The trees of the Tehuantepec Moist Forest reach a height of approximately 25 to 30 m in its highest strata. Three strata are differentiated in the canopy: an inferior with trees 4 to 8 m, including several species of palms; an intermediate, from 10 to 15 m; and a superior, from 20 to 30 m. This type of forest grows predominantly in upland areas that remain well drained throughout the year. It has two characteristic species, one called the Ramón (Brosimum alicastrum), and another Chicozapote (Manilkara zapota). Other prevalent tree species include Swietenia macrophylla, Pimenta dioica, Bursera simaruba, Bucida bucera, Dendropanax arboreus, Maclura tinctoria, Pouteria reticulata, Sabal spp., and Talisia olivaeformis. Plant associations are formed according to drainage and soil composition. The most important characteristic of this forest is its deciduousness, with some tree species losing one fourth of their foliage during the dry season (Pennington and Sarukhán 1998).
Some areas within the biotope are characterized by deep, heavy, and sticky soils that are flooded during the rainy season, and which dry and crack open in the dry season. They are found in small or medium-size lowland depressions. These soils are poorly drained and have a permanent layer of water during the rainy season. They contain low vegetation forest, where the dominant species is palo tinto (Haematoxylum campechianum). This plant association is found in areas that accumulate water from adjacent areas. According to Lundell (1937), the canopy height of this plant association varies between 5 and 11 m, and the number of individual trees increases from the center (flooded areas) to its periphery (inundated periodically). Other species found are the jícaro or guiro (Crescentia alata) and the pucté (Bucidas bursera).
Along with El Mirador-Río Azul National Park, Naatchún Dos Lagunas is considered to have the highest plant diversity in the MBR, with an estimation of more than 200 species per hectare. Within the MBR, the biotope has registered the highest number of mahogany trees per hectare (APESA 1993).
The diversity of fauna is partly the result of its varied terrain, which includes hills, flooded lowlands, and watering holes. There have been very few recent faunal investigations, although field visits and interviews with officials in charge of the area indicate the existence of regional endemic species including the crocodile (Crocodylus moreletii), the Central American river turtle (Dermatemys mawui) classified as an endangered species by IUCN's Red Book, ocellated turkey (Agriocharis ocellata), howler monkey (Alouatta pigra) and red snook (Petenia splendida). There are abundant populations of spider monkeys (Ateles geoffroyi), and, according to park guards, the tapir (Tapirus bairdii) is relatively common. Among those species classified by CONAP as vulnerable, are the jaguar (Panthera onca), margay (Leopardus wiedii), ocelot (Leopardus pardalis), cougar (Puma concolor), jaguarundi (Herpailurus yagouaroundi), pecarí (Dicotyles pecari) and red-brocket deer (Mazama americana). Among the birds are the jabiru (Jabiru mycteria), king vulture (Sarcoramphus papa), hook-billed kite (Chondrohierax uncinatus), peregrin falcon (Falco peregrino), and mealy Amazon parrot (Amazona farinosa) (CONAP 2000a). The Wildlife Conservation Society (WCS) installed five transects at the beginning of 2002 to inventory animal species (Balas 2002, pers. comm.).