Amphibian diversity in Bolivia: a study with special reference to montane forest regions

Abstract

With respect to faunistic studies, Bolivia still has to be regarded one of the least explored countries in South America. Intensified investigations on Bolivian amphibians during the past decade led to an enormous increase of knowledge. However, the still ongoing discoveries of new taxa and biological phenomena clearly demonstrate the defectiveness of this knowledge. The present study provides a first comprehensive analysis of amphibian diversity in Bolivia. Distribution and ecology of species as important components of biodiversity are analyzed. Special emphasize is set on montane forest regions along the eastern Andean slopes (humid Yungas of the Departamentos Cochabamba and Santa Cruz, semi-humid montane forests in southern Departamento Santa Cruz). Fieldwork in the respective regions was conducted during the rainy seasons 1997/98 and 1998/99. In addition, data from museum collections and the literature, as well as from own investigations in 1994 were included in the analysis. An updated checklist of Bolivian amphibians is provided. The known and expected distributions in Bolivia's Departamentos and twelve defined ecoregions are given for all 200 species. Forty-five species are considered Bolivian endemics (=22.5%). The provided checklist is commented concerning taxonomic problems and miscellaneous notes. Eleutherodactylus peruvianus is deleted from the list, Eleutherodactylus dundeei is added, and Eleutherodactylus bisignatus is resurrected from synonymy. In addition, 61 species predicted to occur on Bolivian territory are listed. The spatial distribution of species diversity within Bolivia is analyzed and figured with respect to the distribution in twelve ecoregions. The Amazonian lowland rainforests harbor the largest portion of Bolivian Amphibians (45.1%), followed by the moist forest of the pre-Cambrian shield (35.4%) and the humid transition lowland forests (34.9%). The drier Chiquitania and Chaco forests contain distinctly lower species numbers. Species diversity in the Bolivian lowlands is decreasing when traveling the ecoregions from the north to the south. This general trend is interrupted by the Beni wet savannas exhibiting comparatively low species diversity (15.4%). The per-humid Yungas montane forests harbor the greatest species diversity of all Andean regions by far (32.0%). Regarding political endemism, 69.6% of all species occurring in the cloud forests ("Ceja") are restricted to Bolivia. The per-humid Yungas follow with 54.0% endemic species. In both ecoregions, endemism is distinctly over-represented, whereas political endemism is practically absent in lowland ecoregions. Only the Amazonian rainforests contain 4.5% endemic species. This value is due to species distributed in the humid peri-Andean forests. Ecoregion endemism is great in the lower Yungas forests and the peri-Andean forests (together 51.6%) extending into southern and central Peru, as well as in the upper Yungas montane forests and cloud forests (together 55.7%). Approximately one-fourth of all Bolivian Chaco inhabiting species are restricted to this region which extends to northern Argentina and Paraguay. Besides the total region of humid eastern Andean slopes, four hotspots of diversity are tentatively identified: Alto Madidi region, "La Siberia" region, Samaipata region, and eastern parts of the lower elevations of the Parque Nacional Amboro. The diverse montane forest regions in the Departamentos Cochabamba and Santa Cruz were investigated more detailed. An overview about all 70 montane forest species found in the study area is provided (only species distinctly exceeding 500 m a.s.l. are regarded montane forest species). For every species, information on distribution, natural history, and vocalization is given, as well as notes on its taxonomy, if necessary. The advertisement calls of 33 species are described and figured, many of them for the first time. A brief diagnosis is provided for the unnamed species included. A model of three more or less 'virtual' transects is established to compare and estimate the diversity patterns in montane forests: 1- Chapare transect (roughly equals the old road connecting Paractito and Cochabamba along the Rio San Mateo valley); 2 - Amboro transect (approximately equaling a line from the Samaipata area northeastward to the lowlands); 3 - Rio Seco transect (equaling a line with east-west expansion from Provincia Vallegrande to the village Rio Seco). The transects are described with respect to their alpha and beta diversity. Ecological comparison include activity patterns, habitat use, and reproductive modes for each of three elevational levels per transect. Thirty-six species are recorded from the Chapare transect, with greatest species diversity found between 1300-1700 m a.s.l. Communities are dominated by frogs of the genus Eleutherodactylus. The most common reproductive mode is direct terrestrial development (44%) followed by tadpole development in lotic water (31%). Lentic water bodies are practically absent. Beta diversity along the elevational gradient is limited, but gamma diversity is exceptional high. Species with restricted montane rainforest distributions are clearly dominating, and 53% of the recorded species are Bolivian endemics. With 48 species, the Amboro transect appears more diverse. However, this number is partly due to the larger number of sites sampled covering also a larger variety of habitats. Of the recorded species, 42% are Bolivian endemics. Exceptional high beta diversity is present at the upper elevations of the transect (1700-2300 m a.s.l.). The degree of alpha diversity is comparable to that of the Chapare transect. Direct terrestrial development is the dominating reproductive mode (29%), followed by egg deposition in lentic water (24%). The latter value is mainly due to hylid frogs at the lowermost elevational level (500 m a.s.l.). Thirty-one species are recorded from the Rio Seco transect, distributed at different elevational levels without significant differences in alpha diversity. In total, alpha diversity of investigated sites is lower when compared to the former transects. Beta diversity also is limited. Reproductive modes adapted to dry and distinctly seasonal environments (explosive breeding at temporary ponds, 35%; foam nest production, 32%) are dominant within the recorded communities. Many lowland species of the Chaco-Cerrado domain enter the Andean slopes up to remarkable elevations, reaching their upper limit of vertical distribution in the transect area. The results support close relationships of inter-Andean dry-valleys and Chaco-Cerrado formations in the lowlands. With help of "Parsimony Analysis of Endemism" (PAE) and "Neighbor Joining Analysis of Endemism" (NJAE), amphibian communities of three elevational levels (500, 1300-1600, 1900-2200 m a.s.l.) in each transect are compared to each other as well as to other sites along the eastern Andean slopes and the southwestern Amazon basin. The PAE and NJAE analyses revealed the following results: (1) relationships between the Amboro and Chapare transects are stronger than to the Rio Seco transect; this is true for all elevational levels considered; (2) the two upper elevational levels of all three transects have larger similarities than each of them to the lowermost level; (3) similarities of the 500 m elevational levels of the Amboro and Chapare transects are larger than those of upper elevations between the same transects; (4) the Amboro and Chapare transects are distinguished from more western Andean slopes, namely the Yungas de La Paz and southeastern Peru; (5) the Rio Seco transect has close relationships to the dry and seasonal environments of the Chaco; (6) Mataracu, a site at the eastern edge of Bolivian peri-Andean forests (17degree33'S, 63degree52'W, 500 m a.s.l.), is closer related to sites in northeastern Bolivia than to sites in peri-Andean forests of Peru. Findings on amphibian diversity are estimated and classified. Identified patterns are discussed with respect to possible determinants, including ecological determinism and historical perspectives. The patterns seem to agree best with vicariant