Invasive plants: the process within wetland ecosystems - Invasive Plants: Ecological and Agricultural Aspects - page 97

Numerous ecological factors encourage or allow invasive species to suc-
ceed. Introduced species commonly lack appreciable natural competitors, as is
the case with the wetland emergent purple loosestrife ( Lythrum salicaria ).
Often artificially or naturally disturbed habitats create altered environmental
conditions that favor alien species. Invasion of some highly adaptable alien
species can easily out-compete indigenous species. Among wetlands, the water
hyacinth ( Eichhornia crassipes ), hydrilla ( Hydrilla verticillata ), and water let-
tuce ( Pistia straiotes ) possess remarkably efficient physiological and repro-
ductive capacities that facilitate exclusion of competing species [11-15].
Many species have major ecosystem-level impacts. For example, the growth
of the wetland tree Melaleuca quinquenervia can rapidly alter the topography,
hydrology, and water budgets, and these environmental modifications exclude
many native species. The zebra mussel, such as Dreissena polymorpha , and
similar species can decrease biogenic turbidity of shallow waters, greatly
increase habitat for aquatic plant growth, and thereby strengthen the linkages
between pelagic and benthic processes by their intensive filter feeding activities
[16-19]. In spite of many successful invasions, however, most (90-95%) intro-
ductions of non-indigenous species fail [20, 21] because environmental vari-
ability is too great for existing physiological tolerances of the invasive species.
Major economic repercussions occur and societal values have been lost as a
result of species invasions. Aside from wetland crop losses (e.g., rice), invasive
species can alter nutrient cycles, alter fish and other aquatic animal communi-
ty structure, and reduce the recreational and commercial uses of rivers and
lakes [11, 22, 23]. Many physical problems emerge from excessive develop-
ment of invasive species, including increased sedimentation by production and
by trapping of particles, interference of water movements, navigational trans-
port, and recreational uses. Excessive developments of invasive aquatic plants
have often increased habitats conducive to the development of vectors of
human diseases [24]. Among wetland and littoral plants, hundreds of millions
of dollars are devoted annually to aquatic plant control of non-indigenous spe-
cies [4, 5, 25, 26].
Introduction of alien species in wetlands
Discussion here is directed toward the processes that enhance the effectiveness
of invasive success. One observes, as has been often described, that about two-
thirds of all invaders established in natural wetland vegetation occur in small
water-courses, particularly in plant communities of the Bidens and
Convolvulus groups, and in Azolla, Hydrilla, and Elodea mats. Among aquat-
ic plants and wetland species, the primary means by which habitat expansion
occurs is via physical plant fragmentation and clonal vegetative reproduction.
The observed pattern of invasion is first of population expansion, then con-
traction and consolidation of the invasive species, followed by fragmentation
of the population and integration within indigenous biota [1, 27-34]. The