The beaches of Sian Ka’an constitute a very important habitat and nesting grounds for many species of wildlife, as well as serving as an important transition between land and ocean. Most notably during the months of May through August, the beach is a nesting ground for 4 endangered sea turtle species that have lost important nesting sites along many parts of the Caribbean coast.

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Ocean water and fresh water from inland and the aquifer meet and mingle in the brackish water lagoons along the coast of Sian Ka’an. These lagoons are lined with salt tolerant mangrove and grass species which provide a home to fish and mollusk species that make the area an attractive nesting ground for wading birds and residence of two crocodile species, Crocodylus moreletii and C. acutus.

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Fresh water contained in the Yucatan’s underground aquifer carves away at the limestone above, weakening it and eventually causing it to give way. The result of this phenomenon is a unique habitat known as a cenote or sink hole. Many cenotes remain connected with the aquifer and with other cenotes through underground passageways. Occasionally, however, cenotes are found to be completely isolated from other bodies of water and may contain fish and other wildlife that have evolved over time into completely distinct species.

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The coastal dune is an important protector of inland habitats, especially during storms. Coastal vegetation includes Coccoloba uvifera, Tournefortia gnaphalodes, Suriana maritima, Sesuvium portalacastrum, Ambrosia hispida, Ipomoea and many other species.

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The boundaries of the Sian Ka’an Biosphere Reserve include 110km of the second largest barrier reef in the world, which extends onward past Belize and Honduras. The reef is home to many important and several endangered species. Uncontrolled development along the Caribbean coast of Mexico has placed the reef in grave danger, as it is easily damaged and not so easily restored. In Sian ka'an there have been identified more than seventy species of corals, as the enormous corals brain that end up measuring up to six or seven meters in diameters. There are also hundreds of species of fish that often form tight groups, and an immense quantity of crustaceans, as the thorny lobster and mollusks, some of great size, as the rosy snail or flamenco's tongue.

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The fresh water lagoons of Sian Ka’an are supplied with fresh water from the subterranean aquifer by small springs or cenotes. Found mostly inland, the fresh water filters toward the ocean in channels or through the surrounding wetlands. The lagoons are home to a number of species of fish and vegetation not found in the coastal areas.

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Petenes, or hummocks, are isolated areas of forest from several meters to several kilometers in diameter that are surrounded by swamps or inundated savanna land. These formations are found only in Cuba, the Florida Everglades, Brasil and the Yucatan Peninsula. There is often a cenote at the center of the hummock, which is surrounded by concentric circles of vegetation, from hardwood trees to grasses and rushes. Common species include cedar, mahogany, and bubble gum tree, in the center ring, which are surrounded by various palm trees and the poisonwood tree, and eventually mangrove, rushes, and grasses. Many species of fauna live in hummocks, from insects to reptiles, mammals, and birds.

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Many of the mammal species found within Sian Ka’an reside within the low tropical forest land which is located in the westerly portions of the reserve. The forests contain many hardwood species, including chechem (poison tree), chicozapote, )bubble gum tree), mahogany, tsalam, and other valued hardwoods. The environmental importance of these areas is increased by the international demand for hardwood that is pressuring many countries in the neotropics.

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Coastal savannas are large areas of low-lying land with sparse, oxygen-poor soil that is inundated throughout much of the year. As few tree species have adapted to these conditions, savannas are dominated by species of grass, reeds, and rushes that rarely exceed three meters in height. Inland savannas contain shrubs and occasional trees in addition to grass, are drier and have a lower salinity. These environments are in danger of natural fires during the dry season.

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Mangroves, savannas, swamps and marshes are all included within the classification of “wetland”. Water plays a vital role in the existence of each of these habitats, whether it be fresh, salty, or brackish. Wetlands line the coast of Sian Ka’an, playing an important protective role as a type of buffer between the ocean and the land that can absorb the impact of storms. Wetland areas contain immense biological diversity and are home to a number of endangered species. Evident in all inundated wetlands is a conglomeration of algae known as periphyton that is believed to play an important role in the dissolution of calciferous soils, as well as being an invaluable source of food for many species of fish, mollusks, and insects.
Four species of mangrove line the coastal areas of Sian Ka’an. The mangrove communities are critical for the survival of many species of fish, birds, insects, reptiles, and other plants. Many species of mangrove filter contaminated water and trap loosened sediment, making them protectors of the coral reef and other littoral habitats. The mangrove species found within Sian Ka’an are Rhizophora mangle (red mangrove), Avicennia germinans (black mangrove), Laguncularia racemosa (white mangrove), and Conocarpus erectus (buttonwood mangrove).

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