Even before canaries (特高频噪声) were brought into coal mines to alert workers to the presence of poisonous gas, birds were giving us early warning calls signaling the Earth’s deteriorating environmental health. Global bird populations have shrunk by up to 25% since preagricultural (农业社会前的) times. Over the past 300 years, farmland has expanded from six percent of the Earth’s suce to nearly 33%.
Today, three quarters of threatened bird species depend on forests as their principal habitat; each year, however, some 13,000,000 hectares of forests are destroyed, an area the size of Greece. Nearly half the woodlands lost are relatively undisturbed primary forests that are home to a number of sensitive birds and other creatures.
Direct exploitation, including hunting for food and capture for the pet trade, is the second greatest er after habitat loss, while next is the intentional or accidental introduction of non-native species. As people travel to all parts of the globe, so too do the pests and pets that prey on, out-compete, or alter the habitat of native wildlife.
Pollution poses an additional risk, affecting 12% of the threatened bird species. In addition to direct poisoning from fertilizer and pesticide applications, runoff of chemicals contaminates the wetlands that migrating waterfowl rely on. Persistent organic pollutants accumulate in the food chain and can lead to deformities, reproductive failure, and disease in birds.
Worldwide, one-third of plant and animal species could become extinct by 2050 as a result of climate change, a relatively new threat. Global temperature spikes have brought severe alterations to the migration, breeding, and habitat ranges of some birds.
In addition to these looming ers, seven percent of threatened bird species are at risk from incidental mortality. A rapid decline in seabird populations over the last 15 years corresponds with the growth in commercial longline fisheries. In Europe, Central Asia, and Africa, electrocution on power lines has caused the mass mortality of raptors. Moreover, countless birds die each year from collisions with windows, the number-one cause of U.S. avian mortality.
If birds disappear, so do the economically valuable services they provide. Pring the extinction of additional bird populations depends largely on protecting the world’s remaining wild spaces and preserving the health of our natural and altered ecosystems.
Reports that the ivory-billed woodpecker, long thought to be extinct, is still with us thrilled bird watchers and others, but this sort of second chance seldom occurs in nature. Even with continued habitat protection, once wildlife populations drop dramatically, a rebound is far from guaranteed. Without stabilizing climate and human numbers, putting fences around all the parks in the world will not lie enough to protect threatened species.
Which of the following statement has nothing to do with the drastically declining number of birds