178 Endangered Freshwater Invertebrates often are clustered together into small regions and concentrated in certain taxonomic or ecological groups Causes of Small Ranges A primary cause of small ranges in freshwater invertebrates is the limited dispersal abilities of these animals All freshwater habitats are islands in a sea of terrestrial habitats, and are more or less isolated from other similar habitats Although streams are connected together into drainage networks, the streams of one drainage network are isolated from those in other drainage networks The dispersal abilities of freshwater invertebrates, and thus the perceived isolation of freshwater habitats, vary widely For animals like dragonflies, whose longlived aerial adults are strong fliers, or ectoproct bryozoans, whose tough resting stages (‘‘statoblasts’’) are readily dispersed by migratory waterfowl, the separation of fresh waters probably does not present an important barrier to dispersal or gene flow For other animals, such as fragile groundwater crustaceans that are poor swimmers, avoid the light, and lack tough dispersal stages, adjacent streams or aquifers may be nearly as remote as distant continents, and even small barriers may prevent migration and gene flow For instance, the present-day distribution of microparasellid isopods nearly follows the pattern of marine beaches from over 20 million years ago (Figure 3), where these species presumably arose and from which they subsequently apparently have been unable to disperse The isolation of freshwater habitats may produce small ranges in two ways First, endemic species with small ranges may evolve in place following infrequent crossing of dispersal barriers, resulting in a group of more or less closely related species whose ranges are separated by barriers to dispersal Second, a formerly widespread species may be eliminated from most of its former range, for instance by a changing climate or the arrival of a competitor, stranded in small refuges, and be unable subsequently to disperse out of the refuges This second mechanism may become especially important for the freshwater biota if humans cause large changes in regional or global climate, especially because habitat alterations and pollution have eliminated many of the natural dispersal corridors between freshwater habitats Finally, a species may have a small range because it requires an unusual habitat, which is itself rare For example, the thermosbaenacean crustacean Thermosbaena mirabilis was described from ancient Roman warm baths and is known from only a few thermal springs in Tunisia Its small range presumably owes to its unusual habitat requirements as well as its limited dispersal abilities ‘‘Hot Spots’’ of High Endemism Because the processes of speciation, extinction, and dispersal not occur uniformly over the earth, species richness and endemism vary greatly across the world’s fresh waters Some bodies of water contain more than 1000 invertebrate species, many of them unique to that single body of water At the other extreme, some bodies of water support fewer than 100 invertebrate species, all of them widely distributed We might expect sites of high richness and endemism to be habitats of great age, habitats where dispersal is limited, either by geographic isolation or by characteristics of the habitat, or habitats that harbor animal groups that are prone to speciate Thus, many ancient lakes (Baikal in Siberia, Tanganyika, and Malawi in Africa) and river systems (the Tennessee in the United States, the Mekong in southeast Asia) that have not been Figure Distribution of freshwater microparasellid isopods and Oligocene shorelines (24–37 million years ago) Stippled areas were land during the Oligocene and closed circles show places where freshwater microparasellids have been found