Fire is an important factor in the ecology of the tree-island glades, just as it is in the pineland. Here, too, artificial barriers such as canals and roads have hindered the spread of natural fires. There is some evidence that tree islands were scattered more thinly over the sawgrass prairie a half-century ago, when a single fire might wipe out scores of them and destroy much of the bed of peat that provided a foothold for them. A bird’s-eye view of the glades region today shows many tree islands that have been established in recent decades. But park rangers are now utilizing controlled fires in the glades as well as in the pineland. This tends to prevent new tree islands from taking hold, and thus helps maintain the natural everglades landscape. Driving over the glades toward Florida Bay, you come to a sign reading “Rock Reef Pass—Elevation 3 Feet.” The road then traverses the so-called dwarf cypress forest. The forest is an open area of scattered, stunted baldcypress growing where marl (which, unlike peat, does not burn) has accumulated in small potholes dissolved in the limestone. These marl potholes provide a foothold for the dwarf cypresses in an area that is spotted with cypress heads containing much larger trees. Many of the dwarf cypresses are more than 100 years old, while tall cypresses in the heads may be less than 50 years old. These anomalies can be attributed to varying soil depths and water levels and to the effects of fire. Before you reach the limit of the fresh-water marsh you will come to a side road leading to Mahogany Hammock. (A good foot trail makes it easy to explore this hardwood jungle island.) Just beyond, you will notice the first red mangroves. Small and scattered in this zone, they are a signal that you are approaching a strikingly different plant-and-animal community, the mangrove swamp. Mangrove Swamp (_elevation: sea level to 1 foot above sea level_) [Illustration: PRAIRIE SEDGES] BAY JAMAICA DOGWOOD GUMBO-LIMBO MAHOGANY PRAIRIE SEDGES BUTTONWOOD WHITE-MANGROVE OSPREY NEST BLACK-MANGROVE RED-MANGROVE (very dense growth) ESTUARY [Illustration: BONEFISH Comes in with the tide to feed on crabs and mollusks in shallow water] [Illustration: FLORIDA HORN SHELL Lives in shallow water and feeds upon algae and other aquatic plants] [Illustration: ’COON OYSTER A small (1½″) oyster that lives attached to the roots of mangroves] The southward-creeping waters of the glades eventually meet and mingle with the salty waters of the tidal estuaries. In this transition zone and along the gulf and Florida Bay coasts a group of trees that are tolerant of salty conditions, called “mangroves,” form a vast, watery wilderness. Impenetrable except by boat, it occupies hundreds of square miles, embracing both the shifting zone of brackish water and the saltier coastal waters.