Barrier islands were once covered by maritime forests, but today only a few pockets remain in New Jersey, protected in places like Avalon. On these islands, dry sandy soil, sparse top soil, salt spray, and strong winds keep trees from growing to full potential except in the tertiary dunes, so the forest includes smaller holly, sassafras, oak, black cherry, and red cedar trees along with abundant poison ivy and other vines. These forests are important stopping places for migratory birds and habitat for raccoons, red foxes, squirrels, skunks, snakes, turtles, and toads. Poison ivy thrives there, and thirty three species of birds depend on its berries as an important food source during long cold winters, while higher carbon dioxide levels in the atmosphere make poison ivy more irritating to the skin.