Pigs are not 1. _______ 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 2. _______ freely in search of food. 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 3. _______ millions of dollars in crop damage yearly. They also harbor dozens of diseases that 4. _______ both humans and farm animals. Forest patches with wild pigs have been found to have considerably reduced plant and animal 5. _______ . In addition to either eating other animals or their food supply, wild pigs 6. _______ native habitats by rooting up(根除) grasses and rubbing on trees. Their activities may also create opportunities for invasive plants to colonize these areas. Wild pigs will eat almost anything 7. _______ calories. Mice, deer, birds, snakes and frogs are among their victims. They can also harm other wild species through 8. _______ competition, rather than eating them or shrinking their food supply. On one particular United States island, wild pigs themselves became an attractive food 9. _______ for a species of mainland eagle. The eagles began breeding on the island and also 10. _______ on a species of native fox. The foxes were almost wiped out completely.