The Florida Keys refers to a chain of islands -- called "The Keys" locally -- that are strung out for 170 miles or so in a southwestward arc from the southeast tip of Florida.

A good portion of the islands are developed, incorporated and connected to the mainland by U.S. Highway 1. The Florida Keys are in Monroe County, which also has a large mainland area in Everglades National Park. There are four incorporated towns in the Keys islands, and many unincorporated villages. The 30 or so inhabited islands are connected by 42 bridges. The 100-mile stretch of U.S. 1 running through the Keys is called the Overseas Highway.

West of Key West, uninhabited islands are protected in federal sanctuaries. Along the Keys many small and a few large islands aren't connected by roads. Monroe County estimates that a total of 844 islands make up the Keys.

The islands lie between 24 and 25 degrees north latitude, in the subtropics. More so than South Florida, the Keys are associated with Caribbean habitats. Unlike volcanic islands, however, the Keys were built by plants and animals. In the upper part of the Keys, the islands are remnant, fossilized coral reefs that were exposed as sea level declined. In the Lower Keys, the islands are accumulations of limestone from plants and marine organisms.

The natural habitats of the Keys are characterized as upland forests, inland wetlands and shoreline zones. Soil ranges from sand to marl/silt to rich, decomposed leaf litter. In some places, "caprock" -- the eroded surface of ancient coral formations -- covers the ground. Rain falling through leaf debris becomes acid and gradually dissolves holes in the limestone, where soil accumulates and tree roots find purchase.

The islands' have a subtropical climate; they are the only frost-free places in Florida. There are two main "seasons": heat, rain, and humidity from May or June to December, and somewhat dryer, cooler weather from December to May. Many plants grow slowly or go dormant in the dry season. Some native trees are deciduous, and drop their leaves in the winter or with spring winds.

With its geology, climate, and habitats, the Keys have distinctive plant and animals species, some found nowhere else in America. The Keys define the northern range of many Caribbean plants. The climate also allows a huge range of imported plants to thrive. Nearly any houseplant known to commerce, and most landscape plants of the South, can thrive in the Keys climate (if not in the thin soil). Some exotic species which arrived as landscape plants now invade and threaten natural areas.

Environmental extremes of heat, drought, and infertile soil are matched by adaptations in native plant species. Many are resinous or otherwise armed against drought. Large leaves with drip tips direct rain down to roots. Vegetative reproduction supports regrowth from hurricane damage. Gumbo-Limbo, for one, easily sprouts from cut roots and branches. Trees and plants used for medicine, manufacturing and decorative woodworking are among the natives of the Keys.

Few natives reach very tall. Two that do are palms -- Royal Palm, rare outside the Everglades, which can reach 75-100 feet tall, and Cabbage Palm, the state tree, up to 90 feet. Typically, the canopy of native trees is around 30 to 50 feet.

Interestingly, the natives don't include some well-known plants that may be seen growing wild in the Keys, such as Coconut Palm (Cocus nucifera), Century Plant (Agave decipiens), and Papaya (Carica papaya). None of the most common landscape plants -- Bougainvillea, hibiscus, are native.

As for the history of inhabitation, Key West, the westernmost of the inhabited islands, is an old city by Florida standards. It developed with shipping and trade in the early 19th Century, and was the largest, most properous city in the state at one time. In the rest of the Keys, growth came slowly. Growth accelerated after WWII, when a freshwater pipeline began supplying the Keys and an electric cooperative provided power to the islands of the Upper and Middle Keys. By 2000, Monroe County's -- the Keys' -- population reached nearly 80,000. Obviously, development for residents and a thriving tourism industry has eliminated or altered large parts of the natural environment of the Keys.

Still, some large tracts on the inhabited islands, including hundreds of undeveloped lots in platted subdivisions, remain in more or less natural states. Federal, state, and local regulations protect many plant and animal species. Clearing and development in wetlands and shoreline areas is (theoretically) banned. When development is permitted, rules limit clearing and require replacement of rare and endangered species. The old days, when stripping the land, dredging canals and dumping spoil in wetlands was allowed, even encouraged, are supposedly over.

Some scars of past development are healing under restoration programs, even simple mandated neglect. Local, state and federal agencies manage land-acquisition programs and work with private conservation groups to remove land from development.

Everglades National Park was the first federal program to embrace the Florida Keys natural environment. The park boundary runs just off the Upper Keys, including miles of the bayside waters and islands. Since then, other state and federal parks, reserves, and conservation areas have ensured thousands of acres will remain undeveloped. In the north, a national wildlife refuge, Crocodile Lakes, and an adjacent state reserve protect many square miles of North Key Largo. In the lower Keys, the Key Deer Refuge protects the range of the small native deer on Big Pine Key and nearby islands. Marine environments are protected in large national- and state-designated marine sanctuaries off Key Largo, the Lower Keys and in a national park, Dry Tortugas, 70 miles west of Key West.