Humans as Global Plant Dispersers: Getting More Than We Bargained For

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. These species cause economic losses (e.g., weeds in pastures and crops), adverse effects on human health (such as allergies and toxic reactions), and ecologic losses (reduction of biodiversity and ecosystem services, such as the supply of water ) (Vitousek et al. 1996). This damage has an enormous aggregate cost. For example, each year introduced weeds cost the Australian economy perhaps A$3 billion (ANWS 1997) and the US economy more than $26 billion (Pimentel et al. 2000). In many, perhaps most, regions of the world, the emerging assessment on the origin of weeds comes to two conclusions. First, most weeds in a region are not native but were introduced by humans. Second, the largest single group of these unwelcome intruders was originally introduced deliberately (Panetta 1993). For example, 60% of the more than 600 naturalized taxa listed in Fernald (1950) for the northeastern United States were deliberately introduced; the true percentage is undoubtedly higher because the original use under which many naturalized species were introduced is obscure (Sturtevant 1919). Both conclusions hold major implications for the modes by which a nation’s current weeds might be controlled as well as warnings on the origin of future weed introductions.