Agriculture Reference
In-Depth Information
greater impacts on the world's biota than more widely known aspects of glob-
al environmental change such as rising CO
2
concentrations, climate change,
and decreasing stratospheric ozone levels [5, 6]. Insidious effects of invasive
non-native species include displacement or replacement of native plants and
animals, disruptions in nutrient and fire cycles, and changes in the pattern of
plant successions [7].
In recent years, the impact of invasive species on biodiversity has also
become a major concern. These silent invaders constantly encroach into pre-
serves, parks, crop lands, wildlife refuges, and urban spaces. At a global scale,
invasions by non-native plants, animals, fungi, and microbes are believed to be
responsible for greater losses of biological diversity than any other factor
except habitat loss and direct exploitation of organisms by humans [7]. Non-
native species further threaten fully two-thirds of all endangered species. Non-
natives are now considered by some experts to be the second most important
threat to biodiversity, after habitat destruction [7, 8]. Native species have also
been considered invasive when they spread into human-made habitats such as
farms or gardens [9].
According to the US Congressional Office of Technology Assessment, there
are at least 4,500 species of foreign plants and animals that have established
free-living populations in the United States since the beginning of European
colonization. Of that total, at least 675 species (15%) cause severe harm. In
economic terms, 79 species, or 12% of total harmful species, caused docu-
mented losses of $97 billion from 1906 to 1991 [10].
The nature and distribution of invasive species has no geographical bound-
aries. All living organisms - bacteria, fungi, plants and other organisms - have
evolved in specific areas on the Earth. Local climate, geology, soils, available
water and other natural factors may influence plant or organism's invasion and
subsequent establishment in a particular habitat.
Invasive plants are estimated to infest 40.5 million hectares in the United
States. Every year, they spread across three million additional acres, an area
twice the size of Delaware. Everyday, up to 1,862 hectares of additional
Federal public natural areas in the Western continental United States are neg-
atively impacted by invasive plant species [11].
In 1950, the number of plant introductions into the United States was esti-
mated to be at least 180,000 [12]. In 1975, it was estimated that at least 1,800
introduced plant species had escaped into the wild, with a large proportion
establishing free-living populations [13]. Currently, the Weed Science Society
of America recognizes about 2,100 plant species as weeds in the United States
and Canada [14]. 1,365 or 65% of all weeds in the United States are recog-
nized as non-native in origin by the Weed Science Society of America. This
does not include most weeds of natural areas. Some of the important invasive
weed species in the United States are listed in Table 1.
Of the 6,741 plant species that are recognized as weeds somewhere in the
world, only 2,063 species occur in the contiguous United States [15]. More
than 900 non-native plant species have become established in Florida and they
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