Biology Reference
In-Depth Information
the actual species that have been introduced and their impact on agriculture and
other aspects of the US managed and unmanaged natural systems are not fully
understood. In this article, an assessment of the invasive plants that have been intro-
duced, and their control and damage costs will be estimated.
1.2
Native and Introduced Plants
Most alien plants introduced and established in the US were introduced for food,
fiber, and ornamental purposes. An estimated 5,000 species of plants have been
introduced and are present in the natural or wild ecosystem (Morse et al. 1995;
Audubon 2007). In addition, there are an estimated 17,000 species of native plants
in US (Morin 1995). Florida has the largest number of alien plants that have been
introduced. Most of these 25,000 introduced species of plants were introduced for
ornamental and agricultural purposes (Florida Native Plant Society 2005). An esti-
mated 900 species have escaped and have become established in the natural ecosys-
tems (Refuge Net 2007). Also in California, about 3,000 species of plants have
been introduced and have become established in unmanaged natural ecosystems
(Dowell and Krass 1992; Pimentel et al. 2007).
Some of the nonindigenous plant species that have become established in the
US have displaced several native species of plants (Pimentel et al. 2007). Some
of these plant species are serious weed species and have invaded an estimated
700,000 ha of US wildlife habitat each year (Babbitt 1998). For example, the
European purple loosestrife (
Lythrum salicaria
) that was introduced in the early
nineteenth century as an ornamental plant has been spreading at a rate of
100,000 ha per year. Purple loosestrife is changing the wetland ecosytems that it
has invaded (Costly Invaders 2006). The invading plant has reduced the biomass
of 44 native plant species in various habitats and reduced the numbers of some
animals, including the bog turtle (Glyptemys muhlenbergii) and several duck
species (Costly Invaders 2006).
The invading purple loosestrife now exists in 48 states, and the annual control
costs are estimated to be $45 million per year (Aquatic Invasives 2007). In addition,
several species of biological control insects have been introduced and are providing
partial control of purple loosestrife in the Northeast and Mid-west (University of
Illinois 2007).
Several other species of introduced plants are having an impact on natural fed-
eral lands (Christen 2007). In the Great Smoky Mountains National Park, for exam-
ple, 400 of the estimated 1,500 vascular plant species are exotic, about 26% of the
flora, and ten of these are currently displacing and threatening several native plant
species (Pimentel et al. 2007). The problem of introduced plants is especially
significant in Hawaii, where 946 of the 2,690 plant species are nonindigenous,
about 35% of the flora (Eldredge and Miller 1997). Moreover, Hawaii is particu-
larly vulnerable because it is an island.
