Category FAQs Search : Horticulture
Question: Horticulture as a Pathway of Invasive Plant Introductions in the United States
Answer: The majority of plants used in agriculture, forestry, and horticulture in North America are not native to the continent. Most of the plants that have been introduced are not invasive; they carry out their intended purpose and therefore benefit humans in multiple ways, causing no problems. A small portion of introduced plants, however, escape from cultivation and become pests of natural areas. A recent study found that invasive plants, animals, and fungi are second only to habitat loss and degradation in endangering native plant species (Wilcove et al. 1998 ). Fifty-seven percent of the imperiled species studied were negatively affected by nonnative invasive species. The impacts on native species include competition for resources (Melgoza et al. 1990 , Hester and Hobbs 1992 , Mesléard et al. 1993 , Huenneke and Thomson 1994 ), hybridization (Thompson 1991 ), introduced or increased nitrogen fixation in natural areas (Vitousek et al. 1987 ), changed hydrologic cycles (Carman and Brotherson 1982 ), increased sedimentation (Blackburn et al. 1982 ), and increased frequency or intensity of disturbance cycles (Bock and Bock 1992 , D'Antonio and Vitousek 1992 ). A recent estimate put the economic cost of invasive plants in natural areas, agriculture, and gardens at $35 billion per year (Pimentel et al. 1999 ).
The majority of woody invasive plants in the United States were introduced for horticultural purposes—one study found that 82% of 235 woody plant species identified as colonizing outside of cultivation had been used in landscaping (Reichard 1997 ), and an additional 3% were widely distributed for soil erosion control (virtually all of the latter group were also introduced as ornamentals, however). Herbaceous invasive species are less likely to have been introduced for horticultural purposes; instead, many of these species were introduced through crop seed contaminated with weed seed (Baker 1986 , Mack 1991 ) or through seeds in soil brought over from Europe as ship's ballast and dumped at ports to make room for cargo (Baker 1986 ). The problem is certainly not limited to the United States: Between 57% (Kloot 1987 ) and 65% (Groves 1998 ) of the naturalized flora of Australia, both woody and herbaceous species, were intentionally introduced for horticulture.
We define an invasive plant species as one that has or is likely to spread into native flora and managed plant systems, develop self-sustaining populations, and become dominant or disruptive (or both) to those systems. Invasive species comprise both native and nonnative species, but this article focuses primarily on those invasive plants that are not natives of the areas in which they are invasive. Because these species adversely affect land management, we refer to them as weeds, a term with more managerial than biological overtones, and one that is used in many legal contexts.
Given the number of plant species that have been introduced and established outside their native ranges, it may seem probable that every species is already in the United States. Nonetheless, consider, as Eduardo Rapaport (1991) did, the potential for future introductions: Rapaport estimated that there are about 260,000 vascular plants in the world and, of those, perhaps 10% are colonizing species. Thus, there are 26,000 potential weed species. Of those 26,000, he estimated, 10,000 are seriously invasive species, but only 4,000 have been exchanged among regions of the world. Therefore, 6000 species are considered weeds in their native ranges only and another 16,000 have not become established outside their natural ranges, meaning that 22,000 potential weed species have yet to be moved around the world. Even if these estimates are off by as much as 50%, and even if only 10% of the potential weed species turn out to have serious impacts (Williamson and Brown 1986 ), the potential exists for the introduction of over 1,000 new invasive pest plant species.
The prevention of new introductions of pest plant species is a worthy goal, given the potential for harmful impacts, and a goal that is at least partially attainable. Scientists can examine primary pathways of species introduction and identify ways to modify those pathways to prevent problematic plant introductions. If substantial numbers of weed species may still be introduced, as Rapaport suggests, and if those species are introduced as horticultural plants, as in the past, many more invasive species might well be introduced into the United States by horticultural pathways in the future. Moreover, new and existing species will continue to be spread within the United States by horticultural pathways. In this article we discuss how introductions of pest plants occurred, with a view toward what might happen in the future and what steps might reduce the likelihood that species with high potential to become invasive will be introduced and distributed.
(Source: Pustaka Negeri Sarawak, 10 Jun 2021)