Endangered Freshwater Invertebrates deposits (for gravel or gold, for instance) may have devastating effects both at the site of mining, as well as far downstream through sediment transport and far upstream though headcutting of the stream bed Pollution Water pollution is another widespread activity with severe effects of freshwater invertebrates Rivers and lakes often have been used for waste disposal These wastes include sewage and other organic matter, the decomposition of which may reduce concentrations of dissolved oxygen to levels too low to support most species of invertebrates Other wastes include substances (e.g., mercury used in gold mining, acid precipitation from power plants and automobiles) that are directly toxic to freshwater invertebrates Particularly in industrialized regions, long reaches of streams and rivers have been nearly sterilized of invertebrates as a result of severe, chronic pollution (e.g., Figure 6) In addition to pollution caused by deliberate waste disposal, pollution may arise from a wide range of human activities in the watershed Thus, conversion of forests or native grasslands to agricultural fields or cities typically greatly increases loadings of sediments, nutrients, and toxins that are washed in from the altered watershed These ‘‘nonpointsource’’ pollutants are more difficult to track down and control than point loadings of pollutants from factories, yet may have equally serious effects on freshwater ecosystems While pollution has come under partial control in many developed parts of the world, residual pollution from past releases and inadvertent spills still damage the freshwater biota Spectacular recent examples include a large spill of pesticides into the River Rhine following a fire at the Sandoz chemical plant, which killed fish and invertebrates for hundreds of kilometers, and the overturning of a truck that spilled a rubber accelerant into the Clinch River, Virginia, which killed most aquatic animals in a 10-km reach, including hundreds of endangered mussels, the largest ‘‘take’’ of endangered species in the United States since the Endangered Species Act was passed in 1973 Of course, in many less developed parts of the world, water pollution is still poorly controlled Number of living mussel species 25 Direct Harvest Some freshwater invertebrates are harvested for human use, which may contribute to their endangerment Freshwater mussels have been fished for their shells, pearls, and meat since prehistoric times Especially in the nineteenth and twentieth centuries, harvest rates became so high that many populations were locally depleted in Europe and North America For example, over 13 million kg of shells from living unionids were taken from streams and rivers in the state of Illinois in a single year Harvests of Australian crayfish from the wild reached hundreds of tons yearÀ1, and have contributed to the endangerment of some species Other invertebrates are collected for bait or the pet trade, which may contribute to local depletion of populations Invertebrates sometimes are protected by harvest regulations (e.g., closed seasons, size regulations, bag limits), but such regulations may be inadequately conceived and poorly enforced Fortunately, for economic reasons, harvests usually (but not always) concentrate on common species rather than rare ones Nonnative Species Humans often move species outside of their native ranges Such introductions may be deliberate, such as the stocking of trout throughout much of the temperate world, or inadvertent, such as the widespread movement of species in ships’ ballast water Whatever the cause, these nonnative species often have strong ecological impacts, and sometimes are responsible for the endangerment of freshwater invertebrates A spectacular example is the loss of native unionid mussel populations throughout much of northeastern North America as a result of competition with the introduced zebra mussel (Dreissena polymorpha) It is projected that the zebra mussel will be the final blow that will drive several species of unionids into global extinction Nonnative species may also serve as important predators of freshwater invertebrates, as in the case of the brown trout (Salmo trutta), which when introduced to Tasmania apparently preyed on and reduced the range of the unusual and endemic anaspidacean crustacean Anaspides tasmaniae Because the effects of nonnative species tend to be cumulative and difficult to reverse, this is a difficult and growing problem in invertebrate conservation 1878−1933 1978 20 Global Climate Change Pontiac 15 10 181 20 40 60 80 Kilometers above mouth of river 100 Figure Destruction of the freshwater mussel fauna in the Clinton River, Michigan (USA), in the mid-twentieth century by pollution from the city of Pontiac Humans have changed the chemistry of the earth’s atmosphere so much that large changes in global climate are expected in the twenty-first century At this point, it is difficult to make precise predictions about how these changes will affect specific bodies of fresh water In many bodies of water the changes may be varied and large, involving such diverse characteristics as temperature, hydrology, water level, rising sea level, stratification, the nature and severity of disturbances, increases in damaging ultraviolet light, water chemistry, riparian vegetation, and food quality Further, in regions where fresh water becomes scarcer while human demands for water continue to grow, human destruction and degradation of freshwater habitats probably will become more severe Even though we cannot yet specify the details of global climate