained For: Current introductions of species for aesthetic purposes present the largest single challenge for predicting which plant immigrants will become future pests

Extract

Humans have surpassed natural forces as the principal global disperser of vascular plants. Some of the means of dispersal are accidental: Seeds and other plant disseminules and vegetative propagules are transported inadvertently in clothing; cling to or are ingested by our domesticated animals; and are found within and attached to all manner of commerce, particularly as contaminants in seed lots (Muenscher 1955). Plants are also deliberately transported. Almost all human societies have long been dependent on the deliberate transport of plants as a means to satisfy basic human needs (Mack 1999). Many ancient accounts of plant transport are probably apocryphal, such as the importation of incense trees by Queen Hatshepsut to Egypt in 1500 BC from the Land of Punt (Hodge and Erlanson 1956), but there is nevertheless a verifiable fossil record documenting the cultivation of plants far from their native ranges for thousands of years (Godwin 1975).

Our actions as global plant dispersers can be beneficial, neutral, or detrimental. Establishing plants beyond their native ranges has been not only beneficial but also essential to agriculture (Hodge and Erlanson 1956). Few agricultural economies today operate exclusively with native crops, and none of these support an industrialized society. Many plants that humans transport long distances die en route or soon thereafter, unless carefully protected; the consequence of these plants' dispersal is nil. But some of these immigrants prosper in the new range, even without cultivation, and a few of these wreak much environmental and even economic damage (Vitousek et al. 1996). These species are variously termed, depending on whether they form permanent but nonspreading populations (naturalized species) or prolific, permanent populations that usually spread over large new ranges (invaders) (Mack 1997). The admittedly anthropocentric but familiar term, weed, is often applied to species in both these ecologic categories as well as to species that are destructive in their own native range. Probably no definition of weeds is universally agreed upon, but Baker's (1974) definition has at least wide acceptance: species that not only have no detected human value but actually interfere with human activities. We deal here exclusively with those introduced species that meet this definition in a new range.