8 Invasive Plants in Wetlands I. Characterization of Invasive Plants Wetlands and other water bodies around the world have been drastically altered by invasive species. Wetlands with strictly native vegetation are increasingly rare (Bazzaz 1986; Meffe and Carroll 1994; Cronk and Fuller 1995; Zedler and Rea 1998). Wetland invasives directly affect humans by obstructing water flow, reducing the recreational value of waters (lower accessibility, decreased fish production, clogged boat motors, increased habitat for hosts of parasitic diseases), and blocking hydroelectric and other installations (Van Zon 1977). Before we can begin our discussion of invasive plants in wetlands, it is necessary to define some of the common terms used in this field. Terms used to describe plants that were historically absent from an area include exotic, non-indigenous, alien, adventive, immi- grant, and non-native (Luken 1994), all of which are roughly synonymous. We have chosen to use the term exotic throughout this chapter. The ‘opposite’ category of plants, (i.e., those that originated in an area) are called native or indigenous. We use the term native in this chapter. Since species are naturally in flux and their distributions shift with time or dis- turbance, their status as native or exotic can be difficult to establish. Evidence from fossil and historical records and results from genetic studies are used to determine the origins of plants. Choosing a date or period after which newly arrived plants are considered exotic is problematic. Should we choose the last glaciation as a cutoff date? The introduction of agriculture? Post-colonial settlement? (Schwartz 1997). In this chapter, the plants we describe as exotic have been established as such by many others before us. The focus of our chapter is invasive plants which may be either native or exotic. Invasive plants grow in profusion and produce a significant change in terms of commu- nity composition or ecosystem processes. They grow in agricultural or natural areas; we are mostly concerned with invasions of natural areas. Many use the term weed as a close synonym for invasive. An example of an exotic invasive is the widespread floating plant, Eichhornia crassipes (water hyacinth), which is native to South America, but exotic in waters throughout most of the tropics and subtropics. Another example is the purple- flowered emergent, Lythrum salicaria (purple loosestrife), which is native to Eurasia but exotic in the U.S. and Canada. Several species of Typha (cattail) are native to North America, but grow as invasives in areas that are disturbed, such as the Florida Everglades. Most of the invasives we describe in this chapter are exotics. While the majority of exotic plants become naturalized (integrated into the native flora without monopolizing space or resources or displacing native plants and animals), about 15% of them become invasive (Office of Technology Assessment 1993). L1372 - Chapter 8 04/19/2001 9:09 AM Page 279 © 2001 by CRC Press LLC Often the same plants that are invasive in part of their range are desirable elsewhere. For example, Heteranthera reniformis (mud plantain) is on the list of endangered plants in Connecticut, but is among the worst invasives of northern Italian rice fields. Trapa natans (water chestnut) is extirpated or endangered in many parts of Europe and an important crop in India, yet it is a noxious invasive in eastern North America, and a serious threat to the sturgeon fisheries in the southern part of the Caspian Sea (Cook 1993). Melaleuca quin- quenervia (melaleuca) has invaded and caused damage to wetland ecosystems in Florida, but in its native range in Australia, it has been nearly eliminated by habitat destruction (Bolton and Greenway 1997; Turner et al. 1998). Phragmites australis (common reed) is declining in parts of Europe and resource managers there are striving to understand its decline in order to restore its range. In North America, on the other hand, P. australis is con- sidered an aggressive invasive and controlling it is vital to the restoration of many eastern salt marshes. Wetland invasives are successful in new ranges for a number of reasons: • Invasives usually spread rapidly by both sexual reproduction and vegetative regeneration. Some have prolific seed production, such as Lythrum salicaria (pur- ple loosestrife), which produces up to 2.7 million seeds per plant (Mal et al. 1992). Many of the most noxious invasives, such as several members of the submerged Hydrocharitaceae (frogbit) family, spread entirely by vegetative regeneration in some habitats because only one sex of the plant is present. The vegetative spread of submerged or floating species is most rapid in the tropics and where water lev- els remain constant. In tropical waters, the floating plants Salvinia minima (water fern) and Eichhornia crassipes (water hyacinth) have been observed to double their areal extent in 3.5 and 13 days, respectively (McCann et al. 1996). Salvinia molesta (salvinia) doubles its area in 7 to 17 days. In Lake Kariba between Zimbabwe and Zambia, S. molesta was first reported in 1959. Its area had expanded to 39,000 ha 13 months later and by 1962 it occupied about 100,000 ha (Cook 1993). • The aquatic environment is relatively uniform, and many species, particularly submerged and floating-leaved plants, are cosmopolitan (widely distributed throughout the world). Several species, such as Ceratophyllum demersum (horn- wort), Echinochloa crus-galli (barnyard grass), Eleocharis dulcis (Chinese water chestnut), Ipomoea aquatica (water spinach), Oryza rufipogon (wild red rice), and Pistia stratiotes (water lettuce), grow in many parts of the world and are consid- ered invasive in some habitats (Ashton and Mitchell 1989; Cook 1993). • Many wetland plants have wide ecological tolerances. As generalists, they are capable of becoming dominant under the right circumstances (Cook 1985, 1993; Thompson et al. 1995; Daehler 1998; Pysek 1998). For example, an invasive loosestrife of Californian vernal pools, Lythrum hyssopifolium, is able to germi- nate in a variety of soil moisture and temperature conditions, making it a suc- cessful generalist among native vernal pool plants which require a specific set of conditions for germination (Bliss and Zedler 1998). • Invasive exotics are usually not susceptible to pests or herbivores in the new habitat. The consumers or diseases that evolved in the same location as the exotic plant do not accompany the plant to its new range (Galatowitsch et al. 1999a). • Invasive exotics encounter little competition from native plants in their new ranges (Lugo 1994). Native plants evolved to exploit separate niches, thereby minimizing competition with other plants of the same habitat. Since exotics’ L1372 - Chapter 8 04/19/2001 9:09 AM Page 280 © 2001 by CRC Press LLC competitor plants are usually not present in the new range, exotics are often without direct competitors. They displace native plants because they tend to grow quickly and monopolize resources and light. In New Zealand lakes, a viable shoot of the submerged genus Lagarosiphon (African elodea) may settle on a mixed native community of 15 to 150 cm height in shallow water (2 m). Long roots grow from the Lagarosiphon shoot to the sediment. Once the plant starts to grow side branches fall and produce more roots and small clumps of Lagarosiphon. The clumps may coalesce and eventually smother the native com- munity (Howard-Williams 1993). • Some invasives are resistant to flooding, fire, and drought (Flack and Benton 1998). The evergreen hardwood, Melaleuca quinquenervia, introduced to Florida in the 1880s, is highly flood-tolerant and fire-resistant and therefore capable of rapidly recolonizing burned wetlands (Ewel 1986). The invasive tree, Tamarix ramosissima (saltcedar) is more drought- and salt-tolerant than native inhabitants of many southwest riparian zones such as Pluchea sreicea, Populus fremontii, Prosopis pubescens, Salix exigua, and S. gooddingii, and it is able to dominate when periods of drought are prolonged (Figure 2.14; Busch and Smith 1995; Cleverly et al. 1997). Plants have spread around the world by natural dispersal mechanisms throughout time. Recently, human transport and land use practices have increased the rate at which species are introduced to new habitats. People introduce wetland plants to new habitats in a number of ways: • People introduce species to new habitats unintentionally. Such transport started centuries ago, and each new development in transportation has created new opportunities for the transport of exotic plants. Seeds travel along roads by hitch- ing rides on vehicles. They are also carried by ships in food stores and ballast water. Three noxious invasives of Florida’s waterways, Pistia stratiotes, Salvinia minima, and Alternanthera philoxeroides (alligatorweed), were probably acciden- tally released through the discharge of ship ballast (McCann et al. 1996). • Some invasive wetland exotics are escapes from agriculture. Examples include Trapa natans (water chestnut) and Eleocharis dulcis (Chinese water chestnut). Both are Eurasian species grown as a food source; they are considered to be inva- sive in some North American waters. Rorippa nasturtium-aquaticum (=Nasturtium officinale; water cress), cultivated for its edible leaves, is an invasive in New Zealand (Howard-Williams 1993). The weeds of ricefields, such as Cyperus squar- rosus, Eleocharis olivacea, Lindernia anagallidea, L. dubia, and Najas gracillima, spread to natural areas when their seeds are included in exported rice (Cook 1985). The North American Acorus calumus (sweet flag), grown for its oil that is used in medicine and perfume, is an invasive in Europe and South America (Cook 1996). Arundo donax (giant reed), used for canes and woodwind reeds and as an erosion control on shorelines, colonizes southwest riparian wetlands of the U.S. Several exotic grasses have been cultivated in the U.S. in the search for bet- ter cattle forage. Brachiaria mutica (paragrass), Panicum repens (torpedograss), and Pennisetum purpureum (napier grass) are adapted to wet soils and have become invasive in wetlands of the southeastern states (McCann et al. 1996). • Some exotics, such as Lythrum salicaria, Butomus umbellatus (flowering rush), Hydrocleys nymphoides (water poppy), and Aponogeton distachyos (Cook 1996), L1372 - Chapter 8 04/19/2001 9:09 AM Page 281 © 2001 by CRC Press LLC have escaped from horticultural uses. The tree Schinus terebinthifolius (Brazilian pepper) was intentionally planted throughout southern Florida for its dense masses of scarlet berries and evergreen foliage. It escaped to natural areas where it displaces native vegetation (Ewel 1986; McCann et al. 1996). • People have transported several submerged and floating-leaved plants, such as species of Cabomba, Egeria, Elodea, Hydrilla, and Vallisneria, throughout the world because they have attractive foliage and are used in aquaria (Cook 1996). Most aquarium plants that have become invasive were deliberately stocked in natural waters to create wild populations to be harvested and sold at a later date (McCann et al. 1996). • Sometimes people intentionally introduce an exotic species in the hope of solv- ing a problem. The Australian tree Melaleuca quinquenervia was brought to the U.S. at the beginning of the 1900s because its high evapotranspiration rate low- ers water levels. It was planted in the Everglades of Florida in an effort to make the area suitable for agriculture. Several species of Casuarina (C. equisetifolia, C. glauca, and C. cunninghamiana; Australian pine) were introduced to Florida before 1920 to form windbreaks along coastal areas and are now widespread in southern Florida (Ewel 1986; McCann et al. 1996; Turner et al. 1998). • Some botanically interesting wetland plants have been transported to new habitats for study or teaching, such as species of Azolla, Salvinia, Lagarosiphon, and Lilaeopsis (Cook 1985). Mimosa pellita (commonly called both catclaw mimosa and giant sensitive plant; formerly M. pigra), an emergent South American plant of river banks, may have been introduced to North America as a botanical curiosity because its leaves fold on touch. Its presence in southern Florida is being closely watched as some believe it may displace native vegetation (McCann et al. 1996). Once a species is introduced, its ability to become established and expand its territory depends on whether it has traits that are pre-adapted to the new habitat. If the new species’ seeds or propagules are easily dispersed and dispersal agents such as waterfowl or humans are plentiful, then the likelihood it will spread throughout a region is enhanced (Chambers et al. 1993). Connections between regions such as ditches and canals, and activ- ities such as increased nutrient loading, vegetation removal, altered hydrology, and changed salinity also increase the probability that invasive species will reach new habitats (Galatowitsch et al. 1999a). II. The Extent of Exotic Invasions in Wetland Communities It is estimated that at least 4000 foreign plant species (not including crop plants) and 2300 animal species have become established in the U.S., as well as hundreds of animal and plant pathogens. About 15% are nuisance species (Office of Technology Assessment 1993) and the effort to eradicate them costs U.S. taxpayers billions of dollars each year (the esti- mated annual cost in 1999 was $123 billion). This cost does not include the incalculable effects invasive plants and animals have on native ecosystems such as local extinction of species that are not of economic value (Simberloff 1996). The success of an exotic species in a new range may reflect the conditions of the com- munity being invaded rather than the aggressive traits of the exotic (Lugo 1994). On islands, for example, exotic invasions are especially dramatic (Vitousek 1994). Exotic species amount to as much as 20% of most continental nations’ flora and fauna, but the proportion of exotics on islands is as much as 50% (Vitousek et al. 1996). Islands tend to L1372 - Chapter 8 04/19/2001 9:09 AM Page 282 © 2001 by CRC Press LLC import more plant species than they export. For example, the islands of New Zealand have received 42 wetland plant species and they have exported only one (Cook 1985). Exotics amount to about 20% of New Zealand’s wetland flora, and many species, such as Ceratophyllum demersum, Lagarosiphon major (African elodea), Elodea canadensis (elodea), and Egeria densa (egeria), cause commercial losses to hydropower stations and threaten recreational waters. Species of the Hydrocharitaceae family dominate in almost every lake they have invaded in New Zealand, in part because New Zealand has no native canopy- forming submerged plants (Howard-Williams 1993). In the U.S., the states most impacted by exotic invasives are Hawaii and Florida. Hawaii is the most remote island group in the world, separated from the continents by 4000 km of ocean. Few plant and animal species colonized the islands prior to human set- tlement and from them, thousands of endemic species evolved. Hawaii’s tropical climate means it is subject to invasion by many species that would be eliminated in areas with frost. In addition, Hawaii is a transportation hub between Asia and North America. Heavy volumes of air and naval traffic increase the chances of exotics reaching the islands. While Florida is not an island, it originally had relatively depauperate plant and ani- mal communities since the waters surrounding most of the state excluded entry from trop- ical regions, and plants that thrived in temperate regions to the north were naturally excluded by the climate. Today its many routes of entry and rapidly growing human pop- ulation have made controlling the entry of exotics nearly impossible. The subtropical cli- mate makes it attractive for the year-round growth of ornamental and aquarium plants and many invasives have entered the state’s natural areas as a result of these horticultural industries (Office of Technology Assessment 1993). Disturbed sites are often susceptible to invasion. Disturbance can lead to opportunistic exploitation by invasive species, especially if they were present in small numbers before the disturbance. A disturbance may significantly alter environmental conditions (for example, making the habitat drier or more nutrient-rich) and invasives may be better suited to exploit them. Natural disturbances such as hurricanes and other storms (which affect whole geographic regions), fires (which affect a region or community), or fish nests and turtle trails (which create a disturbance within a community) can make a site suscep- tible to invasion. Humans cause disturbances to wetlands by altering wetland hydrology, developing wetlands or land adjacent to wetlands, and by releasing nutrients and pollu- tants into the air and water (Rejmanek 1989; Chambers et al. 1993; Vitousek 1994). Some examples of human-caused disturbances that may lead to plant invasions are: • Land use changes open formerly vegetated land and the most rapid colonizers (often with weedy tendencies) are the first to take over the open space. Such changes are seen in deforested watersheds, construction sites, abandoned farm land, drained or stressed wetlands, heavily grazed areas, roadsides, canals, and ditches (Rea and Storrs 1999). • The damming and impoundment of nearly all of the major rivers in the U.S. have led to invasive problems by eliminating variations in the rivers’ hydrology to which native species are adapted. In the southwest, the construction of dams along the Colorado River has lowered groundwater tables, and floods no longer scour river banks. Many western riparian communities have shifted from Populus-Salix (cottonwood–willow) forests to stands of Tamarix (Busch and Smith 1995; Vitousek et al. 1996). • The fragmentation of natural habitats with agricultural and urban development has encouraged the spread of exotics. Weeds from farm fields and plants cultivated L1372 - Chapter 8 04/19/2001 9:09 AM Page 283 © 2001 by CRC Press LLC in cities easily move from human-influenced habitats into natural ones (Vitousek et al. 1996). Ambrosia trifida (great ragweed) is a common weed in agricultural and urban landscapes that is also invasive in dry and wet natural areas. • Freshwater inflows into salt marshes change the plant community structure. In California, plant invasions of tidal wetlands are often associated with storm drains, overflows of agricultural irrigation, and sewage spills, which bring about a decrease in salinity. The exotic grass Polypogon monspeliensis has colonized dis- turbed tidal marshes in southern California because it can outcompete the more salt-tolerant native plants (Kuhn and Zedler 1997). • Climate change caused by increasing CO 2 levels may bring about shifts in the species composition of many communities. For example, California’s vernal pool plant communities may be particularly susceptible to climate change. Increasing temperatures during the rainy season, or changes in the timing of the initial rains or in the occurrence of aseasonal rains that saturate the pools, may all bring about conditions to which native plants are not adapted. Vernal pools have already suf- fered enormous losses from human development and land use changes. Climate change may bring about a shift toward more widespread or exotic species and a further decrease in the species richness of vernal pools (Bliss and Zedler 1998). III. Implications of Invasive Plant Infestations in Wetlands Invasive wetland plants pose a serious threat to wetlands and waterways around the world. Invasive plants can replace desirable plants, displace animals, affect ecosystem functions by altering hydrology and nutrient cycling, and negatively affect humans by impeding waterways and harboring disease vectors (Simberloff 1996). A. Changes in Community Structure When exotics invade a new range, native plants, adapted to the environment, are some- times displaced (Mills et al. 1993; Vitousek et al. 1996). Community changes arise through a variety of processes including interspecific competition and disturbance. A general trend is the loss of plant species diversity as communities shift from desirable plants to mono- specific stands of the invasive species.The mechanisms by which invasives outcompete other species are not always known or quantified, but rapid growth and proliferation cer- tainly play an important role. Several introduced wetland tree species illustrate the capacity of invasives to alter the plant community structure and habitat. The Australian tree Melaleuca quinquenervia spreads rapidly. In the 1990s, it was increasing its range in south Florida by about 35 acres each day, replacing Taxodium distichum (bald cypress) and other native plants, particularly wherever cypress trees grow under stressful conditions (Figure 8.1; Myers 1984; Turner et al. 1998). An aggressive evergreen of Florida, Schinus terebinthifolius, typically moves into areas that have been at least partially drained by people. It forms dense stands that elimi- nate the herbaceous understory. S. terebinthifolius seedling survival is unusually high (66 to 100%) and their success impairs competition by native plants. In addition, S. terebinthi- folius appears to be allelopathic, suppressing the growth of other plants (Ewel 1986; McCann et al. 1996). Rhizophora mangle (red mangrove) was planted on the Hawaiian island of Oahu where it has created dense forests up to 22 m high. Mangrove forests have affected native plants by creating shade, and the nearly impenetrable root system has L1372 - Chapter 8 04/19/2001 9:09 AM Page 284 © 2001 by CRC Press LLC altered the animal community and soil oxygen. In U.S. western riparian zones, Tamarix ramosissima (salt cedar) forms new forests or replaces native ones to the detriment of numerous native plant and bird species (Busch 1992; Busch and Smith 1995). Many exotic emergents are able to outcompete native vegetation by rapidly filling in unvegetated areas and crowding out native plants. A native in tropical Asia, the emergent Colocasia esculenta (taro) grows in dense clumps along lake and river margins in Florida and crowds out native vegetation. Brachiaria mutica displaces native plants through rapid growth, and by producing allelopathic chemicals that inhibit other plants’ growth. The Brazilian emergent Alternanthera philoxeroides has become an aggressive plant in many of Florida’s waters. Its hollow stems, which grow up to 15 m in length, extend over the water’s surface and enable these normally emergent plants to form dense floating mats. The mats reduce submerged native plants’ habitat by shading the water column. Mats of the floating plants Pistia stratiotes, Eichhornia crassipes, and Salvinia species also eliminate submerged vegetation habitat by shading the water column (McCann et al. 1996; Rea and Storrs 1999). Exotic species sometimes form hybrids with native plants, thus creating species that become new invasives and altering the genetic makeup of the community. For example, Spartina alterniflora (cordgrass), a native of eastern U.S. salt marshes, formed a hybrid with S. maritima when it was introduced to French and English marshes in the 1800s. The hybrid S. townsendii was sterile, but a mutation yielded a new species, S. anglica, that has proven to be an aggressive invasive along European coastlines (Beeftink 1977). In New Zealand, the exotic shrub Viburnum opulus (guelder rose) has become naturalized in bogs where it breeds with V. americanum. The resulting hybrid grows more rapidly than the original species (Flack and Benton 1998). FIGURE 8.1 (a) A typical gradient in south Florida from dry pine forest to wet bald cypress forest with an intermediate zone that is not particularly favorable for either community. (b) Replacement of this intermediate zone by Melaleuca quinquenervia (melaleuca). (From Myers, R. 1984. Cypress Swamps. K.C. Ewel and H.T. Odum, Eds. Gainesville. University Presses of Florida. Reprinted with permission.) L1372 - Chapter 8 04/19/2001 9:09 AM Page 285 © 2001 by CRC Press LLC The seed banks of areas infested with invasive plants are also altered. In a study of the seed banks of 21 New Zealand lakes with varying degrees of invasion, deWinton and Clayton (1996) found that seed number and seed species richness were significantly lower at sites where the submerged community was dominated by exotics. The exotics, Elodea canadensis, Egeria densa, and Hydrilla verticillata (hydrilla) formed tall canopies with high biomass solely through vegetative regeneration, since only one sex of these dioecious plants was present. As sediments accumulated under the exotics, the seeds of formerly present native species were buried farther below the sediment surface. Even if control measures successfully eradicated the exotic species, the diminished seed bank would limit the revegetation potential of invaded lakes and wetlands. Invasive plants also have negative impacts on wetland animal communities. Dense stands of submerged exotics, such as Hydrilla verticillata, Egeria densa, and Myriophyllum spicatum (Eurasian watermilfoil), provide refuge for young fish and allow high survival rates, which can lead to overpopulation and stunted fish growth. Because predator fish cannot forage as well in dense weed beds, their numbers and biomass decline as sub- merged plant density increases beyond an optimal level (Nichols 1991). Dense Eichhornia crassipes mats shade benthic communities and inhibit the diffusion of oxygen into the water. Low oxygen concentrations below E. crassipes mats can kill fish and dense mats can completely eliminate fish populations in small lakes (McCann et al. 1996). Waterfowl and other bird habitats are also negatively impacted by the presence of wet- land invasives. The Florida Everglades kite (Rostrhamus sociabilis) is endangered, in part because E. crassipes has invaded much of its habitat. E. crassipes outcompetes emergent vegetation which is the habitat of the kite’s preferred food, the apple snail (Pomacea palu- dosa). The roots of E. crassipes can accumulate heavy metals and toxic organic compounds, which may pose a risk for the endangered West Indian manatee (Trichechus manatus) that consumes the plants. Also in Florida, Casuarina species have reduced the habitat area of cotton rats (Sigmodon hispidus), marsh rabbits (Sylvilagus palustris), gopher turtles (Gopherus polyphemus), loggerhead turtles (Caretta caretta caretta), green sea turtles (Chelonia mydas mydas), and American crocodiles (Crocodylus acutus; McCann et al. 1996). Dense stands of Melaleuca quinquenervia eliminate standing water habitats and create a shift in the local wildlife community from aquatic organisms to upland and arboreal species (O’Hare and Dalrymple 1997). B. Changes in Ecosystem Functions Invasive species can alter the abiotic components of their habitat. For example, floating mats of vegetation reduce dissolved oxygen levels in the water by shading the phyto- plankton and submerged plants that produce oxygen. In addition, detritus accumulation can decrease the dissolved oxygen content of the water due to the oxygen demand created by its decomposition (Howard and Harley 1998). Floating plants can also alter the normal succession of a wetland. Salvinia molesta forms floating mats on which herbaceous plants grow. Eventually woody shrubs and small trees grow there as well. The larger plants have a higher water demand and thereby eliminate open water plants and animals (Cook 1993). Hydrology can also be altered by plants with high evapotranspiration rates. Melaleuca quinquenervia lowers water tables through high evapotranspiration and now infests over 200,000 ha of South Florida, posing one of the greatest threats to the Everglades (U.S. Army Corps of Engineers 1999). Tamarix ramosissima, T. chinensis, and several other saltcedar species were originally introduced to the U.S. as a source of wood, shade, and erosion con- trol, and are now considered nuisance species over nearly 400,000 ha of western riparian L1372 - Chapter 8 04/19/2001 9:09 AM Page 286 © 2001 by CRC Press LLC areas. T. ramosissima transpires water at a greater rate than native plants. It roots deeply and lowers water tables, thus eliminating surface water habitats that are vital in the arid southwest. When rain falls, the tree promotes flooding by blocking water channels with its dense growth (Busch 1992; Busch and Smith 1995; Flack and Benton 1998). Fire regimes may also be altered when exotics take over a habitat. In the riparian areas of southwestern U.S., the Eurasian grass Arundo donax forms tall, dense monospecific stands. In the autumn, the dry leaves and stems can fuel intense fires. A. donax is fire-tol- erant and quickly resprouts from rhizomes. The effect is to transform riparian swamps from flood- to fire-dominated systems where native species cannot survive. Because of the lack of trees and other natives, areas infested with A. donax suffer from increased erosion and reduced habitat value and biological diversity (Flack and Benton 1998). In the Australian tree Melaleuca quinquenervia, fire induces massive seed release which can cre- ate dense stands with up to 250,000 seedlings per hectare. In Florida, M. quinquenervia is able to replace less fire-tolerant native vegetation (Turner et al. 1998). C. Effects on Human Endeavors Wetland weeds are a nuisance to many human activities and their capacity to harbor dis- ease vectors can seriously threaten human life, particularly in tropical countries. Vectors of human and animal diseases, such as malaria, schistosomiasis, and lymphatic filariasis of the brughian type (also called elephantiasis), have long been serious problems in tropical regions. Floating weeds such as Eichhornia crassipes, Pistia stratiotes, and Salvinia auricu- lata exacerbate the situation by expanding the disease vectors’ habitat and inhibiting the movement of their fish predators (Hill et al. 1997). Wetland weeds negatively impact human enterprise in a number of other ways as well (Bos 1997; Hoyer and Canfield 1997; Madsen 1997; Kay and Rice 1999): • Due to their rapid growth, floating and submerged species fill water bodies and clog water intakes and distribution systems used for irrigation, public water sup- plies, and hydroelectric generating plants. If plants block water control gates during floods, there may be damage to crops, buildings, and equipment, and possibly loss of life. • The roots of floating species bind suspended sediments and keep them within reservoirs. When they decompose, sedimentation in flood control reservoirs is increased, thus decreasing their holding capacity. A change in the sediment type (sand, clay, silt, and organic matter) affects plant establishment and growth, invertebrate populations, and fish spawning and feeding. • Floating invasives interfere with aquaculture because they shade submerged plant refuges and the phytoplankton that fish eat. Oxygen levels are decreased beneath floating mats, resulting in fish kills. • Both herbaceous and woody exotics can impede boating access and navigation by blocking boat ramps and boat trails. Floating and submerged plants hinder boat travel by covering or filling entire water bodies. • Piles of live or dead vegetation along residential shorelines, on boat ramps, in swimming areas, and in commercial boating areas create odor problems and can provide a breeding location for mosquitoes and other nuisance organisms. • Recreational activities such as swimming, boating, waterskiing, and sport fish- ing are difficult, if not impossible, in the presence of dense weed infestations. L1372 - Chapter 8 04/19/2001 9:09 AM Page 287 © 2001 by CRC Press LLC IV. The Control of Invasive Plants in Wetlands The control of wetland invasives entails eradicating or reducing the plant’s growth and preventing its spread. Control also includes restoring native species and habitats to pre- vent further invasions (Clinton 1999). The most effective control for exotic species is to eliminate their introduction to new ranges entirely. Keeping exotics out of a new range requires legislation and its enforcement and such laws are not in practice worldwide. In the U.S. the need for legislation regarding aquatic exotics became apparent in the late 1800s when Eichhornia crassipes began to impede river traffic in Florida and Louisiana. The U.S. Congress initiated the Removal of Aquatic Growths Project within the Rivers and Harbors Act of 1899. Since then the project has been renewed several times and today it is funded under the Water Resources Development Act of 1986. The state and federal gov- ernments generally share the cost of controls (U.S. Army Corps of Engineers 1999). The acts passed by the U.S. Congress that have a bearing on wetland plant exotics are the Federal Noxious Weed Act of 1974 and the Non-Indigenous Aquatic Nuisance Prevention and Control Act of 1990. The Federal Noxious Weed Act of 1974 is administered by the Animal and Plant Health Inspection Service of the U.S. Department of Agriculture whose task is to identify actual and potential noxious weeds, prevent their entry into the U.S., and to detect and eradicate infestations in their early stages. The Non-Indigenous Aquatic Nuisance Prevention and Control Act of 1990 authorizes the U.S. Fish and Wildlife Service and the National Oceanic and Atmospheric Administration to regulate introduc- tions of both plant and animal aquatic nuisance species such as the zebra mussel (Dreissena polymorpha; Hoyer and Canfield 1997). Wetland invasives are usually controlled using a combination of methods which reduce the plant’s growth rather than eliminate it entirely. Controlling invasives can bring about negative consequences if dead plants are left in place to decompose because decay- ing vegetation reduces oxygen levels, releases plant nutrients, and deposits large amounts of detritus (Nichols 1991). In addition, control of one plant can lead to the success of another unwanted species (Harris 1988). In some Florida waters, when Eichhornia crassipes was controlled, the exotic species Pistia stratiotes and Alternanthera philoxeroides moved in to exploit the newly opened habitat and caused similar problems (Schmitz et al. 1993; McCann et al. 1996). Wherever the decision is made to manage wetland invasives, plan- ners must set specific goals and adapt them to the local situation (Luken 1997). Invasives are controlled using habitat alteration and mechanical, chemical, and biological controls. A. Habitat Alterations Habitat alterations such as shading the water or sediment surface, dredging the top layer of sediment, or changing the hydrologic regime of the water body can impede plant growth. Because none of these alterations is specific to nuisance species, they are used to control monospecific stands of a nuisance plant or all of the plant growth in an area. 1. Shading the Water’s Surface The growth of submerged plants can be inhibited by decreasing the amount of light in the water column. Water bodies may be shaded by planting trees, shrubs, or tall herbaceous plants along the shoreline. Tall plants at the edge of the water can provide an effective light barrier; however, their shade only reaches a narrow area near the shore, so this method is only effective in streams or small water bodies. Tall shade plants are an attractive solution L1372 - Chapter 8 04/19/2001 9:09 AM Page 288 © 2001 by CRC Press LLC [...]... Crowell.) FIGURE 8. 8 The flowers of Lythrum salicaria are trimorphic, i.e., they exhibit three different levels of anthers (which bear pollen) and stigmas (which receive pollen) The flowers are categorized according to stylar morphs as short-, medium- and long-styled (the style connects the stigma to the ovary) From left to right, the flowers are long-styled, medium-styled, and short-styled Long-styled flowers... well as rare and endangered species L salicaria infestations extirpated Scirpus longii (Long’s bulrush) in Massachusetts and the rare Eleocharis parvula (dwarf spike rush) in New York (Harris 1 988 ) L salicaria eliminates the animal habitat and food that other wetland plants provide and thereby endangers wetland animals such as the bog turtle (Clemmys muhlenbergii; Malecki and Rawinski 1 985 ) Its seeds... LLC L1372 - Chapter 8 04/19/2001 9:09 AM Page 295 TABLE 8. 4 The Advantages and Disadvantages of Herbicide Use to Control Wetland Invasives Advantages Herbicides Are usually easy to apply Usually act rapidly to remove nuisance plants Can be used in a variety of water depths and wetland types Are often less expensive than other control methods Can easily be applied around underwater obstructions and structures... sediment disturbance It is effective in rapidly clearing areas From Howard-Williams 1993; U.S Army Corps of Engineers 1999 © 2001 by CRC Press LLC L1372 - Chapter 8 TABLE 8. 3 The Susceptibility of Selected Wetland Plants to Various Herbicides Emergent and Floating-Leaved Plants Alternanthera philoxeroides (alligatorweed) 2,4-D Dimethylamine (DMS) Diquat Diquat + Complexed Copper Endothal Dipotassium... rapidly expanding M spicatum populations (Painter and McCabe 1 988 ; Creed and Sheldon 1993, 1994; Sheldon and Creed 1995; Creed 19 98) B Hydrilla verticillata (Hydrilla) 1 Biology Hydrilla verticillata, a native of southeast Asia, is a monocotyledon in the Hydrocharitaceae It is a rooted, submerged perennial with leaves from 5 to 15 mm long and 2 to 4 mm wide, arranged in pairs on the lower nodes and in... its rapid spread and the many problems it causes, E crassipes is widely considered the worst aquatic weed in the world (Figure 8. 6; Cook 1993) © 2001 by CRC Press LLC L1372 - Chapter 8 04/19/2001 9:09 AM Page 3 08 FIGURE 8. 6 The world distribution of Eichhornia crassipes (water hyacinth) It is invasive outside of its native range in South America (From Ashton, P.J and Mitchell, D.S 1 989 Biological Invasions... (Martyn 1 985 ) The effectiveness and feasibility of other fungi that cause disease in E crassipes are still under study (Charudattan 1997) © 2001 by CRC Press LLC L1372 - Chapter 8 04/19/2001 9:09 AM Page 310 D Lythrum salicaria (Purple Loosestrife) 1 Biology Lythrum salicaria, of the family Lythraceae, is an emergent herbaceous perennial that grows in freshwater wetlands in temperate areas (Figure 8. 7)... The flowers are both insect- and self-pollinated Seed production depends on age, size, and vigor of the plant A single stem produces 900 to 1000 capsules and the number of seeds in each capsule varies from 80 to 130 The average number of seeds produced per plant is 2.7 million The seeds are dispersed by wind and water and they adhere to aquatic wildlife as well as to people and their vehicles The seeds... perennial and woody plants and they have more selectivity than contact herbicides Dichlobenil, 2,4-D, fluridone, and glyphosate are systemic herbicides (Nichols 1991; Hoyer and Canfield 1997) While herbicides are often effective and easy to use, concerns regarding the environmental safety and human health risks of herbicides and the other potential drawbacks of their use sometimes make planners and managers... in the east and midwest (Stuckey 1 980 ) Today the worst L salicaria infestations are in the midwest and east, with the St Lawrence River watershed and the Great Lakes region particulary affected It is now found in all of the contiguous U.S except Florida and in all of the Canadian provinces (Weeden et al 1996a) © 2001 by CRC Press LLC L1372 - Chapter 8 04/19/2001 9:09 AM Page 312 FIGURE 8. 9 Lythrum . example, the islands of New Zealand have received 42 wetland plant species and they have exported only one (Cook 1 985 ). Exotics amount to about 20% of New Zealand’s wetland flora, and many species,. hydrology, developing wetlands or land adjacent to wetlands, and by releasing nutrients and pollu- tants into the air and water (Rejmanek 1 989 ; Chambers et al. 1993; Vitousek 1994). Some examples of human-caused. flooding, fire, and drought (Flack and Benton 19 98) . The evergreen hardwood, Melaleuca quinquenervia, introduced to Florida in the 188 0s, is highly flood-tolerant and fire-resistant and therefore