Location and General Description
This ecoregion located at the juncture of Central and South America. Condensation over the warm land produced by moisture-laden air from the Caribbean Sea hitting the mountains produces constant high humidity and precipitation (DeVries 1987). Rainfall ranges from about 2,500 mm in central Panama (Ridgely 1976) to over 5,000 mm in southern Nicaragua. These forests are distinguished from the cooler subtropical Central American Atlantic Moist Forest ecoregion to the north by their distinct geologic history and consistent annual temperatures above 24o C (Holdridge 1968).
Until recent geologic times, the isthmus south of central Nicaragua was discontinuous, volcanically active and topographically and environmentally diverse. Basalt bedrock is the parent material of the residual and often unconsolidated soils covering the hilly areas of this ecoregion. Old alluvial terraces form the base of the swamp forests and flat lands in the lowest elevations and near the Caribbean Coast (Hartshorn 1983; Vásquez Morera 1983). The northern section of this ecoregion is formed of a wide, relatively flat alluvial plain, with a gradual elevation change from sea level to 500 m, while south into Panama, steep slopes rise up from the Atlantic Ocean, significantly narrowing the ecoregion to only 5-10 km in width.
This ecoregion is characterized by a lush, tall tropical evergreen forest of huge, buttressed canopy trees reaching 40 m in height and an extremely rich epiphyte flora. The palm component includes many subcanopy and understory species. Abundant subcanopy palm species are Welfia georgii, Socratea durissima and Iriatea gigantea and in permanently flooded areas, Raphia taedigera (Hartshorn 1983). Seasonal swamp forests occur in the lowest and flattest areas in Nicaragua and northern Costa Rica, particularly along the coast where they grade into mangrove forests. In these forests, Gavilán (Pentaclethra macroloba) dominates the canopy, along with Caobilla (Carapa nicaraguensis). The almendro (Dipteryx panamensis) and the monkey pot tree (Lecythis ampla) are two outstanding and rapidly disappearing canopy emergents, which are regional endemics of the lowlands, below 250 m.
While biologically very diverse, this ecoregion supports low levels of endemism. The high species richness is derived in great part from the mixing of North and South American floras and faunas on this land bridge (Rich and Rich 1983; Raven 1985). The resident fauna, including butterfly, reptile, amphibian, bird, and mammalian taxa are for the most part wide-ranging representative species of a wet tropical forest ecoregion that extends from southern Mexico to northern South America (DeVries 1987; Stiles 1985; Wilson 1990; Guyer 1990). Strict endemism among fauna is almost non-existent: between 80-100% of the mammal species that occur in northern Costa Rica also occur in Panama, Nicaragua, Honduras, and Colombia (Stiles 1985; Wilson 1990). However, a number of restricted range birds are shared with the Central American Atlantic moist forests ecoregion to the north, together forming an Endemic Bird Area (Stattersfield et al 1999). The Caribbean slope is a major migration route (Stiles 1983); neotropical and altitudinal migrants comprise about 30% of the avifauna, particularly against the foothills (adapted from Stiles 1985 and 1989).
Few large expanses of primary rainforest remain intact, occurring only in large reserves, particularly the Indio-Maiz Biological Reserve (approx. 400,000 ha) along Nicaragua's coast (Cardenal Sevilla 1990) and in eastern Panama along La Amistad International Park. These blocks retain nearly all vertebrate species of this ecoregion, including most large predators though increasing isolation threatens their long-term viability (Powell pers. comm.; Stiles 1985). Although small in size, the 1,500 ha La Selva Biological Station in Northeastern Costa Rica hosts permanent populations of large predators (Panthera onca) and herbivores (Tapirus bairdii) probably because its connection to the upper montane forests of Braulio Carrillo National Park. In fact, this connection represents the last intact gradient of primary forest from near sea level to 2,900 m elevation in Central America. (Lieberman et al. 1996). Tortuguero National Park, along the Caribbean coast of northern Costa Rica acts as an isolated refuge to many species, as does Barra de Colorado Wildlife Refuge – although enforcement of protection remains a challenge to both areas.
Logging and clearing of remaining forests threaten many species of slow-growing trees, such as the almendro and monkey pot tree. Four species of marine turtles nest on the coastal beaches. The Atlantic lowlands and middle elevations contain some of the rarest butterfly species in Central America and some of the world's highest butterfly species richness (DeVries 1987). Scarlet and great green macaws (Ara macao, A. ambigua) nest in the lowland forests. The ecoregion is extremely important for great green macaw, a vulnerable species, which moves seasonally to higher elevations within the ecoregion. Other endangered restricted range species that migrate seasonally to this ecoregion are the three-wattled bellbird (Procnias tricarunculata) and the bare-necked umbrella bird (Cephalopterus glabricollis).
Although a few large blocks of intact habitat still exist, the once vast Atlantic lowland forests have been seriously fragmented in recent years (Sánchez-Azofeifa et al. 1999). The tropical evergreen forests are among the least well represented in Costa Rica's protected areas system (Stiles, 1985), although large reserves exist in southern Nicaragua and eastern Panama. Most lowland wet forest parks are too small and isolated to maintain viable populations of large-ranging species; the only one connected to highland forest is La Selva in Costa Rica (just over 1,700 ha), which is too small to protect much of the avifauna (Boza 1996 and pers. comm, Stiles and Skutch 1989) and other larger taxa. Only about 130,000 ha in the lowland Atlantic zone are currently protected and difficult economic conditions offer little likelihood that the area in protection will be significantly expanded (Powell et al 1992).
The lack of protection of the Atlantic lowlands and the heavy bias toward deforestation at elevations of < 1,000 m (Powell et al 1992) contribute to the fragmentation and elimination of these forests. With gradual slopes and relatively good access, much of Costa Rica's remaining Atlantic slope forest has been intervened or exists in small fragments. Nicaragua's lack of access and the until-recently inaccessible steeper slopes of western Panama's Atlantic lowlands and foothills have left these areas relatively forested (Mendez 1994; based on Landsat images 1992).
Types and Severity of Threats
Flat areas with alluvial soils are under banana cultivation, while the less fertile hilly basaltic soils have more recently been logged and converted to cattle pasture. New access roads, malaria control, and incentives for migration to these areas in the last few decades have encouraged settlement and resource extraction from the area. The last remaining intact forests in this ecoregion are currently under tremendous logging pressure and are being felled, rapidly (ProAmbiente 1998). Squatting and other property claims are resulting in unregulated destruction in many areas despite the legislation in place to protect forests. Clear-cuts have even been made illegally within many parks - including Tortuguero National Park – which now provides ready access for poachers to the once isolated second largest Green Turtle (Chelonia midas) nesting beach in the world.
While future threats in this ecoregion vary among the three countries, destruction of forest habitat through logging and conversion to cattle pasture are the most widespread and significant. Illegal logging and squatting is making in-roads into the remaining large forest blocks in Nicaragua (Powell pers. comm.), while in the already fragmented Costa Rican forests; the principal threat is clearing for cattle production (Wille pers. comm). In Panama, the Atlantic slopes are steeper and less likely to be converted to agriculture; however, new roads in this region will certainly increase human settlement and cutting of accessible forests. The Mesoamerican Biological Corridor project, which aims to enhance connectivity among the Atlantic slope habitats, may provide some needed support for new or expanded protected areas or for payments for environmental services provided by private lands.
Justification of Ecoregion Delineation
The delineation’s for this ecoregion were derived from two separate national vegetation maps. In Costa Rica we followed the delineation’s of life zones by Tosi (1969). In this case we lumped the following ecoregion of the Pacific slope catchment, south of the Central Valley: Tropical Wet Forest, Premontane Wet Forest, Tropical Moist Forest (TMF), TMF Perhumid Province Transition, and TMF Premontane Transition Belt. The western delineation is marked by either the continental divide or where it abuts to montane forests at higher elevations. In Panama we relied on the UNDP (1970) vegetation map, again, lumping the following lowland and premontane moist forest components on the southern portion of the continental divide: Tropical Moist Forest, Premontane Rain Forest, and Tropical Wet Forest. The northern delineation, which separates this ecoregion from the Central American Atlantic moist forests to the north due to distinct species associations and seasonal climatic factors (Tosi et al. unpublished). This ecoregion also hosts several endemic species (Stattersfield et al 1998) (see description above for further details).
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Prepared by: George Powell, Sue Palminteri, and Jan Schipper
Reviewed by: In process