Structure et organisation du peuplement d ’Anoures de Cocha Cashu, Parc National Manu, Amazonie péruvienne

Résumé En

The structure, relative abundance and density of the anuran community of Cocha Cashu, in the amazonian rainforest of Peru (350 m, 11° 55' S, 71° 18' W, Manu National Park) was studied during 395 days of field work, from September 1985 to November 1989. The study compared the two major types of upper amazonian habitats, floodplain forest and upland forest. The entire assemblage is composed of 81 species (Table I). There are more diurnal species in the upland forest than in the floodplain forest. Although each type of habitat has twenty terrestrial species and twelve food specialists, mainly ant eaters, these guilds account for significantly different proportions on each of the assemblages (Tables II and III). Sixty six species (out of 71) were observed in 250 days of transect sampling in the floodplain, whereas 49 species were counted in only 55 days on the uplands (Fig. 9) ; ten of these species do not occur in the floodplain and 39 are common to both habitats. Although relative abundance curves of both habitats are similar (Fig. 1 0), relative abundances of the 39 shared species differ significantly for each type of habitat (Fig. 1 1). Most of the Anuran species are small. Mean males’ Snout-Vent-Length ranges from 13.5 mm in Eleutherodactylus cf. carvalhoi to 136 mm in Leptodac-tylus pentadactylus, and at least 80 % weigh less than 10 g (Table V). None of the species accounts for more than 1 5 % of the relative abundances in floodplain or in the upland, suggesting a rather equitable distribution of abundance. No significant differences between the two types of habitats were found in terms of numbers of species, grouped by mode of reproduction (Table V). However, when relative abundance of individuals grouped by mode of reproduction were compared, all but the species laying their eggs in vegetation overhanging water and those building terrestrial foam nests without free tadpoles, showed significant differences (Table VI). The Cocha Cashu floodplain data are compared to those from Santa Cecilia (Ecuador, 350 m, 0° 03' N 77°11'W), the richest site in Anuran species (87 species) until now. There are no differences in the number of species by mode of reproduction, but, except for the Dendrobatidae, all relative abundances of individuals by mode of reproduction differ significantly between the two sites (Tab. VII). Densities were estimated by two methods : 219 censuses of male calling activity of ten forest breeding species (Table VIII), and 80 forest litter plots. Densities estimated by these two methods did not coincide. Plots revealed an uneven distribution of frogs in the forest and densities similar to other amazonian sites, but lower than those estimated for Central America (Table XI). Although most species were observed on transects, pit-fall traps were effective on catching active nocturnal terrestrial species not easily observed by other methods (Table XII). None of the methods used here to estimate diversity or density are efficient enough to sample adequately the entire community. The possible causes of the anuran species richness at Cocha Cashu are briefly discussed. The number of species at Cocha Cashu and Santa Cecilia being very similar (respectively 81 and 87 species) despite the striking differences in rainfall regime between the two sites, high atmospheric humidity cannot be considered as the only determinant of the high species diversity of anurans in tropical rainforests. The importance of historical factors, and more particularly of the effect of river dynamics on the heterogeneity of the Upper Amazonian environment, is emphasized.