Environmental Problems, Their Causes, and Sustainability 1

To sum it up: Many species are overfished, big fish arebecoming scarce, smaller fish are next, and we throw away30% of the fish we catch.How Have Human Activities Affected theSurvival of Aquatic Species? Many Extinctionson the HorizonMarine and aquatic fish are threatened with prematureextinction by human activities more than anyother group of species.Human activities such as overfishing, habitat destructionand degradation, invasions by nonnative species,and pollution are endangering a number of aquaticspecies. According to marine biologists, at least 1,200marine species have become extinct in the past fewhundred years, and many thousands of additional marinespecies could disappear during this century. Indeed,fish are threatened with extinction by human activitiesmore than any other group of species (Figure 12-5, p. 228).Also, according to the UN Food and AgricultureOrganization and the World Wildlife Fund, at least afifth of the world’s 10,000 known freshwater fishspecies (37% in the United States) are threatened withextinction or have already become extinct. Indeed,freshwater animals are disappearing five times fasterthan land animals.The tiny seahorse is vulnerable to global extinctionchiefly because in dried form it is used in traditionalChinese medicine to treat heart disease, asthma,impotence, and a host of other ills.How Have Nonnative Species Affected FishPopulations and Species? Alien InvadersHave Hit the WaterNonnative species are an increasing threat to marineand freshwater biodiversity.Another problem is the deliberate or accidental introductionof hundreds of nonnative species (Figure 12-9,p. 235) into coastal waters, wetlands, and lakesthroughout the world. These bioinvaders can displaceor cause the extinction of native species and disruptecosystem functions, as happened to Lake Victoria(p. 251). Bioinvaders are blamed for about two-thirdsof fish extinctions in the United States between 1900and 2000. Invasive aquatic species cost the UnitedStates an average of about $16 million per hour!Many aquatic invaders arrive in the ballast waterof ships when it is discharged in the waters of ports.One way to reduce this threat is to require ships todischarge their ballast water and replace it with saltwater at sea before entering ports. Other ways are torequire ships to sterilize their ballast water or pumpnitrogen into it (Individuals Matter, p. 257).Let us take a look at two aquatic invader species.The Asian swamp eel has invaded the waterways ofsouth Florida, probably after escaping from a homeaquarium. This rapidly reproducing eel eats almostanything—including many prized fish species—bysucking them in like a vacuum cleaner. It can eludecold weather, drought, fires, and predators (includinghumans with nets) by burrowing into mud banks. It isalso resistant to waterborne poisons because it canbreathe air and can wriggle across dry land to invadenew waterways, ditches, canals, and marshes. Eventuallyit could take over much of the waterways of thesoutheastern United States as far north as ChesapeakeBay. You have to admire a species with such an arrayof survival skills.The purple loosestrife (Figure 12-9, p. 235) is a perennialplant that grows in wetlands in parts of Europe. Inthe early 1880s, it was imported into the United Statesas an ornamental plant. It was also released accidentallyinto U.S. waterways in ballast water contaminatedwith its seeds.A single plant can produce more than 2.5 millionseeds a year. The seeds are spread by water, in mud,and by becoming attached to wildlife, livestock, people,and tire treads.Few native plants can compete with this prolificand highly productive plant. This explains why it hasspread to temperate and boreal wetlands in 35 states(Figure 13-5) and into southeastern Canada.As it spreads, it reduces wetland biodiversity bydisplacing native vegetation and reducing habitat forsome forms of wetland wildlife. Some conservationistscall this plant the “purple plague.”Hopeful news. Some states have recently introducedtwo natural predators of loosestrife from Europe: aweevil species and a leaf-eating beetle. It will take sometime to determine the effectiveness of this biologicalPresentNot presentNo dataFigure 13-5 Natural capital degradation: distribution of purpleloosestrife in wetlands in the lower 48 states. This nonnativespecies from Eurasia was introduced deliberately into theUnited States in the early 1980s and has spread to wetlands in35 states. (U.S. Department of Agriculture)256 CHAPTER 13 Sustaining Aquatic Biodiversity