Diversidade de aranhas (Arachnida-Araneae) em dois gradientes altitudinais na Amazônia, Amazonas, Brasil

Abstract

Mountains probably represent the most obvious example of environmental influence on biotic communities. In this work we studied the altitudinal distribution of a spider community at the Pico da Neblina (AM-Brazil). We sampled six altitudes, 100 m, 400 m, 860 m, 2000 m e 2400 m, and in each of them three sites were investigated. Spiders were sampled with a beating tray (sampling unit = 20 beating events), during the day, and through active search (sampling unit = one hour of search), during the night. We obtained 54 samples by altitude, half with each method, totaling 324 samples for the whole gradient. We collected 3140 spiders, sorted to 528 morphospecies, from 39 families. Most species are rare, and 197 (37%) were represented by just one individual. Richness by altitude ranged from 224 (at 100 m) to 24 (at 2400 m) species and presented a negative relation with altitude, decreasing in a monotonic way. The observed pattern presented a poor fit with that generated by the mid-domain effect (MDE), which predicts higher richness at intermediate altitudes. Our data didn’t support a Rapoport effect either, which predicts a positive relation between altitude and altitudinal range size. These two variables were not related to each other and most species (333 species or 63%) were recorded at just one altitude. Only 25 species (25%) presented a large range, encompassing more than half of the altitudinal gradient. The distribution of individuals along the range of each species varied in a specific way, which is not in accordance with hypothesis based on rescue effects to explain the occurrence of Rapoport effect. The composition of species presented a great variation along the gradient and even for the sampling sites within each altitude. Beta diversity calculated for the whole gradient was 3,45, which means that the spider fauna from the Pico da Neblina includes three and a half different communities. The result of a NMDS seems to support this result as it present three main groups, one composed by the three first altitudes, another by the two highest altitudes and the fourth altitude (1550 m) situated in an intermediate position between the two groups. This result does not support the altitudinal division proposed for the Guaiana region, where our study site is located. Dominance pattern drastically increased at the two last altitudes. Finally, we described eight new species of the genus Chrysometa (Tetragnathidae), seven from the Pico da Neblina and one from another mountain sampled in the region, the Serra do Tapirapecó (AM). Diversity obtained at the Pico da Neblina was very high (12 species and 336 individuals) and its richness and especially abundance and relative importance increased with altitude. At the Serra do Tapirapecó the diversity of the genus was much lower (4 species and 40 individuals), which can imputed to the smaller altitude of the localities sampled there. The analysis of the pattern of altitudinal distribution at the study areas and in a larger scale (based on the literature), indicates that the genus reaches its maximum diversity at high altitude sites, and that species from highlands tend to have a narrower distribution than species from lowlands.