Environmental Problems, Their Causes, and Sustainability 1

Nonnative species are the second greatest threat tobirds. They include bird-eating cats, rats, brown-treesnakes, and mongooses.Birds can also be loved to death. A third of theworld’s 330 parrot species are threatened from a combinationof habitat loss and capture for the pet trade (oftenillegal), especially in Europe and the United States.At least 23 species of seabirds face extinction becausethey are being drowned after becoming hookedon miles of baited lines put out by fishing boats. Millionsof migrating birds are also killed each year whenthey collide with power lines, communications towers,and skyscrapers that we have erected in the middle oftheir migration routes. For example, each year U.S.hunters kill about 121 million birds. But about 1 billionbirds are killed in the U.S. each year by flying intoglass windows.Other threats to birds are oil spills, exposure topesticides, herbicides that destroy their habitats, andswallowing toxic lead shotgun pellets left in wetlandsand lead sinkers left by anglers. Poorly regulated illegalhunting and capture also take a heavy toll.Conservation biologists view this decline of birdspecies as an early warning of the greater loss of biodiversityto come. The reason is that birds are excellentenvironmental indicators because they live in every climateand biome, respond quickly to environmentalchanges in their habitats, and are easy to track andcount.Besides serving as indicator species, birds play importantecological roles. These include helping controlpopulations of rodents and insects (which decimatemany tree species), pollinating a variety of floweringplants, spreading plants throughout their habitats byconsuming and excreting plant seeds, and scavengingdead animals. Conservation biologists urge us to listenmore carefully to what birds are telling us about thestate of the environment.provide more than 98% of the U.S. food supply. Similarly,nonnative tree species are grown in about 85% ofthe world’s tree plantations. Some deliberately introducedspecies have also helped control pests.The problem is that some introduced species haveno natural predators, competitors, parasites, or pathogensto help control their numbers in their new habitats.Such species can reduce or wipe out populationsof many native species and trigger ecological disruptions.Figure 12-9 shows some of the estimated 50,000nonnative species deliberately or accidentally introducedinto the United States that have caused ecologicaland economic harm.After habitat loss and degradation, the deliberateor accidental introduction of nonnative species intoecosystems is the biggest cause of animal and plantextinctions. Nonnative species threaten almost half ofthe more than 1,260 endangered and threatened speciesin the United States and 95% of those in the stateof Hawaii, according to the U.S. Fish and WildlifeService. They are also blamed for about two-thirds offish extinctions in the United States between 1900 and2000. One example of a deliberately introduced plantspecies is the kudzu (“CUD-zoo”) vine, which growsrampant in the southeastern United States (see CaseStudy below).Deliberately introduced animal species have alsocaused ecological and economic damage. An exampleis the estimated 1 million European wild (feral) boars, orhogs (Figure 12-9), found in parts of Florida, Texas, andother states. They breed like rabbits, have razor-sharptusks, compete for food with endangered animals, rootup farm fields, and cause traffic accidents. Game andwildlife officials have had little success in controllingtheir numbers with hunting and trapping and saythere is no way to stop them. Another example is theestimated 30 million feral cats and 41 million outdoor petcats introduced into the United States; they kill about568 million birds per year!12-4 EXTINCTION THREATSFROM NONNATIVE SPECIESWhat Is the Role of Deliberately IntroducedSpecies? Good and Bad NewsMany nonnative species provide us with food,medicine, and other benefits but a few can wipe outsome native species, disrupt ecosystems, and causelarge economic losses.We depend heavily on nonnative organisms for ecosystemservices, food, shelter, medicine, and aestheticenjoyment.According to a 2000 study by ecologist DavidPimentel, introduced species such as corn, wheat, rice,other food crops, cattle, poultry, and other livestockCase Study: Deliberate Introduction of theKudzu Vine: Unintended ConsequencesThe rapidly growing kudzu vine has spread throughoutmuch of the southern United States and is almostimpossible to control.In the 1930s the kudzu vine was imported from Japanand planted in the southeastern United States to helpcontrol soil erosion. It does control erosion. But it is soprolific and difficult to kill that it engulfs hillsides, gardens,trees, abandoned houses and cars, stream banks,Figure 12-9 (facing page) Threats to natural capital: somenonnative species that have been deliberately or accidentallyintroduced into the United States.234 CHAPTER 12 Sustaining Biodiversity: The Species Approach