enormous economic effects (Hutchinson, 1975). Its populations extended widely throughout canals that were major arteries for shipping industries at the time and made their navigation by horse-drawn barges very difficult. The favor was returned when the Eurasian water milfoil, Myriophyllum spicatum, was introduced to North America in the early twentieth century. This species has proliferated throughout shallow water regions of many lakes, limiting their recreational use and leading to the development of extensive and costly aquatic weed control programs. it also has caused substantial reductions in the populations of many native plants. A range of problems has been associated with the human-caused spread of the water hyacinth, Eichhornia crassipes , from South America throughout much of the world (Sculthorpe, 1967). This free-floating plant proliferates quickly and has seriously clogged many waterways and lakes in the southeastern United States. Similar problems have followed the introduction of other floating plant species, such as Pistia stratiotes (water lettuce) and Salvinia auriculata, which also can cause public health problems by providing refuges for disease-bearing mosquitoes. Purple loosestrife, Lythrum salicaria , which was introduced to North America from Eurasia in the early 1800s, has monopolized wetlands and displaced native plant species throughout the United States and Canada (Stuckey, 1980; Thompson et al., 1987). The plant has attractive flowers that, along with their use as a pollen source for honeybees, are probably responsible for the plant's spread. Community changes caused by such nonnative plants have important deleterious effects on many waterfowl and insect species, which lose access to plant species that had provided their forage base; thus, they are the subjects of active limnological research.
An important lesson to be derived from exotic species invasions and introductions is that aquatic communities often are delicately balanced. Shifts caused by invading species can have substantial effects on ecosystem and community structure, as well as on fundamental ecosystem processes, and often these shifts have impacts that are not desirable. For example, Lamarra (1975) showed that common carp, perhaps the most widely distributed nonnative fish in North America, vastly enhances phosphorus return from lake sediments, enhancing eutrophication. Limnologists, along with fisheries biologists, are essential in evaluating such effects and predicting the occurrence and results of future invasions. Addressing the problems caused by exotic species will require a fundamental understanding of a wide range of aquatic ecosystem features, including factors dictating community structures and ecosystem processes that are the basis for much limnological research.
Taken to the extreme, the net impact of the varied forms of human effects on inland aquatic ecosystems can threaten entire species with