Ría Celestún stands out for its rich ecosystems, communities and species found in a relatively small area. These are relevant in conservation terms as they are located in an inter-dependent coastal environment: the shallow marine continental shelf, coastal sand dunes, mangroves, coastal lagoons, swamps, hillocks, flooded pastures, the flooded lowland jungle, and a type of lowland jungle subcaducifolia in a prime state of conservation (Tun et al. 1998; Rzendowski 1986).
Endemic plant species in the coastal sand dunes of Celestún include: Matelea yucatanensis, Exostema caribaeum and Spermacoce confusa. Other noteworthy endangered species include palm trees: chi'it (Thrinax radiata), nakax (Coccothrinax readii), kuka (Pseudophoenix sargentii) and Sabal gretheridae, which is classified as rare. Of the agavaceous family: Beaucarnea pliabilis is considered endangered on a region-wide scale (Tun et al. 1998).
The wide diversity of fauna in Ría Celestún includes 304 bird species, including resident and migratory (García & Vigilante 1989). Of this group, coastal and swamp species include egrets (Ardeidae), ducks (Anatidae), gulls and a variety of migratory species such as sandpipers that fly south from the North American continent (the United States and Canada) during the winter. The region is a resting, feeding and nesting area for the pink flamingo (Phoenicopterus ruber ruber), similar to Ría Lagartos Biosphere Reserve, where it tends to nest (Espino-Barros 1989; Espino-Barros 1989; Hernández & García 1976). There are around 28,000 flamingoes which have been counted during December and February in their natural grounds along the Yucatán Peninsula (Espino-Barros 1989).
According to the 1994 Official Mexican Ecology Regulation 059 (NOM-059-ECOL-1994) which lists birds in endangered categories (1994 Official Gazette), in Ría, seven species are considered in need of special protection, 21 are rare, 18 are endangered and five on the verge of extinction, including the royal teal (Cairina moschata), lesser sea petrel (Sterna antillarum), Jabirú (Jabiru mycteria), king vulture (Sarcoramphus papa) and the white-fronted goose (Anser albifrons). Endangered species include the Yucatán parrot (Amazona xantholora), rufous egret (Egretta rufescens), lesser yellow-headed vulture (Cathartes burrovianus), wood stork (Mycteria americana) and the peregrine falcon (Falco peregrinus), also listed in CITES Appendix I. The International Union for the Conservation of Nature (IUCN) lists Charadrius melodus as vulnerable. Endemic species include the Yucatan wren (Campylorhynchus yucatanicus), which appears to depend on the coastal sand dune clumps, where it is more numerous than in any other kind of vegetation, and the Mexican Sheartail (Calothorax eliza) (Birdlife International 2001; IUCN 2002).
Some 75 mammal species have been registered to date, broken down in 11 orders and 26 families. Chiropters account for most of these species, followed by rodents and carnivores. This group is rarely endemic, and only rodents (Peromyscus yucatanicus) are found throughout the peninsula. According to the NOM-059-ECOL-1994 list, five of these species are rare, including the shrew (Cryptotis mayensis), cacomistle (Bassariscus sumichrasti) and the kinkajou (Potos flavus); eight species are endangered, including the anteater (Tamandua mexicana) and jaguarundi (Herpailurus yagouarundi); and seven species are on the verge of extinction: the spider monkey (Ateles geoffroyi), the tayra (Eira barbara), ocelot (Leopardus pardalis), margay (L. wiedii), jaguar (Panthera onca), tapir (Tapirus bairdii) and the manatee (Trichechus manatus) (Batllori 1986; Herrera & Trejo 1991; IUCN 2002).
In terms of amphibians and reptiles reported in the reserve, there are 13 species of amphibians in seven families and 64 reptile species in 18 families, 36 species of which are snakes. Four of these species qualify for special protection, 12 are considered rare, six are endangered, and three are in danger of becoming extinct (NOM-059.ECOL-1994). The hawksbill turtle (Eretmochelys imbricata) and Carey turtle (Caretta caretta) are other species on the verge of extinction (IUCN 2002; Rodríguez & Durán 1993), and a rare species is the Morelet's crocodile (Crocodylus moreletii). There are 15 endemic species which are fairly common throughout the peninsula, plus a Caribbean species that has only been found on the coast of the Yucatán in Mexico.
In Ría Celestún, 140 fish species have been registered, divided into 18 orders, 48 families of teleosteum and one suborder with six families of elasmobranchiae. The Scianidae, Sparidae, Gerreidae and Lutjanidae families are the most representative as they are the most numerous of species. Some species are endemic, such as Gambusia yucatana and others are subject to special protection, such as Poecilia velifera and Cichlasoma urophthalmus (NOM-059-ECOL-1994).
The reed marshes of Ría Celestún are an important haven, breeding, and feeding grounds for a wide variety of fish, mollusks and crustaceans, which are the basis of fishing, one of the principal productive activities in the area. Some key species for trading purposes are shrimp (Farfantepenaeus spp.), octopus (Octopus maya), blue crab (Callinectes sapidus), Mayan cichlids (Cichlasoma urophthalmus), catfish (Arius melanopus) and mullets (Mugil spp.). Species such as sea bass (Epeniphelus morio), durophagous stone crab (Menipe mercenaria) and octopus (Octopus maya) are being over-extracted through the use of non-regulatory equipment and failure to follow established fishing bans (Gío 1996).
The macrocosmic ecological importance of the region where the reserve is located stems from the fact it defines a biological corridor split in relatively narrow strips between the eco-systems represented by Pantanos de Centla Biosphere Reserve in Tabasco; Laguna de Términos Flora & Fauna Protection Area and Los Petenes Biosphere Reserve in Campeche; the Ría Celestún, Campeche and Yucatán Biosphere Reserves; and El Palmar State Reserve in Yucatán (Herrera-Silveira 1991; Contreras 1993).