A Special Kind of Diversity
Since south Florida was the subtropical extremity of a temperate landmass, its mix of flora and fauna was eclectic as well as abundant. Temperate species from the north, including hawks, raccoons, oaks, bobcats, and white-tailed deer, joined tropical species from the south, including roseate spoonbills that flew in, loggerhead turtles that swam in, tree snails that floated in on branches, mahoganies whose seeds blew in during storms, and cocoplums whose seeds were dropped in by birds. Gators came down from the north; crocs came up from the south. They all came together in the Everglades—and nowhere else. 
Biodiversity can be defined in a number of ways. Three of these are particularly interesting when thinking about The Everglades. Michael Grunwald, in the above quotation from his book, The Swamp, suggests that the Everglades is unique in that it boasts an unusual mix of species not normally found together. As Grunwald notes, because South Florida is located on the boundary between tropical and temperate zones, many species, while not unique to the everglades, are found coexisting with other groups in unusual circumstances. Probably the most famous example is the coexistence of Alligators and crocodiles.
The most obvious definition is, of course, by the number of species in the animal and plant taxa of an ecosystem. By that standard, compared with other wetlands, marshes and deltas in the world, the Everglades is highly diverse but not uniquely so. For example, the Okavongo Delta wetland in Botswana and the Pantanal wetland in Brazil shelter at least as many or more species. A third important element of biodiversity could be found in the variety of habitats found within the greater ecosystem. In this sense, the Greater Everglades Ecosystem encompasses some of the worlds most diverse and distinctive wetland landscapes. The combination of high species diversity along with the extraordinary size and complexity of the numerous habitats of which the Everglades is composed does certainly make it a special place.