Pigs are not native to North America. They were first introduced to California by Spanish and Russian explorers and settlers many centuries ago. In the early times, pigs were allowed to wander freely in search of food.  16  This practice also allowed many pigs to escape from farms and live in the wild, which became a problem. In fact, they are one of the most damaging invasive species on the continent. Wild pigs caused millions of dollars in crop damage yearly.  17  They also harbored dozens of diseases that threaten both humans and farm animals. Forest patches with wild pigs have been found to have considerably reduced plant and animal diversity. In addition to either eating other animals or their food supply, wild pigs damaged native habitats by rooting up grasses and rubbing on trees. Their activities may also create opportunities for invasive plants to colonize these areas. Wild pigs will eat almost anything containing calories. Mice, deer, birds, snakes and frogs are among their victims. They can also harm other wild species through indirect competition rather than eating them or shrinking their food supply. On one particular United States island, wild pigs themselves became an attractive food source for a species of Mainland Eagle.  18  The eagles began breeding on the island and also feeding on a species of native fox. The foxes were almost wiped out completely.

猪并非是美国的本土物种。多个世纪前，西班牙和俄罗斯的探险家和定居者首次将它们引入加利福尼亚州。早期，猪被允许自由游荡和寻找食物。 16  这种做法也让许多猪逃离农场，在野外生活，而这成为了问题。事实上，作为非洲大陆最具破坏性的入侵物种之一，野猪每年造成数百万美元的农作物损失。 17  它们还携带着数十种威胁人类和农场动物的疾病。有野猪生活的森林斑块被发现植物和动物多样性大幅减少。除了吃其他动物或它们的食物供应，野猪还通过杂交和在树上摩擦来破坏当地的栖息地。它们的活动也可能为入侵植物在这些地区定居创造机会。野猪几乎可以吃任何含热量的食物。老鼠、鹿、鸟、蛇和青蛙都是受害者。它们也可以通过间接竞争而不是吃掉或减少食物供应来伤害其他野生物种。在美国一座小岛上，野猪本身就是一种大陆鹰的诱人食物来源。 18  鹰开始在岛上繁殖，它们也以一种本土狐狸为食。这导致狐狸几乎被彻底消灭。

16.关于北美早期的猪，我们了解到什么？
A)许多（猪）逃离农场，变成野生的（猪）。
B)它们实际上是北美本土的。
C)许多（猪）在野外寻找食物时被杀。
D)它们被西班牙和俄国探险家猎捕。

17.为什么野猪对人类是一种威胁？
A)它们经常对人类发动突然袭击。
B)它们破坏了自然的食物供应链。
C)它们造成了大量的环境污染。
D)它们携带多种疾病。

18.关于美国一个岛上的本土狐狸，本篇短文说了什么？
A)它们与野猪和平共处。
B)它们把食物全吃光了。
C)它们成为了老鹰的食物。
D)它们繁殖的速度很快。