As a center for trade and travel between the U.S., the Caribbean, and South America, South Florida is especially vulnerable to invasive species, or species of plants and animals that adapt aggressively to conditions in the Everglades, allowing them to reproduce faster and grow larger than they would naturally in their native environments. Approximately 26% of all species of fish, reptiles, birds, and mammals in South Florida are exotic—more than in any other part of the U.S.—and the region hosts one of the highest numbers of exotic plant species in the world.[156] Controlling invasive species in 1,700,000 acres (6,900 km2) of infested land in South Florida costs authorities about $500 million a year.[157]