Trinidad and Tobago is a twin island country situated off the northern edge of South America mainland, lying 11 kilometres (6.8 miles) off the coast of Venezuela . Because Trinidad and Tobago lie on the continental shelf of South America, their biological diversity is unlike that of most other Caribbean islands, and has much in common with that of Venezuela. The main ecosystems are: coastal and marine (coral reefs, mangrove swamps, open ocean and seagrass beds); forest; freshwater (rivers and streams); karst; man-made ecosystems (agricultural land, freshwater dams, secondary forest); and savanna.