Pieterse (1990a,b) and Pieterse and Murphy (1990a) discussed the ecology and management of nuisance aquatic plants. Cook (1990) described the origin, autoecology, and spread of most plants that are considered worldwide pests. Of the more than 700 aquatic plants in the world, fewer than 20 are considered major pests or weeds. The term "weed" may be defined as a plant that is undesirable in one area but beneficial in other areas. The negative effects of excessive plant growth can be direct or indirect. Direct harmful effects include impeding the movement of water, hindering navigation, interfering with hydroelectric facilities, increasing sedimentation, decreasing human-food production in aquatic habitats (crops and fisheries), and adversely affecting recreation such as swimming, waterskiing, and fishing. The indirect negative effects include loss of water by evapotranspiration and increase of health hazards by the development of vectors of human diseases such as malaria and schistosomiasis (bilharzia; Pieterse 1990a). Holm et al. (1969) and Holm and Yeo (1980) discussed many of the international problems caused by the failure to control nuisance aquatic plants.
Whereas Klose (1950) estimated that at least 180,000 nonindigenous plant were introduced into the United States prior to 1950, Bates (1964) estimated that 200,000 exotic plants have been brought into the United States in the past few decades. Ripley (1975) estimated that as many as 1,800 exotic plant species have escaped into the nation's ecosystems and that a large portion of these species have become naturalized (Morton 1976; Austin 1978). Many were imported into the United States through Miami, Florida (Austin 1978). Florida has been particularly susceptible to exotic-plant invasions because of its semitropical climate and abundance of aquatic habitats. Florida is also home to a large exotic aquarium and ornamental foliage-plant industry that imports millions of non-native plants each year and cultures native and exotic species for export.
McLane (1969) and Long (1974) described the early history--which started in the late 1920's--of the introduction of nonindigenous aquatic plants into Florida by the aquatic plant industry. During that time, stocking open waters of the state of Florida with nonindigenous plants was common practice. The purpose of most of the stocking of Florida's waterways with nonindigenous plants was to create wild populations for later harvest and sale. Because this activity is now illegal and because the industry realized that additional introductions could lead to more regulations, the rate of new introductions of plants was reduced in recent years (Schmitz et al. 1991).
The first attempts to count the number of plants in Florida was by Small (1933, 1938). He recognized 3,356 seed-plant and 133 fern species. More recent studies revealed that Florida is inhabited by approximately 3,450 species of plants, as many as 925 of which are considered exotic (Ward 1990).
Freshwater ecosystems in Florida were probably the first in the United States that experienced invasions by nonindigenous vegetation (Schmitz et al. 1993). Austin (1978) discussed the early introduction of many of today's worst pest species such as the kudzu-vine (Pueraria thunbergiana), Brazilian peppertree (Schinus terebinthifolius), melaleuca (Melaleuca quinquenervia), Australian pine (Casuarina spp.), sugar-cane (Socchorum officinarum) and oranges (Citrus aurantium and Citrus sinensis) by well-meaning individuals. Among those who promoted the importation and planting of nonindigenous plants was John C. Gifford, a forester at the University of Miami, bank official, nurseryman, and land-development company entrepreneur (Austin 1978), and Dr. Henry Nehrling, a well-known scholar and horticulturist (Nehrling 1944).
More recently, Tarver et al. (1986) compiled an identification manual with descriptions, habitats, and values to wildlife of common native, naturalized, and noxious nonindigenous aquatic and wetland plant species of Florida. Presently, statewide surveys of aquatic plants are conducted every other year to determine the status of nuisance plants and to identify new introductions (Schardt 1980, 1986, 1988; Schardt and Nall 1989). Nineteen nonindigenous plant species were reported in Florida's public waters during 1990 (Schardt and Schmitz 1990). This placed Florida into second or third place behind Hawaii and perhaps California (Westman 1990) in the number of introduced plant species.
Of the 925 exotic terrestrial and aquatic plants in Florida, approximately 33 are considered highly invasive (Exotic Pest Plant Council 1993). The 20 nonindigenous aquatic plant species established in Florida (Table 4) include invasive species such as hydrilla (Hydrilla verticillata), waterlettuce (Pistia stratiotes), alligatorweed (Alternanthera philoxeroides), torpedograss (Panicum repens), and waterhyacinth (Eichhornia crassipes); the latter species has the reputation of being the world's worst aquatic weed (Cook 1990). The remaining plants are nuisances, which cause occasional problems or seem to be innocuous in Florida's navigable rivers and public lakes. Although the 1990 survey (Schardt and Schmitz 1990) dealt only with the natural waters of the state, the additional species are also invasive in ditches or in flood-control canals. The surveys did not include the nonindigenous plants established in drainage systems and in other created water bodies.
Most Florida residents who spend any time near water in Florida are well aware of the state's problems with aquatic nuisance plants. However, many residents of the state are unaware of the magnitude of the Florida aquatic-plant industry. The cumulative 1992-93 database of the Bureau of Aquatic Plant Management, Tallahassee, Florida, now under the Florida Department of Environmental Protection, contained the names and addresses of all known entities in the collection, culture, sale, research, or restoration of aquatic plants and listed 1,051 entries. In 1991, the Aquaculture Program of the Florida Department of Agriculture and Consumer Services, Tallahassee, Florida, produced the Florida Aquatic Plant Locator (Aquaculture Program 1991). The locator is a list of sources of Florida aquatic plants for the aquarium, garden pool, and wetland restoration, for mitigation, and for human consumption. The locator listed 31 wholesalers and 90 retail dealers of aquarium plants; and 67 wholesalers and 62 retail dealers of aquatic plants for garden pools. In addition, wholesalers and retail dealers that specialize in aquatic plants for restoration (91), exporters (8), aquatic plant installers (42), maintenance services (41), and landscape architects who specialize in designing the indoor and outdoor facilities with aquatic plants (8) were listed. Joyce (1990) discussed the potential uses of aquatic nuisance plants for biofertilizers, compost, animal foods, human drugs, human foods, paper and fibre production, ornamentation, waste water treatment, and biogas production. Most uses are presently limited in scope or still under study. Rataj and Horeman (1977) discussed the identification, cultivation, and ecology of the plants that are most frequently used in aquariums.
Plants are considered nonindigenous to Florida if information suggests they were not present in the state before the colonization of the continent by Europeans. Only few exotic aquatic plants are suspected to have been accidentally released in Florida, usually from the discharge of ship ballast (Schardt and Schmitz 1990). These introductions included waterlettuce (Pistia stratiotes), salvinia (Salvinia minima) and alligatorweed (Alternanthera philoxeroides; Schardt and Schmitz 1990).
Horticultural interests have been paramount in the invasion of Florida by exotic plants. Foy et al.(1983) summarized the history and impacts of exotic plants and the introduction of pests on North American aquaculture. Waterhyacinth (Eichhornia crassipes) was introduced because of its showy flower. Brazilian pepper (Schinus terebinthifolius) was intentionally planted throughout southern Florida to display its brilliant red berries and evergreen foliage. The advent of airfreight and the return of military personnel that were stationed across the globe during World War II precipitated a boom in the aquarium industry. McLane (1969) described the relation of the aquatic plant business with the introduction of exotic aquatic plants in Florida and potential problems in the late 1960's. Most plants can quickly be transported to anywhere in the world. Some were subsequently cultivated in public waters of Florida to provide a ready source of plants. Nonindigenous plants in Florida that were introduced by the aquarium trade include limnophila (Limnophila sessiliflora), hygrophila (Hygrophila polysperma), and the state's worst aquatic weed, hydrilla (Hydrilla verticillata). Regardless of the reason for an introduction, none of the plants was screened or investigated for its potential harm to the receiving aquatic ecosystems before its introduction into the open waters of the state (Schardt and Schmitz 1990).
Most plant species were deliberately imported into Florida almost exclusively for agricultural or horticultural purposes. Many emersed exotic plants were spread in the early twentieth century to support agricultural interests. In search for better cattle forage that could withstand the wet climate of southern Florida, grasses such as paragrass (Brachiaria mutica), napier grass (Pennisetum purpureum), and the invasive torpedograss (Panicum repens) were introduced. Melaleuca (Melaleuca quinquenervia) was deliberately planted in the Everglades to dry these wetlands and make them suitable for agriculture (Schardt and Schmitz 1990). Austin (1978) discussed the effects of some nonindigenous plants on southeastern Florida. Williams (1980) reported the number of introduced plants and the increased efforts to stop further introductions. He also pointed out that with even limited studies the undesirability of the introduced plants and the toxicity of some of these plants to humans and other animals could have been determined. The aquatic-plant industry is now sensitive to further unplanned introductions and diligent about containing nonindigenous plants.
Possession of the 24 plant species that are considered pest species is prohibited in Florida (Anonymous 1993). Routine examination of importation and nurseries facilitated the identification and destruction of exotic problem species such as Salvinia molesta, Eichhornia azurea (rooted waterhyacinth), and the male genotype of hydrilla. None of these species has yet become established. The introduction of any of these species into waterways of Florida is prohibited (Schardt and Schmitz 1990).
The state of Florida is considered to have one of the best organized aquatic plant programs (surveys, inspections, control, law enforcement, legislation, and documentation; Steward 1990). As part of this program, the Bureau of Aquatic Plant Management of the Florida Department of Environmental Protection has for 12 years conducted an annual survey of the state waterways to provide early warning of new introductions, determine the most critical control problems, obtain funding for control, and assess the success of control in earlier years.
A plant's invasive capacity is determined by physiological factors such as rapid growth rate, multiple reproductive strategies, and tolerance to a broad range of environmental parameters. The conditions of the site into which a plant is introduced also determines the plant's degree of invasiveness (Westman 1990). If endemic plants are stressed or removed or if natural controls of an nonindigenous plant such as predators, diseases, or other limiting environmental conditions are not present, the nonindigenous plant may have a distinct advantage over endemic species (Schardt and Schmitz 1990).
The nonindigenous plants with the greatest potential for invasion are the floating and submersed species because they can harm nearly all of Florida's shallow waters (Schardt and Schmitz 1990). From 1980 through 1991, more than $98 million were spent for the control of waterhyacinth, waterlettuce, and hydrilla in Florida public waters (Schmitz et al. 1993). State records show that the management of all other plants--native and exotic plants--cost less than 5% of that sum (Schardt and Schmitz 1990).
The advantage of using biological control agents to manage nuisance aquatic plants is significant. "Advantage of using effective, safe organisms to manage aquatic weeks biologically include (1) longevity of the method once it is established; (2) constant feeding activities against the growing weed; (3) low longterm costs; (4) high effectiveness in some plants; and (5) in the case of fish, the potential for conversion of the week to a useful protein product (fish flesh)" (Sutton and Vandiver 1986:2).
Morton (1976) summarized the spread of many ornamental and fruit-bearing plant species in Florida and the environmental and ecological problems it created. She stated (p. 348), "The great increase in our naturalized flora is attributable mainly to the escape from cultivation of trees, shrubs, vines, and other plants deliberately imported as ornamentals or as sources of food, timber, fibre or forage." Schmitz et al. (1991) reported the history and efforts to prevent new introductions of exotic invasive aquatic and wetland plants. Gunderson (1983) described the distribution and effect of nonindigenous woody plants in the Big Cypress National Preserve and the influences of fire, hurricanes, frost, and anthropogenic disturbance on the establishment of such plants.
Chemical or mechanical methods were traditionally used to control nuisance-plant populations. More recently, biological control was developed. Frank and McCoy (1993) listed the nuisance aquatic or semi-aquatic nonindigenous plants that were identified as targets for biological control. They are waterlettuce (Pistia stratiotes L., Arales:Araceae); alligatorweed (Alternanthera philoxeroides (Martius) Grisebach, Caryophyllales:Amaranthaceae); Eurasian watermilfoil (Myriophyllum spicatum L.,Haloragales:Haloragaceae); hydrilla (Hydrilla verticillata (Lf.) Royle, Hydrocharitales:Hydrocharitaceae); waterhyacinth (Eichhornia crassipes (Martius) Solms, Lilliales:Pontederiaceae); melaleuca (Melaleuca quinquenervia (Cavanilles) S.T. Blake, Myrtales:Myrtaceae); and Brazilian Peppertree (Schinus terebinthifolius Raddi, Sapindales:Anacardiaceae).
Biological control agents are also considered for the control of limnophila (Limnophila sessiflora), hygrophila (Hygrophila polysperma), water morning glory (Ipomoea aquatica), and parrotfeather (Myriophyllum aquaticum) (Buckingham and Habeck 1990). The time, resources, and cost of finding, evaluating, and testing a biological control agent and obtaining approval for its release into the field are considerable. Cost estimates for biological controls of the mentioned plants could not be obtained.
The following descriptions of nonindigenous plants of Florida are from Schardt and Schmitz (1990), Schmitz et al. (1991), and Schmitz et al. (1993). Information, especially on plant control agents and on significant effects on the environment and on the economy, was added. The distributional maps of plants (Figs. 2-19) were taken by computer scan from Schardt and Schmitz (1990).
